X-Git-Url: https://hackdaworld.org/gitweb/?a=blobdiff_plain;f=moldyn.c;h=7ea5d5080ab1ab185fa46b2137894239e9233902;hb=1d83ceb2ce2ff5150fd079f1066b7f583e38c8f4;hp=9d782d657fbc89c5ab800ed7705de48c87739a1e;hpb=76f807f6dda48b6d606309cea79005e612e4f665;p=physik%2Fposic.git

diff --git a/moldyn.c b/moldyn.c
index 9d782d6..7ea5d50 100644
--- a/moldyn.c
+++ b/moldyn.c
@@ -71,9 +71,18 @@ int set_cutoff(t_moldyn *moldyn,double cutoff) {
 	return 0;
 }
 
-int set_temperature(t_moldyn *moldyn,double t) {
-	
-	moldyn->t=t;
+int set_temperature(t_moldyn *moldyn,double t_ref) {
+
+	moldyn->t_ref=t_ref;
+
+	return 0;
+}
+
+int set_pt_scale(t_moldyn *moldyn,u8 ptype,double ptc,u8 ttype,double ttc) {
+
+	moldyn->pt_scale=(ptype|ttype);
+	moldyn->t_tc=ttc;
+	moldyn->p_tc=ptc;
 
 	return 0;
 }
@@ -93,6 +102,13 @@ int set_dim(t_moldyn *moldyn,double x,double y,double z,u8 visualize) {
 	return 0;
 }
 
+int set_nn_dist(t_moldyn *moldyn,double dist) {
+
+	moldyn->nnd=dist;
+
+	return 0;
+}
+
 int set_pbc(t_moldyn *moldyn,u8 x,u8 y,u8 z) {
 
 	if(x)
@@ -123,6 +139,14 @@ int set_potential2b(t_moldyn *moldyn,pf_func2b func,void *params) {
 	return 0;
 }
 
+int set_potential2b_post(t_moldyn *moldyn,pf_func2b_post func,void *params) {
+
+	moldyn->func2b_post=func;
+	moldyn->pot2b_params=params;
+
+	return 0;
+}
+
 int set_potential3b(t_moldyn *moldyn,pf_func3b func,void *params) {
 
 	moldyn->func3b=func;
@@ -131,38 +155,56 @@ int set_potential3b(t_moldyn *moldyn,pf_func3b func,void *params) {
 	return 0;
 }
 
-int moldyn_set_log(t_moldyn *moldyn,u8 type,char *fb,int timer) {
+int moldyn_set_log_dir(t_moldyn *moldyn,char *dir) {
+
+	strncpy(moldyn->vlsdir,dir,127);
+
+	return 0;
+}
+	
+int moldyn_set_log(t_moldyn *moldyn,u8 type,int timer) {
+
+	char filename[128];
+	int ret;
 
 	switch(type) {
 		case LOG_TOTAL_ENERGY:
 			moldyn->ewrite=timer;
-			moldyn->efd=open(fb,O_WRONLY|O_CREAT|O_TRUNC);
+			snprintf(filename,127,"%s/energy",moldyn->vlsdir);
+			moldyn->efd=open(filename,
+			                 O_WRONLY|O_CREAT|O_EXCL,
+			                 S_IRUSR|S_IWUSR);
 			if(moldyn->efd<0) {
-				perror("[moldyn] efd open");
+				perror("[moldyn] energy log fd open");
 				return moldyn->efd;
 			}
 			dprintf(moldyn->efd,"# total energy log file\n");
 			break;
 		case LOG_TOTAL_MOMENTUM:
 			moldyn->mwrite=timer;
-			moldyn->mfd=open(fb,O_WRONLY|O_CREAT|O_TRUNC);
+			snprintf(filename,127,"%s/momentum",moldyn->vlsdir);
+			moldyn->mfd=open(filename,
+			                 O_WRONLY|O_CREAT|O_EXCL,
+			                 S_IRUSR|S_IWUSR);
 			if(moldyn->mfd<0) {
-				perror("[moldyn] mfd open");
+				perror("[moldyn] momentum log fd open");
 				return moldyn->mfd;
 			}
 			dprintf(moldyn->efd,"# total momentum log file\n");
 			break;
 		case SAVE_STEP:
 			moldyn->swrite=timer;
-			strncpy(moldyn->sfb,fb,63);
 			break;
 		case VISUAL_STEP:
 			moldyn->vwrite=timer;
-			strncpy(moldyn->vfb,fb,63);
-			visual_init(&(moldyn->vis),fb);
+			ret=visual_init(&(moldyn->vis),moldyn->vlsdir);
+			if(ret<0) {
+				printf("[moldyn] visual init failure\n");
+				return ret;
+			}
 			break;
 		default:
-			printf("unknown log mechanism: %02x\n",type);
+			printf("[moldyn] unknown log mechanism: %02x\n",type);
 			return -1;
 	}
 
@@ -185,17 +227,15 @@ int create_lattice(t_moldyn *moldyn,u8 type,double lc,int element,double mass,
 	int count;
 	int ret;
 	t_3dvec origin;
-	t_atom *atom;
 
 	count=a*b*c;
-	atom=moldyn->atom;
 
 	if(type==FCC) count*=4;
 
 	if(type==DIAMOND) count*=8;
 
-	atom=malloc(count*sizeof(t_atom));
-	if(atom==NULL) {
+	moldyn->atom=malloc(count*sizeof(t_atom));
+	if(moldyn->atom==NULL) {
 		perror("malloc (atoms)");
 		return -1;
 	}
@@ -204,10 +244,10 @@ int create_lattice(t_moldyn *moldyn,u8 type,double lc,int element,double mass,
 
 	switch(type) {
 		case FCC:
-			ret=fcc_init(a,b,c,lc,atom,&origin);
+			ret=fcc_init(a,b,c,lc,moldyn->atom,&origin);
 			break;
 		case DIAMOND:
-			ret=diamond_init(a,b,c,lc,atom,&origin);
+			ret=diamond_init(a,b,c,lc,moldyn->atom,&origin);
 			break;
 		default:
 			printf("unknown lattice type (%02x)\n",type);
@@ -223,15 +263,18 @@ int create_lattice(t_moldyn *moldyn,u8 type,double lc,int element,double mass,
 	}
 
 	moldyn->count=count;
+	printf("[moldyn] created lattice with %d atoms\n",count);
 
 	while(count) {
-		atom[count-1].element=element;
-		atom[count-1].mass=mass;
-		atom[count-1].attr=attr;
-		atom[count-1].bnum=bnum;
 		count-=1;
+		moldyn->atom[count].element=element;
+		moldyn->atom[count].mass=mass;
+		moldyn->atom[count].attr=attr;
+		moldyn->atom[count].bnum=bnum;
+		check_per_bound(moldyn,&(moldyn->atom[count].r));
 	}
 
