potentials/tersoff.c

   1 /*
   2  * tersoff.c - tersoff potential
   3  *
   4  * author: Frank Zirkelbach <frank.zirkelbach@physik.uni-augsburg.de>
   5  *
   6  */
   7
   8 #define _GNU_SOURCE
   9 #include <stdio.h>
  10 #include <stdlib.h>
  11 #include <string.h>
  12 #include <sys/types.h>
  13 #include <sys/stat.h>
  14 #include <fcntl.h>
  15 #include <unistd.h>
  16 #include <math.h>
  17
  18 #include "../moldyn.h"
  19 #include "../math/math.h"
  20 #include "tersoff.h"
  21
  22 /* create mixed terms from parameters and set them */
  23 int tersoff_mult_set_params(t_moldyn *moldyn,int element1,int element2) {
  24
  25         t_tersoff_mult_params *p;
  26
  27         // set cutoff before parameters (actually only necessary for some pots)
  28         if(moldyn->cutoff==0.0) {
  29                 printf("[tersoff] WARNING: no cutoff!\n");
  30                 return -1;
  31         }
  32
  33         /* alloc mem for potential parameters */
  34          moldyn->pot_params=malloc(sizeof(t_tersoff_mult_params));
  35          if(moldyn->pot_params==NULL) {
  36                 perror("[tersoff] pot params alloc");
  37                 return -1;
  38         }
  39
  40         /* these are now tersoff parameters */
  41         p=moldyn->pot_params;
  42
  43         // only 1 combination by now :p
  44         switch(element1) {
  45                 case SI:
  46                         /* type: silicon */
  47                         p->S[0]=TM_S_SI;
  48                         p->R[0]=TM_R_SI;
  49                         p->A[0]=TM_A_SI;
  50                         p->B[0]=TM_B_SI;
  51                         p->lambda[0]=TM_LAMBDA_SI;
  52                         p->mu[0]=TM_MU_SI;
  53                         p->beta[0]=TM_BETA_SI;
  54                         p->n[0]=TM_N_SI;
  55                         p->c[0]=TM_C_SI;
  56                         p->d[0]=TM_D_SI;
  57                         p->h[0]=TM_H_SI;
  58                         switch(element2) {
  59                                 case C:
  60                                         p->chi=TM_CHI_SIC;
  61                                         break;
  62                                 default:
  63                                         printf("[tersoff] WARNING: element2\n");
  64                                         return -1;
  65                         }
  66                         break;
  67                 default:
  68                         printf("[tersoff] WARNING: element1\n");
  69                         return -1;
  70         }
  71
  72         switch(element2) {
  73                 case C:
  74                         /* type carbon */
  75                         p->S[1]=TM_S_C;
  76                         p->R[1]=TM_R_C;
  77                         p->A[1]=TM_A_C;
  78                         p->B[1]=TM_B_C;
  79                         p->lambda[1]=TM_LAMBDA_C;
  80                         p->mu[1]=TM_MU_C;
  81                         p->beta[1]=TM_BETA_C;
  82                         p->n[1]=TM_N_C;
  83                         p->c[1]=TM_C_C;
  84                         p->d[1]=TM_D_C;
  85                         p->h[1]=TM_H_C;
  86                         break;
  87                 default:
  88                         printf("[tersoff] WARNING: element1\n");
  89                         return -1;
  90         }
  91
  92         printf("[tersoff] parameter completion\n");