+
 	return ret;
 }
 
@@ -270,7 +313,7 @@ int destroy_atoms(t_moldyn *moldyn) {
 	return 0;
 }
 
-int thermal_init(t_moldyn *moldyn) {
+int thermal_init(t_moldyn *moldyn,u8 equi_init) {
 
 	/*
 	 * - gaussian distribution of velocities
@@ -290,7 +333,7 @@ int thermal_init(t_moldyn *moldyn) {
 	/* gaussian distribution of velocities */
 	v3_zero(&p_total);
 	for(i=0;i<moldyn->count;i++) {
-		sigma=sqrt(2.0*K_BOLTZMANN*moldyn->t/atom[i].mass);
+		sigma=sqrt(2.0*K_BOLTZMANN*moldyn->t_ref/atom[i].mass);
 		/* x direction */
 		v=sigma*rand_get_gauss(random);
 		atom[i].v.x=v;
@@ -313,16 +356,17 @@ int thermal_init(t_moldyn *moldyn) {
 	}
 
 	/* velocity scaling */
-	scale_velocity(moldyn);
+	scale_velocity(moldyn,equi_init);
 
 	return 0;
 }
 
-int scale_velocity(t_moldyn *moldyn) {
+int scale_velocity(t_moldyn *moldyn,u8 equi_init) {
 
 	int i;
-	double e,c;
+	double e,scale;
 	t_atom *atom;
+	int count;
 
 	atom=moldyn->atom;
 
@@ -330,17 +374,44 @@ int scale_velocity(t_moldyn *moldyn) {
 	 * - velocity scaling (E = 3/2 N k T), E: kinetic energy
 	 */
 
-	if(moldyn->t==0.0) {
-		printf("[moldyn] no velocity scaling for T = 0 K\n");
-		return -1;
+	/* get kinetic energy / temperature & count involved atoms */
+	e=0.0;
+	count=0;
+	for(i=0;i<moldyn->count;i++) {
+		if((equi_init&TRUE)||(atom[i].attr&ATOM_ATTR_HB)) {
+			e+=0.5*atom[i].mass*v3_absolute_square(&(atom[i].v));
+			count+=1;
+		}
+	}
+	if(count!=0) moldyn->t=(2.0*e)/(3.0*count*K_BOLTZMANN);
+	else return 0;	/* no atoms involved in scaling! */
+	
+	/* (temporary) hack for e,t = 0 */
+	if(e==0.0) {
+	moldyn->t=0.0;
+		if(moldyn->t_ref!=0.0) {
+			thermal_init(moldyn,equi_init);
+			return 0;
+		}
+		else
+			return 0; /* no scaling needed */
 	}
 
-	e=0.0;
-	for(i=0;i<moldyn->count;i++)
-		e+=0.5*atom[i].mass*v3_absolute_square(&(atom[i].v));
-	c=sqrt((2.0*e)/(3.0*moldyn->count*K_BOLTZMANN*moldyn->t));
-	for(i=0;i<moldyn->count;i++)
-		v3_scale(&(atom[i].v),&(atom[i].v),(1.0/c));
+
+	/* get scaling factor */
+	scale=moldyn->t_ref/moldyn->t;
+	if(equi_init&TRUE)
+		scale*=2.0;
+	else
+		if(moldyn->pt_scale&T_SCALE_BERENDSEN)
+			scale=1.0+(scale-1.0)/moldyn->t_tc;
+	scale=sqrt(scale);
+
+	/* velocity scaling */
+	for(i=0;i<moldyn->count;i++) {
+		if((equi_init&TRUE)||(atom[i].attr&ATOM_ATTR_HB))
+			v3_scale(&(atom[i].v),&(atom[i].v),scale);
+	}
 
 	return 0;
 }
@@ -397,12 +468,6 @@ double estimate_time_step(t_moldyn *moldyn,double nn_dist) {
 
 	/* nn_dist is the nearest neighbour distance */
 
-	if(moldyn->t==5.0) {
-		printf("[moldyn] i do not estimate timesteps below %f K!\n",
-		       MOLDYN_CRITICAL_EST_TEMP);
-		return 23.42;
-	}
-
 	tau=(0.05*nn_dist*moldyn->atom[0].mass)/sqrt(3.0*K_BOLTZMANN*moldyn->t);
 
 	return tau;	
@@ -492,7 +557,6 @@ int link_cell_neighbour_index(t_moldyn *moldyn,int i,int j,int k,t_list *cell) {
 	count2=27;
 	a=nx*ny;
 
-
 	cell[0]=lc->subcell[i+j*nx+k*a];
 	for(ci=-1;ci<=1;ci++) {
 		bx=0;
@@ -526,10 +590,9 @@ int link_cell_neighbour_index(t_moldyn *moldyn,int i,int j,int k,t_list *cell) {
 		}
 	}
 
-	lc->dnlc=count2;
-	lc->countn=27;
+	lc->dnlc=count1;
 
-	return count2;
+	return count1;
 }
 
 int link_cell_shutdown(t_moldyn *moldyn) {
@@ -596,9 +659,9 @@ int moldyn_integrate(t_moldyn *moldyn) {
 	t_3dvec p;
 	t_moldyn_schedule *schedule;
 	t_atom *atom;
-
 	int fd;
-	char fb[128];
+	char dir[128];
+	double ds;
 
 	schedule=&(moldyn->schedule);
 	atom=moldyn->atom;
@@ -619,9 +682,24 @@ int moldyn_integrate(t_moldyn *moldyn) {
 	/* calculate initial forces */
 	potential_force_calc(moldyn);
 
+	/* some stupid checks before we actually start calculating bullshit */
+	if(moldyn->cutoff>0.5*moldyn->dim.x)
+		printf("[moldyn] warning: cutoff > 0.5 x dim.x\n");
+	if(moldyn->cutoff>0.5*moldyn->dim.y)
+		printf("[moldyn] warning: cutoff > 0.5 x dim.y\n");
+	if(moldyn->cutoff>0.5*moldyn->dim.z)
+		printf("[moldyn] warning: cutoff > 0.5 x dim.z\n");
+	ds=0.5*atom[0].f.x*moldyn->tau_square/atom[0].mass;
+	if(ds>0.05*moldyn->nnd)
+		printf("[moldyn] warning: forces too high / tau too small!\n");
+
 	/* zero absolute time */
 	moldyn->time=0.0;
 
+	/* debugging, ignore */
+	moldyn->debug=0;
+
+	/* executing the schedule */
 	for(sched=0;sched<moldyn->schedule.content_count;sched++) {
 
 		/* setting amount of runs and finite time step size */
@@ -636,14 +714,15 @@ int moldyn_integrate(t_moldyn *moldyn) {
 		/* integration step */
 		moldyn->integrate(moldyn);
 
-		/* increase absolute time */
-		moldyn->time+=moldyn->tau;
+		/* p/t scaling */
+		if(moldyn->pt_scale&(T_SCALE_BERENDSEN|T_SCALE_DIRECT))
+			scale_velocity(moldyn,FALSE);
 
 		/* check for log & visualization */
 		if(e) {
 			if(!(i%e))
 				dprintf(moldyn->efd,
-				        "%.15f %.45f %.45f %.45f\n",
+				        "%f %f %f %f\n",
 				        moldyn->time,update_e_kin(moldyn),
 				        moldyn->energy,
 				        get_total_energy(moldyn));
@@ -652,15 +731,14 @@ int moldyn_integrate(t_moldyn *moldyn) {
 			if(!(i%m)) {
 				p=get_total_p(moldyn);
 				dprintf(moldyn->mfd,
-				        "%.15f %.45f\n",moldyn->time,
-				        v3_norm(&p));
+				        "%f %f\n",moldyn->time,v3_norm(&p));
 			}
 		}
 		if(s) {
 			if(!(i%s)) {
-				snprintf(fb,128,"%s-%f-%.15f.save",moldyn->sfb,
-				         moldyn->t,i*moldyn->tau);
-				fd=open(fb,O_WRONLY|O_TRUNC|O_CREAT);
+				snprintf(dir,128,"%s/s-%07.f.save",
+				         moldyn->vlsdir,moldyn->time);
+				fd=open(dir,O_WRONLY|O_TRUNC|O_CREAT);
 				if(fd<0) perror("[moldyn] save fd open");
 				else {
 					write(fd,moldyn,sizeof(t_moldyn));
@@ -674,17 +752,24 @@ int moldyn_integrate(t_moldyn *moldyn) {
 			if(!(i%v)) {
 				visual_atoms(&(moldyn->vis),moldyn->time,
 				             moldyn->atom,moldyn->count);
-				printf("\rsched: %d, steps: %d",sched,i);
+				printf("\rsched: %d, steps: %d, debug: %d",
+				       sched,i,moldyn->debug);
 				fflush(stdout);
 			}
 		}
 
+		/* increase absolute time */
+		moldyn->time+=moldyn->tau;
+
 	}
 
 		/* check for hooks */
 		if(schedule->hook)
 			schedule->hook(moldyn,schedule->hook_params);
 
+		/* get a new info line */
+		printf("\n");
+
 	}
 
 	return 0;
@@ -722,7 +807,6 @@ int velocity_verlet(t_moldyn *moldyn) {
 
 	/* forces depending on chosen potential */
 	potential_force_calc(moldyn);
-	//moldyn->potential_force_function(moldyn);
 
 	for(i=0;i<count;i++) {
 		/* again velocities */
@@ -745,109 +829,124 @@ int velocity_verlet(t_moldyn *moldyn) {
 int potential_force_calc(t_moldyn *moldyn) {
 
 	int i,j,k,count;
-	t_atom *atom,*btom,*ktom;
+	t_atom *itom,*jtom,*ktom;
 	t_linkcell *lc;
-	t_list neighbour[27];
-	t_list *this,*thisk,*neighbourk;
-	u8 bc,bck;
-	int countn,dnlc;
+	t_list neighbour_i[27];
+	t_list neighbour_i2[27];
+	//t_list neighbour_j[27];
+	t_list *this,*that;
+	u8 bc_ij,bc_ik;
+	int dnlc;
 
 	count=moldyn->count;
-	atom=moldyn->atom;
+	itom=moldyn->atom;
 	lc=&(moldyn->lc);
 
 	/* reset energy */
 	moldyn->energy=0.0;
 
+	/* get energy and force of every atom */
 	for(i=0;i<count;i++) {
-	
+
 		/* reset force */
-		v3_zero(&(atom[i].f));
+		v3_zero(&(itom[i].f));
 
 		/* single particle potential/force */
-		if(atom[i].attr&ATOM_ATTR_1BP)
-			moldyn->func1b(moldyn,&(atom[i]));
+		if(itom[i].attr&ATOM_ATTR_1BP)
+			moldyn->func1b(moldyn,&(itom[i]));
+
+		if(!(itom[i].attr&(ATOM_ATTR_2BP|ATOM_ATTR_3BP)))
+			continue;
 
 		/* 2 body pair potential/force */
-		if(atom[i].attr&(ATOM_ATTR_2BP|ATOM_ATTR_3BP)) {
 	
-			link_cell_neighbour_index(moldyn,
-				(atom[i].r.x+moldyn->dim.x/2)/lc->x,
-				(atom[i].r.y+moldyn->dim.y/2)/lc->y,
-				(atom[i].r.z+moldyn->dim.z/2)/lc->z,
-				neighbour);
+		link_cell_neighbour_index(moldyn,
+		                          (itom[i].r.x+moldyn->dim.x/2)/lc->x,
+		                          (itom[i].r.y+moldyn->dim.y/2)/lc->y,
+		                          (itom[i].r.z+moldyn->dim.z/2)/lc->z,
+		                          neighbour_i);
 
-			countn=lc->countn;
-			dnlc=lc->dnlc;
+		dnlc=lc->dnlc;
 
-			for(j=0;j<countn;j++) {
+		for(j=0;j<27;j++) {
 
-				this=&(neighbour[j]);
-				list_reset(this);
+			this=&(neighbour_i[j]);
+			list_reset(this);
 
-				if(this->start==NULL)
-					continue;
+			if(this->start==NULL)
+				continue;
 
-				bc=(j<dnlc)?0:1;
+			bc_ij=(j<dnlc)?0:1;
 
-				do {
-					btom=this->current->data;
+			do {
+				jtom=this->current->data;
 
-					if(btom==&(atom[i]))
-						continue;
+				if(jtom==&(itom[i]))
+					continue;
 
-					if((btom->attr&ATOM_ATTR_2BP)&
-					   (atom[i].attr&ATOM_ATTR_2BP))
-						moldyn->func2b(moldyn,
-						               &(atom[i]),
-						               btom,
-						               bc);
+				if((jtom->attr&ATOM_ATTR_2BP)&
+				   (itom[i].attr&ATOM_ATTR_2BP))
+					moldyn->func2b(moldyn,
+					               &(itom[i]),
+					               jtom,
+					               bc_ij);
 
-					/* 3 body potential/force */
+				/* 3 body potential/force */
 
-					if(!(atom[i].attr&ATOM_ATTR_3BP)||
-					   !(btom->attr&ATOM_ATTR_3BP))
-						continue;
+				if(!(itom[i].attr&ATOM_ATTR_3BP)||
+				   !(jtom->attr&ATOM_ATTR_3BP))
+					continue;
 