  93         p->S2[0]=p->S[0]*p->S[0];
  94         p->S2[1]=p->S[1]*p->S[1];
  95         p->Smixed=sqrt(p->S[0]*p->S[1]);
  96         p->S2mixed=p->Smixed*p->Smixed;
  97         p->Rmixed=sqrt(p->R[0]*p->R[1]);
  98         p->Amixed=sqrt(p->A[0]*p->A[1]);
  99         p->Bmixed=sqrt(p->B[0]*p->B[1]);
 100         p->lambda_m=0.5*(p->lambda[0]+p->lambda[1]);
 101         p->mu_m=0.5*(p->mu[0]+p->mu[1]);
 102
 103         printf("[tersoff] mult parameter info:\n");
 104         printf("  S (A)  | %f | %f | %f\n",p->S[0],p->S[1],p->Smixed);
 105         printf("  R (A)  | %f | %f | %f\n",p->R[0],p->R[1],p->Rmixed);
 106         printf("  A (eV) | %f | %f | %f\n",p->A[0]/EV,p->A[1]/EV,p->Amixed/EV);
 107         printf("  B (eV) | %f | %f | %f\n",p->B[0]/EV,p->B[1]/EV,p->Bmixed/EV);
 108         printf("  lambda | %f | %f | %f\n",p->lambda[0],p->lambda[1],
 109                                           p->lambda_m);
 110         printf("  mu     | %f | %f | %f\n",p->mu[0],p->mu[1],p->mu_m);
 111         printf("  beta   | %.10f | %.10f\n",p->beta[0],p->beta[1]);
 112         printf("  n      | %f | %f\n",p->n[0],p->n[1]);
 113         printf("  c      | %f | %f\n",p->c[0],p->c[1]);
 114         printf("  d      | %f | %f\n",p->d[0],p->d[1]);
 115         printf("  h      | %f | %f\n",p->h[0],p->h[1]);
 116         printf("  chi    | %f \n",p->chi);
 117
 118         return 0;
 119 }
 120
 121 /* tersoff 1 body part */
 122 int tersoff_mult_1bp(t_moldyn *moldyn,t_atom *ai) {
 123
 124         int brand;
 125         t_tersoff_mult_params *params;
 126         t_tersoff_exchange *exchange;
 127
 128         brand=ai->brand;
 129         params=moldyn->pot_params;
 130         exchange=&(params->exchange);
 131
 132         /*
 133          * simple: point constant parameters only depending on atom i to
 134          *         their right values
 135          */
 136
 137         exchange->beta_i=&(params->beta[brand]);
 138         exchange->n_i=&(params->n[brand]);
 139         exchange->c_i=&(params->c[brand]);
 140         exchange->d_i=&(params->d[brand]);
 141         exchange->h_i=&(params->h[brand]);
 142
 143         exchange->betaini=pow(*(exchange->beta_i),*(exchange->n_i));
 144         exchange->ci2=params->c[brand]*params->c[brand];
 145         exchange->di2=params->d[brand]*params->d[brand];
 146         exchange->ci2di2=exchange->ci2/exchange->di2;
 147
 148         return 0;
 149 }
 150
 151 /* tersoff 2 body part */
 152 int tersoff_mult_2bp(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 153
 154         t_tersoff_mult_params *params;
 155         t_3dvec dist_ij,force;
 156         double d_ij,d_ij2;
 157         double A,R,S,S2,lambda;
 158         double f_r,df_r;
 159         double f_c,df_c;
 160         int brand;
 161         double s_r;
 162         double arg;
 163         double energy;
 164
 165         printf("WARNING! - tersoff_mult_2bp is obsolete.\n");
 166         printf("WARNING! - repulsive part handled in 3bp/j2 routine.\n");
 167
 168         /* use newtons third law */
 169         if(ai<aj) return 0;
 170
 171         params=moldyn->pot_params;
 172         brand=aj->brand;
 173
 174         /* determine cutoff square */
 175         if(brand==ai->brand)
 176                 S2=params->S2[brand];
 177         else
 178                 S2=params->S2mixed;
 179
 180         /* dist_ij, d_ij2 */
 181         v3_sub(&dist_ij,&(aj->r),&(ai->r));
 182         if(bc) check_per_bound(moldyn,&dist_ij);