-					link_cell_neighbour_index(moldyn,
-					   (btom->r.x+moldyn->dim.x/2)/lc->x,
-					   (btom->r.y+moldyn->dim.y/2)/lc->y,
-					   (btom->r.z+moldyn->dim.z/2)/lc->z,
-					   neighbourk);
+				/* copy the neighbour lists */
+				memcpy(neighbour_i2,neighbour_i,
+				       27*sizeof(t_list));
 
-					for(k=0;k<lc->countn;k++) {
+				/* get neighbours of i */
+				for(k=0;k<27;k++) {
 
-						thisk=&(neighbourk[k]);
-						list_reset(thisk);
+					that=&(neighbour_i2[k]);
+					list_reset(that);
 					
-						if(thisk->start==NULL)
-							continue;
+					if(that->start==NULL)
+						continue;
 
-						bck=(k<lc->dnlc)?0:1;
+					bc_ik=(k<dnlc)?0:1;
 
-						do {
+					do {
 
-			ktom=thisk->current->data;
+						ktom=that->current->data;
 
-			if(!(ktom->attr&ATOM_ATTR_3BP))
-				continue;
+						if(!(ktom->attr&ATOM_ATTR_3BP))
+							continue;
 
-			if(ktom==btom)
-				continue;
+						if(ktom==jtom)
+							continue;
 
-			if(ktom==&(atom[i]))
-				continue;
+						if(ktom==&(itom[i]))
+							continue;
+
+						moldyn->func3b(moldyn,
+						               &(itom[i]),
+						               jtom,
+						               ktom,
+						               bc_ik|bc_ij);
 
-			moldyn->func3b(moldyn,&(atom[i]),btom,ktom,bck);
+					} while(list_next(that)!=\
+					        L_NO_NEXT_ELEMENT);
 
-						} while(list_next(thisk)!=\
-						        L_NO_NEXT_ELEMENT);
+				}
 
-					}
+				/* 2bp post function */
+				if(moldyn->func2b_post) {
+					moldyn->func2b_post(moldyn,
+					                    &(itom[i]),
+					                    jtom,bc_ij);
+				}
 					
-				} while(list_next(this)!=L_NO_NEXT_ELEMENT);
-			}
+			} while(list_next(this)!=L_NO_NEXT_ELEMENT);
+		
 		}
+
 	}
 
 	return 0;
@@ -902,14 +1001,15 @@ int harmonic_oscillator(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 	sc=params->spring_constant;
 	equi_dist=params->equilibrium_distance;
 
-	v3_sub(&distance,&(ai->r),&(aj->r));
+	v3_sub(&distance,&(aj->r),&(ai->r));
 	
 	if(bc) check_per_bound(moldyn,&distance);
 	d=v3_norm(&distance);
 	if(d<=moldyn->cutoff) {
 		/* energy is 1/2 (d-d0)^2, but we will add this twice ... */
 		moldyn->energy+=(0.25*sc*(d-equi_dist)*(d-equi_dist));
-		v3_scale(&force,&distance,-sc*(1.0-(equi_dist/d)));
+		/* f = -grad E; grad r_ij = -1 1/r_ij distance */
+		v3_scale(&force,&distance,sc*(1.0-(equi_dist/d)));
 		v3_add(&(ai->f),&(ai->f),&force);
 	}
 
@@ -930,7 +1030,7 @@ int lennard_jones(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 	sig6=params->sigma6;
 	sig12=params->sigma12;
 
-	v3_sub(&distance,&(ai->r),&(aj->r));
+	v3_sub(&distance,&(aj->r),&(ai->r));
 	if(bc) check_per_bound(moldyn,&distance);
 	d=v3_absolute_square(&distance);	/* 1/r^2 */
 	if(d<=moldyn->cutoff_square) {
@@ -946,7 +1046,7 @@ int lennard_jones(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 		h1*=12*sig12;
 		d=+h1-h2;
 		d*=eps;
-		v3_scale(&force,&distance,d);
+		v3_scale(&force,&distance,-1.0*d); /* f = - grad E */
 		v3_add(&(ai->f),&(ai->f),&force);
 	}
 
@@ -957,6 +1057,35 @@ int lennard_jones(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
  * tersoff potential & force for 2 sorts of atoms
  */
 
+/* create mixed terms from parameters and set them */
+int tersoff_mult_complete_params(t_tersoff_mult_params *p) {
+
+	printf("[moldyn] tersoff parameter completion\n");
+	p->Smixed=sqrt(p->S[0]*p->S[1]);
+	p->Rmixed=sqrt(p->R[0]*p->R[1]);
+	p->Amixed=sqrt(p->A[0]*p->A[1]);
+	p->Bmixed=sqrt(p->B[0]*p->B[1]);
+	p->lambda_m=0.5*(p->lambda[0]+p->lambda[1]);
+	p->mu_m=0.5*(p->mu[0]+p->mu[1]);
+
+	printf("[moldyn] tersoff mult parameter info:\n");
+	printf("  S (A)  | %f | %f | %f\n",p->S[0],p->S[1],p->Smixed);
+	printf("  R (A)  | %f | %f | %f\n",p->R[0],p->R[1],p->Rmixed);
+	printf("  A (eV) | %f | %f | %f\n",p->A[0]/EV,p->A[1]/EV,p->Amixed/EV);
+	printf("  B (eV) | %f | %f | %f\n",p->B[0]/EV,p->B[1]/EV,p->Bmixed/EV);
+	printf("  lambda | %f | %f | %f\n",p->lambda[0],p->lambda[1],
+	                                  p->lambda_m);
+	printf("  mu     | %f | %f | %f\n",p->mu[0],p->mu[1],p->mu_m);
+	printf("  beta   | %.10f | %.10f\n",p->beta[0],p->beta[1]);
+	printf("  n      | %f | %f\n",p->n[0],p->n[1]);
+	printf("  c      | %f | %f\n",p->c[0],p->c[1]);
+	printf("  d      | %f | %f\n",p->d[0],p->d[1]);
+	printf("  h      | %f | %f\n",p->h[0],p->h[1]);
+	printf("  chi    | %f \n",p->chi);
+
+	return 0;
+}
+
 /* tersoff 1 body part */
 int tersoff_mult_1bp(t_moldyn *moldyn,t_atom *ai) {
 
@@ -973,16 +1102,16 @@ int tersoff_mult_1bp(t_moldyn *moldyn,t_atom *ai) {
 	 *         their right values
 	 */
 
-	exchange->beta=&(params->beta[num]);
-	exchange->n=&(params->n[num]);
-	exchange->c=&(params->c[num]);
-	exchange->d=&(params->d[num]);
-	exchange->h=&(params->h[num]);
+	exchange->beta_i=&(params->beta[num]);
+	exchange->n_i=&(params->n[num]);
+	exchange->c_i=&(params->c[num]);
+	exchange->d_i=&(params->d[num]);
+	exchange->h_i=&(params->h[num]);
 