 183         d_ij2=v3_absolute_square(&dist_ij);
 184
 185         /* if d_ij2 > S2 => no force & potential energy contribution */
 186         if(d_ij2>S2) {
 187                 return 0;
 188         }
 189
 190         /* now we will need the distance */
 191         d_ij=sqrt(d_ij2);
 192
 193         /* more constants */
 194         if(brand==ai->brand) {
 195                 S=params->S[brand];
 196                 R=params->R[brand];
 197                 A=params->A[brand];
 198                 lambda=params->lambda[brand];
 199         }
 200         else {
 201                 S=params->Smixed;
 202                 R=params->Rmixed;
 203                 A=params->Amixed;
 204                 lambda=params->lambda_m;
 205         }
 206
 207         /* f_r_ij, df_r_ij */
 208         f_r=A*exp(-lambda*d_ij);
 209         df_r=lambda*f_r/d_ij;
 210
 211         /* f_c, df_c */
 212         if(d_ij<R) {
 213                 f_c=1.0;
 214                 df_c=0.0;
 215                 v3_scale(&force,&dist_ij,-df_r);
 216         }
 217         else {
 218                 s_r=S-R;
 219                 arg=M_PI*(d_ij-R)/s_r;
 220                 f_c=0.5+0.5*cos(arg);
 221                 df_c=0.5*sin(arg)*(M_PI/(s_r*d_ij));
 222                 v3_scale(&force,&dist_ij,-df_c*f_r-df_r*f_c);
 223         }
 224
 225         /* add forces */
 226         v3_add(&(ai->f),&(ai->f),&force);
 227         v3_sub(&(aj->f),&(aj->f),&force); // reason: dri rij = - drj rij
 228
 229 #ifdef DEBUG
 230         if((ai==&(moldyn->atom[0]))|(aj==&(moldyn->atom[0]))) {
 231                 printf("force 2bp: [%d %d]\n",ai->tag,aj->tag);
 232                 printf("adding %f %f %f\n",force.x,force.y,force.z);
 233                 if(ai==&(moldyn->atom[0]))
 234                         printf("total i: %f %f %f\n",ai->f.x,ai->f.y,ai->f.z);
 235                 if(aj==&(moldyn->atom[0]))
 236                         printf("total j: %f %f %f\n",aj->f.x,aj->f.y,aj->f.z);
 237         }
 238 #endif
 239
 240         /* virial */
 241         virial_calc(ai,&force,&dist_ij);
 242
 243         /* energy 2bp contribution */
 244         energy=f_r*f_c;
 245         moldyn->energy+=energy;
 246         ai->e+=0.5*energy;
 247         aj->e+=0.5*energy;
 248
 249         return 0;
 250 }
 251
 252 /* tersoff 3 body potential function (first ij loop) */
 253 int tersoff_mult_3bp_j1(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 254
 255         t_tersoff_mult_params *params;
 256         t_tersoff_exchange *exchange;
 257         unsigned char brand;
 258         double S2;
 259         t_3dvec dist_ij;
 260         double d_ij2,d_ij;
 261
 262         params=moldyn->pot_params;
 263         exchange=&(params->exchange);
 264
 265         /* reset zeta sum */
 266         exchange->zeta_ij=0.0;
 267
 268         /*
 269          * set ij depending values
 270          */
 271
 272         brand=ai->brand;
 273
 274         if(brand==aj->brand)
 275                 S2=params->S2[brand];
 276         else
 277                 S2=params->S2mixed;
 278
 279         /* dist_ij, d_ij2 */
 280         v3_sub(&dist_ij,&(aj->r),&(ai->r));
 281         if(bc) check_per_bound(moldyn,&dist_ij);
 282         d_ij2=v3_absolute_square(&dist_ij);
 283
 284         /* if d_ij2 > S2 => no force & potential energy contribution */
 285         if(d_ij2>S2) {
 286                 moldyn->run3bp=0;
 287                 return 0;