-	exchange->betan=pow(*(exchange->beta),*(exchange->n));
-	exchange->c2=params->c[num]*params->c[num];
-	exchange->d2=params->d[num]*params->d[num];
-	exchange->c2d2=exchange->c2/exchange->d2;
+	exchange->betaini=pow(*(exchange->beta_i),*(exchange->n_i));
+	exchange->ci2=params->c[num]*params->c[num];
+	exchange->di2=params->d[num]*params->d[num];
+	exchange->ci2di2=exchange->ci2/exchange->di2;
 
 	return 0;
 }
@@ -1000,92 +1129,232 @@ int tersoff_mult_2bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 	int num;
 	double s_r;
 	double arg;
-	double scale;
 
 	params=moldyn->pot2b_params;
-	num=ai->bnum;
+	num=aj->bnum;
 	exchange=&(params->exchange);
 
+	/* clear 3bp and 2bp post run */
 	exchange->run3bp=0;
+	exchange->run2bp_post=0;
+
+	/* reset S > r > R mark */
+	exchange->d_ij_between_rs=0;
 	
 	/*
-	 * we need: f_c, df_c, f_r, df_r
+	 * calc of 2bp contribution of V_ij and dV_ij/ji
+	 *
+	 * for Vij and dV_ij we need:
+	 * - f_c_ij, df_c_ij
+	 * - f_r_ij, df_r_ij
+	 *
+	 * for dV_ji we need:
+	 * - f_c_ji = f_c_ij, df_c_ji = df_c_ij
+	 * - f_r_ji = f_r_ij; df_r_ji = df_r_ij
 	 *
-	 * therefore we need: R, S, A, lambda
 	 */
 
-	v3_sub(&dist_ij,&(ai->r),&(aj->r));
-
+	/* dist_ij, d_ij */
+	v3_sub(&dist_ij,&(aj->r),&(ai->r));
 	if(bc) check_per_bound(moldyn,&dist_ij);
+	d_ij=v3_norm(&dist_ij);
 
-	/* save for use in 3bp */ /* REALLY ?!?!?! */
+	/* save for use in 3bp */
+	exchange->d_ij=d_ij;
 	exchange->dist_ij=dist_ij;
 
 	/* constants */
-	if(num==aj->bnum) {
+	if(num==ai->bnum) {
 		S=params->S[num];
 		R=params->R[num];
 		A=params->A[num];
+		B=params->B[num];
 		lambda=params->lambda[num];
-		/* more constants depending of atoms i and j, needed in 3bp */
-		params->exchange.B=&(params->B[num]);
-		params->exchange.mu=&(params->mu[num]);
 		mu=params->mu[num];
-		params->exchange.chi=1.0;
+		exchange->chi=1.0;
 	}
 	else {
 		S=params->Smixed;
 		R=params->Rmixed;
 		A=params->Amixed;
+		B=params->Bmixed;
 		lambda=params->lambda_m;
-		/* more constants depending of atoms i and j, needed in 3bp */
-		params->exchange.B=&(params->Bmixed);
-		params->exchange.mu=&(params->mu_m);
 		mu=params->mu_m;
 		params->exchange.chi=params->chi;
 	}
 
-	d_ij=v3_norm(&dist_ij);
-
-	/* save for use in 3bp */
-	exchange->d_ij=d_ij;
-
+	/* if d_ij > S => no force & potential energy contribution */
 	if(d_ij>S)
 		return 0;
 
+	/* more constants */
+	exchange->beta_j=&(params->beta[num]);
+	exchange->n_j=&(params->n[num]);
+	exchange->c_j=&(params->c[num]);
+	exchange->d_j=&(params->d[num]);
+	exchange->h_j=&(params->h[num]);
+	if(num==ai->bnum) {
+		exchange->betajnj=exchange->betaini;
+		exchange->cj2=exchange->ci2;
+		exchange->dj2=exchange->di2;
+		exchange->cj2dj2=exchange->ci2di2;
+	}
+	else {
+		exchange->betajnj=pow(*(exchange->beta_j),*(exchange->n_j));
+		exchange->cj2=params->c[num]*params->c[num];
+		exchange->dj2=params->d[num]*params->d[num];
+		exchange->cj2dj2=exchange->cj2/exchange->dj2;
+	}
+
+	/* f_r_ij = f_r_ji, df_r_ij = df_r_ji */
 	f_r=A*exp(-lambda*d_ij);
-	df_r=-lambda*f_r/d_ij;
+	df_r=lambda*f_r/d_ij;
 
-	/* f_a, df_a calc + save for 3bp use */
+	/* f_a, df_a calc (again, same for ij and ji) | save for later use! */
 	exchange->f_a=-B*exp(-mu*d_ij);
 	exchange->df_a=-mu*exchange->f_a/d_ij;
 
+	/* f_c, df_c calc (again, same for ij and ji) */
 	if(d_ij<R) {
 		/* f_c = 1, df_c = 0 */
 		f_c=1.0;
 		df_c=0.0;
-		v3_scale(&force,&dist_ij,df_r);
+		/* two body contribution (ij, ji) */
+		v3_scale(&force,&dist_ij,-df_r);
 	}
 	else {
 		s_r=S-R;
 		arg=M_PI*(d_ij-R)/s_r;
 		f_c=0.5+0.5*cos(arg);
 		df_c=-0.5*sin(arg)*(M_PI/(s_r*d_ij));
-		scale=df_c*f_r+df_r*f_c;
-		v3_scale(&force,&dist_ij,scale);
+		/* two body contribution (ij, ji) */
+		v3_scale(&force,&dist_ij,-df_c*f_r-df_r*f_c);
+		/* tell 3bp that S > r > R */
+		exchange->d_ij_between_rs=1;
 	}
 
-	/* add forces */
+	/* add forces of 2bp (ij, ji) contribution
+	 * dVij = dVji and we sum up both: no 1/2) */
 	v3_add(&(ai->f),&(ai->f),&force);
-	/* energy is 0.5 f_r f_c, but we will sum it up twice ... */
-	moldyn->energy+=(0.25*f_r*f_c);
+
+	/* energy 2bp contribution (ij, ji) is 0.5 f_r f_c ... */
+	moldyn->energy+=(0.5*f_r*f_c);
 
 	/* save for use in 3bp */
 	exchange->f_c=f_c;
 	exchange->df_c=df_c;
 