 288         }
 289
 290         /* d_ij */
 291         d_ij=sqrt(d_ij2);
 292
 293         /* store values */
 294         exchange->dist_ij=dist_ij;
 295         exchange->d_ij2=d_ij2;
 296         exchange->d_ij=d_ij;
 297
 298         /* reset k counter for first k loop */
 299         exchange->kcount=0;
 300
 301         return 0;
 302 }
 303
 304 /* tersoff 3 body potential function (first k loop) */
 305 int tersoff_mult_3bp_k1(t_moldyn *moldyn,
 306                         t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 307
 308         t_tersoff_mult_params *params;
 309         t_tersoff_exchange *exchange;
 310         unsigned char brand;
 311         double R,S,S2;
 312         t_3dvec dist_ij,dist_ik;
 313         double d_ik2,d_ik,d_ij;
 314         double cos_theta,h_cos,d2_h_cos2,frac,g,dg,s_r,arg;
 315         double f_c_ik,df_c_ik;
 316         int kcount;
 317
 318         params=moldyn->pot_params;
 319         exchange=&(params->exchange);
 320         kcount=exchange->kcount;
 321
 322         if(kcount>TERSOFF_MAXN) {
 323                 printf("FATAL: neighbours = %d\n",kcount);
 324                 printf("  -> %d %d %d\n",ai->tag,aj->tag,ak->tag);
 325         }
 326
 327         /* ik constants */
 328         brand=ai->brand;
 329         if(brand==ak->brand) {
 330                 R=params->R[brand];
 331                 S=params->S[brand];
 332                 S2=params->S2[brand];
 333         }
 334         else {
 335                 R=params->Rmixed;
 336                 S=params->Smixed;
 337                 S2=params->S2mixed;
 338         }
 339
 340         /* dist_ik, d_ik2 */
 341         v3_sub(&dist_ik,&(ak->r),&(ai->r));
 342         if(bc) check_per_bound(moldyn,&dist_ik);
 343         d_ik2=v3_absolute_square(&dist_ik);
 344
 345         /* store data for second k loop */
 346         exchange->dist_ik[kcount]=dist_ik;
 347         exchange->d_ik2[kcount]=d_ik2;
 348
 349         /* return if not within cutoff */
 350         if(d_ik2>S2) {
 351                 exchange->kcount++;
 352                 return 0;
 353         }
 354
 355         /* d_ik */
 356         d_ik=sqrt(d_ik2);
 357
 358         /* dist_ij, d_ij */
 359         dist_ij=exchange->dist_ij;
 360         d_ij=exchange->d_ij;
 361
 362         /* cos theta */
 363         cos_theta=v3_scalar_product(&dist_ij,&dist_ik)/(d_ij*d_ik);
 364
 365         /* g_ijk */
 366         h_cos=*(exchange->h_i)-cos_theta;
 367         d2_h_cos2=exchange->di2+(h_cos*h_cos);
 368         frac=exchange->ci2/d2_h_cos2;
 369         g=1.0+exchange->ci2di2-frac;
 370         dg=-2.0*frac*h_cos/d2_h_cos2;
 371
 372         /* zeta sum += f_c_ik * g_ijk */
 373         if(d_ik<=R) {
 374                 exchange->zeta_ij+=g;
 375                 f_c_ik=1.0;
 376                 df_c_ik=0.0;
 377         }
 378         else {
 379                 s_r=S-R;
 380                 arg=M_PI*(d_ik-R)/s_r;
 381                 f_c_ik=0.5+0.5*cos(arg);
 382                 df_c_ik=0.5*sin(arg)*(M_PI/(s_r*d_ik));
 383                 exchange->zeta_ij+=f_c_ik*g;
 384         }
 385
 386 #ifdef DEBUG
 387         if((ai==&(moldyn->atom[0]))|
 388            (aj==&(moldyn->atom[864]))|
 389            (ak==&(moldyn->atom[1003]))) {
 390                 printf(" -> %f %f %f\n",exchange->ci2di2,frac,h_cos);
 391         }
 392 #endif
 393
 394         /* store even more data for second k loop */
 395         exchange->g[kcount]=g;