-	/* enable the run of 3bp function */
+	/* enable the run of 3bp function and 2bp post processing */
 	exchange->run3bp=1;
+	exchange->run2bp_post=1;
+
+	/* reset 3bp sums */
+	exchange->zeta_ij=0.0;
+	exchange->zeta_ji=0.0;
+	v3_zero(&(exchange->dzeta_ij));
+	v3_zero(&(exchange->dzeta_ji));
+
+	return 0;
+}
+
+/* tersoff 2 body post part */
+
+int tersoff_mult_post_2bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
+
+	/*
+	 * here we have to allow for the 3bp sums
+	 *
+	 * that is:
+	 * - zeta_ij, dzeta_ij
+	 * - zeta_ji, dzeta_ji
+	 *
+	 * to compute the 3bp contribution to:
+	 * - Vij, dVij
+	 * - dVji
+	 *
+	 */
+
+	t_tersoff_mult_params *params;
+	t_tersoff_exchange *exchange;
+
+	t_3dvec force,temp;
+	t_3dvec *dist_ij;
+	double b,db,tmp;
+	double f_c,df_c,f_a,df_a;
+	double chi,ni,betaini,nj,betajnj;
+	double zeta;
+
+	params=moldyn->pot2b_params;
+	exchange=&(params->exchange);
+
+	/* we do not run if f_c_ij was detected to be 0! */
+	if(!(exchange->run2bp_post))
+		return 0;
+
+	f_c=exchange->f_c;
+	df_c=exchange->df_c;
+	f_a=exchange->f_a;
+	df_a=exchange->df_a;
+	betaini=exchange->betaini;
+	betajnj=exchange->betajnj;
+	ni=*(exchange->n_i);
+	nj=*(exchange->n_j);
+	chi=exchange->chi;
+	dist_ij=&(exchange->dist_ij);
+	
+	/* Vij and dVij */
+	zeta=exchange->zeta_ij;
+	if(zeta==0.0) {
+		moldyn->debug++;		/* just for debugging ... */
+		db=0.0;
+		b=chi;
+		v3_scale(&force,dist_ij,df_a*b*f_c);
+	}
+	else {
+		tmp=betaini*pow(zeta,ni-1.0);		/* beta^n * zeta^n-1 */
+		b=(1+zeta*tmp);				/* 1 + beta^n zeta^n */
+		db=chi*pow(b,-1.0/(2*ni)-1);		/* x(...)^(-1/2n - 1) */
+		b=db*b;					/* b_ij */
+		db*=-0.5*tmp;				/* db_ij */
+		v3_scale(&force,&(exchange->dzeta_ij),f_a*db);
+		v3_scale(&temp,dist_ij,df_a*b);
+		v3_add(&force,&force,&temp);
+		v3_scale(&force,&force,f_c);
+	}
+	v3_scale(&temp,dist_ij,df_c*b*f_a);
+	v3_add(&force,&force,&temp);
+	v3_scale(&force,&force,-0.5);
+
+	/* add force */
+	v3_add(&(ai->f),&(ai->f),&force);
+
+	/* add energy of 3bp sum */
+	moldyn->energy+=(0.5*f_c*b*f_a);
+
+	/* dVji */
+	zeta=exchange->zeta_ji;
+	if(zeta==0.0) {
+		moldyn->debug++;
+		b=chi;
+		v3_scale(&force,dist_ij,df_a*b*f_c);
+	}
+	else {
+		tmp=betajnj*pow(zeta,nj-1.0);		/* beta^n * zeta^n-1 */
+		b=(1+zeta*tmp);				/* 1 + beta^n zeta^n */
+		db=chi*pow(b,-1.0/(2*nj)-1);		/* x(...)^(-1/2n - 1) */
+		b=db*b;					/* b_ij */
+		db*=-0.5*tmp;				/* db_ij */
+		v3_scale(&force,&(exchange->dzeta_ji),f_a*db);
+		v3_scale(&temp,dist_ij,df_a*b);
+		v3_add(&force,&force,&temp);
+		v3_scale(&force,&force,f_c);
+	}
+	v3_scale(&temp,dist_ij,df_c*b*f_a);
+	v3_add(&force,&force,&temp);
+	v3_scale(&force,&force,-0.5);
+
+	/* add force */
+	v3_sub(&(ai->f),&(ai->f),&force);
 
 	return 0;
 }
@@ -1097,60 +1366,74 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 	t_tersoff_mult_params *params;
 	t_tersoff_exchange *exchange;
 	t_3dvec dist_ij,dist_ik,dist_jk;
-	t_3dvec temp,force;
+	t_3dvec temp1,temp2;
+	t_3dvec *dzeta;
 	double R,S,s_r;
-	double d_ij,d_ij2,d_ik,d_jk;
-	double f_c,df_c,b_ij,f_a,df_a;
+	double B,mu;
+	double d_ij,d_ik,d_jk;
+	double rr,dd;
+	double f_c,df_c;
 	double f_c_ik,df_c_ik,arg;
-	double scale;
-	double chi;
-	double n,c,d,h,beta,betan;
+	double f_c_jk;
+	double n,c,d,h;
 	double c2,d2,c2d2;
-	double numer,denom;
-	double theta,cos_theta,sin_theta;
-	double d_theta,d_theta1,d_theta2;
-	double h_cos,h_cos2,d2_h_cos2;
-	double frac1,bracket1,bracket2,bracket2_n_1,bracket2_n;
-	double bracket3,bracket3_pow_1,bracket3_pow;
+	double cos_theta,d_costheta1,d_costheta2;
+	double h_cos,d2_h_cos2;
+	double frac,g,zeta,chi;
+	double tmp;
 	int num;
 
 	params=moldyn->pot3b_params;
-	num=ai->bnum;
 	exchange=&(params->exchange);
 
 	if(!(exchange->run3bp))
 		return 0;
 
 	/*
-	 * we need: f_c, d_fc, b_ij, db_ij, f_a, df_a
+	 * calc of 3bp contribution of V_ij and dV_ij/ji/jk &
+	 * 2bp contribution of dV_jk
+	 *
+	 * for Vij and dV_ij we still need:
+	 * - b_ij, db_ij (zeta_ij)
+	 *   - f_c_ik, df_c_ik, constants_i, cos_theta_ijk, d_costheta_ijk
 	 *
-	 * we got f_c, df_c, f_a, df_a from 2bp calculation
+	 * for dV_ji we still need:
+	 * - b_ji, db_ji (zeta_ji)
+	 *   - f_c_jk, d_c_jk, constants_j, cos_theta_jik, d_costheta_jik
+	 *
+	 * for dV_jk we need:
+	 * - f_c_jk
+	 * - f_a_jk
+	 * - db_jk (zeta_jk)
+	 *   - f_c_ji, df_c_ji, constants_j, cos_theta_jki, d_costheta_jki
+	 *
+	 */
+
+	/*
+	 * get exchange data 
 	 */
 
+	/* dist_ij, d_ij - this is < S_ij ! */
+	dist_ij=exchange->dist_ij;
 	d_ij=exchange->d_ij;
-	d_ij2=exchange->d_ij2;
 
-	f_a=params->exchange.f_a;
-	df_a=params->exchange.df_a;
-	
-	/* d_ij is <= S, as we didn't return so far! */
+	/* f_c_ij, df_c_ij (same for ji) */
+	f_c=exchange->f_c;
+	df_c=exchange->df_c;
 
 	/*
-	 * calc of b_ij (scalar) and db_ij (vector)
-	 *
-	 * - for b_ij: chi, beta, f_c_ik, w(=1), c, d, h, n, cos_theta
-	 *
-	 * - for db_ij: d_theta, sin_theta, cos_theta, f_c_ik, df_c_ik,
-	 *              w_ik,
-	 *
+	 * calculate unknown values now ...
 	 */
 
-	
-	v3_sub(&dist_ik,&(ai->r),&(ak->r));
+	/* V_ij and dV_ij stuff (in b_ij there is f_c_ik) */
+
+	/* dist_ik, d_ik */
+	v3_sub(&dist_ik,&(ak->r),&(ai->r));
 	if(bc) check_per_bound(moldyn,&dist_ik);
 	d_ik=v3_norm(&dist_ik);
 
-	/* constants for f_c_ik calc */
+	/* ik constants */
+	num=ai->bnum;
 	if(num==ak->bnum) {
 		R=params->R[num];
 		S=params->S[num];
@@ -1160,93 +1443,206 @@ int tersoff_mult_3bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 		S=params->Smixed;
 	}
 
-	/* calc of f_c_ik */
-	if(d_ik>S)
-		return 0;
+	/* zeta_ij/dzeta_ij contribution only for d_ik < S */
+	if(d_ik<S) {
+
+		/* get constants_i from exchange data */
+		n=*(exchange->n_i);
+		c=*(exchange->c_i);
+		d=*(exchange->d_i);
+		h=*(exchange->h_i);
+		c2=exchange->ci2;
+		d2=exchange->di2;
+		c2d2=exchange->ci2di2;
+
+		/* cosine of theta_ijk by scalaproduct */
+		rr=v3_scalar_product(&dist_ij,&dist_ik);
+		dd=d_ij*d_ik;
+		cos_theta=rr/dd;
+
+		/* d_costheta */
+		tmp=1.0/dd;
+		d_costheta1=cos_theta/(d_ij*d_ij)-tmp;
+		d_costheta2=cos_theta/(d_ik*d_ik)-tmp;
+
+		/* some usefull values */
+		h_cos=(h-cos_theta);
+		d2_h_cos2=d2+(h_cos*h_cos);
+		frac=c2/(d2_h_cos2);
+
+		/* g(cos_theta) */
+		g=1.0+c2d2-frac;
+
+		/* d_costheta_ij and dg(cos_theta) - needed in any case! */
+		v3_scale(&temp1,&dist_ij,d_costheta1);
+		v3_scale(&temp2,&dist_ik,d_costheta2);
+		v3_add(&temp1,&temp1,&temp2);
+		v3_scale(&temp1,&temp1,-2.0*frac*h_cos/d2_h_cos2); /* dg */
+
+		/* f_c_ik & df_c_ik + {d,}zeta contribution */
+		dzeta=&(exchange->dzeta_ij);
+		if(d_ik<R) {
+			/* {d,}f_c_ik */
+			// => f_c_ik=1.0;
+			// => df_c_ik=0.0; of course we do not set this!
+
+			/* zeta_ij */
+			exchange->zeta_ij+=g;
+
+			/* dzeta_ij */
+			v3_add(dzeta,dzeta,&temp1);
+		}
+		else {
+			/* {d,}f_c_ik */
+			s_r=S-R;
+			arg=M_PI*(d_ik-R)/s_r;
+			f_c_ik=0.5+0.5*cos(arg);
+			df_c_ik=-0.5*sin(arg)*(M_PI/(s_r*d_ik));
+
+			/* zeta_ij */
+			exchange->zeta_ij+=f_c_ik*g;
+
+			/* dzeta_ij */
+			v3_scale(&temp1,&temp1,f_c_ik);
+			v3_scale(&temp2,&dist_ik,g*df_c_ik);
+			v3_add(&temp1,&temp1,&temp2);
+			v3_add(dzeta,dzeta,&temp1);
+		}
+	}
+
+	/* dV_ji stuff (in b_ji there is f_c_jk) + dV_jk stuff! */
+
+	/* dist_jk, d_jk */
+	v3_sub(&dist_jk,&(ak->r),&(aj->r));
+	if(bc) check_per_bound(moldyn,&dist_jk);
+	d_jk=v3_norm(&dist_jk);
 
-	if(d_ik<R) {
-		/* f_c_ik = 1, df_c_ik = 0 */
-		f_c_ik=1.0;
-		df_c_ik=0.0;
+	/* jk constants */
+	num=aj->bnum;
+	if(num==ak->bnum) {
+		R=params->R[num];
+		S=params->S[num];
+		B=params->B[num];
+		mu=params->mu[num];
+		chi=1.0;
 	}
 	else {
-		s_r=S-R;
-		arg=M_PI*(d_ik-R)/s_r;
-		f_c_ik=0.5+0.5*cos(arg);
-		df_c_ik=-0.5*sin(arg)*(M_PI/(s_r*d_ik));
+		R=params->Rmixed;
+		S=params->Smixed;
+		B=params->Bmixed;
+		mu=params->mu_m;
+		chi=params->chi;
 	}
-	
-	v3_sub(&dist_jk,&(aj->r),&(ak->r));
-	if(bc) check_per_bound(moldyn,&dist_jk);
-	d_jk=v3_norm(&dist_jk);
 