 396         exchange->dg[kcount]=dg;
 397         exchange->d_ik[kcount]=d_ik;
 398         exchange->cos_theta[kcount]=cos_theta;
 399         exchange->f_c_ik[kcount]=f_c_ik;
 400         exchange->df_c_ik[kcount]=df_c_ik;
 401
 402         /* increase k counter */
 403         exchange->kcount++;
 404
 405         return 0;
 406 }
 407
 408 int tersoff_mult_3bp_j2(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
 409
 410         t_tersoff_mult_params *params;
 411         t_tersoff_exchange *exchange;
 412         t_3dvec force;
 413         double f_a,df_a,b,db,f_c,df_c;
 414         double f_r,df_r;
 415         double scale;
 416         double mu,B,chi;
 417         double lambda,A;
 418         double d_ij;
 419         unsigned char brand;
 420         double ni,tmp;
 421         double S,R,s_r,arg;
 422         double energy;
 423
 424         params=moldyn->pot_params;
 425         exchange=&(params->exchange);
 426
 427         brand=aj->brand;
 428         if(brand==ai->brand) {
 429                 S=params->S[brand];
 430                 R=params->R[brand];
 431                 B=params->B[brand];
 432                 A=params->A[brand];
 433                 mu=params->mu[brand];
 434                 lambda=params->lambda[brand];
 435                 chi=1.0;
 436         }
 437         else {
 438                 S=params->Smixed;
 439                 R=params->Rmixed;
 440                 B=params->Bmixed;
 441                 A=params->Amixed;
 442                 mu=params->mu_m;
 443                 lambda=params->lambda_m;
 444                 chi=params->chi;
 445         }
 446
 447         d_ij=exchange->d_ij;
 448
 449         /* f_c, df_c */
 450         if(d_ij<R) {
 451                 f_c=1.0;
 452                 df_c=0.0;
 453         }
 454         else {
 455                 s_r=S-R;
 456                 arg=M_PI*(d_ij-R)/s_r;
 457                 f_c=0.5+0.5*cos(arg);
 458                 df_c=0.5*sin(arg)*(M_PI/(s_r*d_ij));
 459         }
 460
 461         /* f_a, df_a */
 462         f_a=-B*exp(-mu*d_ij);
 463         df_a=mu*f_a/d_ij;
 464
 465         /* f_r, df_r */
 466         f_r=A*exp(-lambda*d_ij);
 467         df_r=lambda*f_r/d_ij;
 468
 469         /* b, db */
 470         if(exchange->zeta_ij==0.0) {
 471                 b=chi;
 472                 db=0.0;
 473         }
 474         else {
 475                 ni=*(exchange->n_i);
 476                 tmp=exchange->betaini*pow(exchange->zeta_ij,ni-1.0);
 477                 b=(1.0+exchange->zeta_ij*tmp);
 478                 db=chi*pow(b,-1.0/(2.0*ni)-1.0);
 479                 b=db*b;
 480                 db*=-0.5*tmp;
 481         }
 482
 483         /* force contribution */
 484         scale=-0.5*(f_c*(df_r+b*df_a)+df_c*(f_r+b*df_a));
 485         v3_scale(&force,&(exchange->dist_ij),scale);
 486         v3_add(&(ai->f),&(ai->f),&force);
 487         v3_sub(&(aj->f),&(aj->f),&force); // dri rij = - drj rij
 488
 489 #ifdef DEBUG
 490         if((ai==&(moldyn->atom[0]))|(aj==&(moldyn->atom[0]))) {
 491                 printf("force 3bp (j2): [%d %d sum]\n",ai->tag,aj->tag);
 492                 printf("adding %f %f %f\n",force.x,force.y,force.z);
 493                 if(ai==&(moldyn->atom[0]))
 494                         printf("total i: %f %f %f\n",ai->f.x,ai->f.y,ai->f.z);
 495                 if(aj==&(moldyn->atom[0]))
 496                         printf("total j: %f %f %f\n",aj->f.x,aj->f.y,aj->f.z);