-	beta=*(exchange->beta);
-	betan=exchange->betan;
-	n=*(exchange->n);
-	c=*(exchange->c);
-	d=*(exchange->d);
-	h=*(exchange->h);
-	c2=exchange->c2;
-	d2=exchange->d2;
-	c2d2=exchange->c2d2;
-
-	numer=d_ij2+d_ik*d_ik-d_jk*d_jk;
-	denom=2*d_ij*d_ik;
-	cos_theta=numer/denom;
-	sin_theta=sqrt(1.0-(cos_theta*cos_theta));
-	theta=acos(cos_theta);
-	d_theta=(-1.0/sqrt(1.0-cos_theta*cos_theta))/(denom*denom);
-	d_theta1=2*denom-numer*2*d_ik/d_ij;
-	d_theta2=2*denom-numer*2*d_ij/d_ik;
-	d_theta1*=d_theta;
-	d_theta2*=d_theta;
-
-	h_cos=(h-cos_theta);
-	h_cos2=h_cos*h_cos;
-	d2_h_cos2=d2-h_cos2;
-
-	/* some usefull expressions */
-	frac1=c2/(d2-h_cos2);
-	bracket1=1+c2d2-frac1;
-	bracket2=f_c_ik*bracket1;
-	bracket2_n_1=pow(bracket2,n-1.0);
-	bracket2_n=bracket2_n_1*bracket2;
-	bracket3=1+betan*bracket2_n;
-	bracket3_pow_1=pow(bracket3,(-1.0/(2.0*n))-1.0);
-	bracket3_pow=bracket3_pow_1*bracket3;
-
-	/* now go on with calc of b_ij and derivation of b_ij */
-	b_ij=chi*bracket3_pow;
-
-	/* derivation of theta */
-	v3_scale(&force,&dist_ij,d_theta1);
-	v3_scale(&temp,&dist_ik,d_theta2);
-	v3_add(&force,&force,&temp);
+	/* zeta_ji/dzeta_ji contribution only for d_jk < S_jk */
+	if(d_jk<S) {
+
+		/* constants_j from exchange data */
+		n=*(exchange->n_j);
+		c=*(exchange->c_j);
+		d=*(exchange->d_j);
+		h=*(exchange->h_j);
+		c2=exchange->cj2;
+		d2=exchange->dj2;
+		c2d2=exchange->cj2dj2;
+
+		/* cosine of theta_jik by scalaproduct */
+		rr=v3_scalar_product(&dist_ij,&dist_jk); /* times -1 */
+		dd=d_ij*d_jk;
+		cos_theta=rr/dd;
+
+		/* d_costheta */
+		d_costheta1=1.0/(d_jk*d_ij);
+		d_costheta2=cos_theta/(d_ij*d_ij); /* in fact -cos(), but ^ */
+
+		/* some usefull values */
+		h_cos=(h-cos_theta);
+		d2_h_cos2=d2+(h_cos*h_cos);
+		frac=c2/(d2_h_cos2);
+
+		/* g(cos_theta) */
+		g=1.0+c2d2-frac;
+
+		/* d_costheta_ij and dg(cos_theta) - needed in any case! */
+		v3_scale(&temp1,&dist_jk,d_costheta1);
+		v3_scale(&temp2,&dist_ij,-d_costheta2); /* ji -> ij => -1 */
+		v3_add(&temp1,&temp1,&temp2);
+		v3_scale(&temp1,&temp1,-2.0*frac*h_cos/d2_h_cos2); /* dg */
+
+		/* f_c_jk + {d,}zeta contribution (df_c_jk = 0) */
+		dzeta=&(exchange->dzeta_ji);
+		if(d_jk<R) {
+			/* f_c_jk */
+			f_c_jk=1.0;
+
+			/* zeta_ji */
+			exchange->zeta_ji+=g;
+
+			/* dzeta_ji */
+			v3_add(dzeta,dzeta,&temp1);
+		}
+		else {
+			/* f_c_jk */
+			s_r=S-R;
+			arg=M_PI*(d_jk-R)/s_r;
+			f_c_jk=0.5+0.5*cos(arg);
+
+			/* zeta_ji */
+			exchange->zeta_ji+=f_c_jk*g;
+
+			/* dzeta_ij */
+			v3_scale(&temp1,&temp1,f_c_jk);
+			v3_add(dzeta,dzeta,&temp1);
+		}
 
-	/* part 1 of derivation of b_ij */
-	v3_scale(&force,&force,sin_theta*2*h_cos*f_c_ik*frac1);
+		/* dV_jk stuff | add force contribution on atom i immediately */
+		if(exchange->d_ij_between_rs) {
+			zeta=f_c*g;
+			v3_scale(&temp1,&temp1,f_c);
+			v3_scale(&temp2,&dist_ij,df_c);
+			v3_add(&temp1,&temp1,&temp2);
+		}
+		else {
+			zeta=g;
+			// dzeta_jk is simply dg, which is temp1
+		}
+		/* betajnj * zeta_jk ^ nj-1 */
+		tmp=exchange->betajnj*pow(zeta,(n-1.0));
+		tmp=-chi/2.0*pow(1+tmp*zeta,-1.0/(2.0*n)-1)*tmp;
+		v3_scale(&temp1,&temp1,tmp*B*exp(-mu*d_jk)*f_c_jk*0.5);
+		v3_add(&(ai->f),&(ai->f),&temp1); /* -1 skipped in f_a calc ^ */
+		                                  /* scaled with 0.5 ^ */
+	}
 
-	/* part 2 of derivation of b_ij */
-	v3_scale(&temp,&dist_ik,df_c_ik*bracket1);
+	return 0;
+}
 
-	/* sum up and scale ... */
-	v3_add(&temp,&temp,&force);
-	scale=bracket2_n_1*n*betan*(1+betan*bracket3_pow_1)*chi*(1.0/(2.0*n));
-	v3_scale(&temp,&temp,scale);
 
-	/* now construct an energy and a force out of that */
-	v3_scale(&temp,&temp,f_a);
-	v3_scale(&force,&dist_ij,df_a*b_ij);
-	v3_add(&temp,&temp,&force);
-	v3_scale(&temp,&temp,f_c);
-	v3_scale(&force,&dist_ij,df_c*b_ij*f_a);
-	v3_add(&force,&force,&temp);
+/*
+ * debugging / critical check functions
+ */
+
+int moldyn_bc_check(t_moldyn *moldyn) {
+
+	t_atom *atom;
+	t_3dvec *dim;
+	int i;
+
+	atom=moldyn->atom;
+	dim=&(moldyn->dim);
+
+	for(i=0;i<moldyn->count;i++) {
+		if(atom[i].r.x>=dim->x/2||-atom[i].r.x>dim->x/2)
+			printf("FATAL: atom %d: x: %.20f (%.20f)\n",
+			       i,atom[i].r.x*1e10,dim->x/2*1e10);
+		if(atom[i].r.y>=dim->y/2||-atom[i].r.y>dim->y/2)
+			printf("FATAL: atom %d: y: %.20f (%.20f)\n",
+			       i,atom[i].r.y*1e10,dim->y/2*1e10);
+		if(atom[i].r.z>=dim->z/2||-atom[i].r.z>dim->z/2)
+			printf("FATAL: atom %d: z: %.20f (%.20f)\n",
+			       i,atom[i].r.z*1e10,dim->z/2*1e10);
+	}
 
-	/* add forces */
-	v3_add(&(ai->f),&(ai->f),&force);
-	/* energy is 0.5 f_r f_c, but we will sum it up twice ... */
-	moldyn->energy+=(0.25*f_a*b_ij*f_c);
-				
 	return 0;
 }