 497                 printf("energy: %f = %f %f %f %f\n",0.5*f_c*(b*f_a+f_r),
 498                                                     f_c,b,f_a,f_r);
 499                 printf("        %f %f %f\n",exchange->zeta_ij,.0,.0);
 500         }
 501 #endif
 502
 503         /* virial */
 504         if(aj<ai)
 505                 virial_calc(ai,&force,&(exchange->dist_ij));
 506
 507         /* dzeta prefactor = - 0.5 f_c f_a db */
 508         exchange->pre_dzeta=-0.5*f_a*f_c*db;
 509
 510         /* energy contribution */
 511         energy=0.5*f_c*(b*f_a+f_r);
 512         moldyn->energy+=energy;
 513         ai->e+=energy;
 514
 515         /* reset k counter for second k loop */
 516         exchange->kcount=0;
 517
 518         return 0;
 519 }
 520
 521 /* tersoff 3 body potential function (second k loop) */
 522 int tersoff_mult_3bp_k2(t_moldyn *moldyn,
 523                         t_atom *ai,t_atom *aj,t_atom *ak,u8 bc) {
 524
 525         t_tersoff_mult_params *params;
 526         t_tersoff_exchange *exchange;
 527         int kcount;
 528         t_3dvec dist_ik,dist_ij;
 529         double d_ik2,d_ik,d_ij2,d_ij;
 530         unsigned char brand;
 531         double S2;
 532         double g,dg,cos_theta;
 533         double pre_dzeta;
 534         double f_c_ik,df_c_ik;
 535         double dijdik_inv,fcdg,dfcg;
 536         t_3dvec dcosdri,dcosdrj,dcosdrk;
 537         t_3dvec force,tmp;
 538
 539         params=moldyn->pot_params;
 540         exchange=&(params->exchange);
 541         kcount=exchange->kcount;
 542
 543         if(kcount>TERSOFF_MAXN)
 544                 printf("FATAL: neighbours!\n");
 545
 546         /* d_ik2 */
 547         d_ik2=exchange->d_ik2[kcount];
 548
 549         brand=ak->brand;
 550         if(brand==ai->brand)
 551                 S2=params->S2[brand];
 552         else
 553                 S2=params->S2mixed;
 554
 555         /* return if d_ik > S */
 556         if(d_ik2>S2) {
 557                 exchange->kcount++;
 558                 return 0;
 559         }
 560
 561         /* prefactor dzeta */
 562         pre_dzeta=exchange->pre_dzeta;
 563
 564         /* dist_ik, d_ik */
 565         dist_ik=exchange->dist_ik[kcount];
 566         d_ik=exchange->d_ik[kcount];
 567
 568         /* f_c_ik, df_c_ik */
 569         f_c_ik=exchange->f_c_ik[kcount];
 570         df_c_ik=exchange->df_c_ik[kcount];
 571
 572         /* dist_ij, d_ij2, d_ij */
 573         dist_ij=exchange->dist_ij;
 574         d_ij2=exchange->d_ij2;
 575         d_ij=exchange->d_ij;
 576
 577         /* g, dg, cos_theta */
 578         g=exchange->g[kcount];
 579         dg=exchange->dg[kcount];
 580         cos_theta=exchange->cos_theta[kcount];
 581
 582         /* cos_theta derivatives wrt i,j,k */
 583         dijdik_inv=1.0/(d_ij*d_ik);
 584         v3_scale(&dcosdrj,&dist_ik,dijdik_inv);
 585         v3_scale(&tmp,&dist_ij,-cos_theta/d_ij2);
 586         v3_add(&dcosdrj,&dcosdrj,&tmp);
 587         v3_scale(&dcosdrk,&dist_ij,dijdik_inv);
 588         v3_scale(&tmp,&dist_ik,-cos_theta/d_ik2);
 589         v3_add(&dcosdrk,&dcosdrk,&tmp);
 590         v3_add(&dcosdri,&dcosdrj,&dcosdrk);
 591         v3_scale(&dcosdri,&dcosdri,-1.0);
 592
 593         /* f_c_ik * dg, df_c_ik * g */
 594         fcdg=f_c_ik*dg;
 595         dfcg=df_c_ik*g;
 596
 597         /* derivative wrt i */
 598         v3_scale(&force,&dist_ik,dfcg);
 599         v3_scale(&tmp,&dcosdri,fcdg);
 600         v3_add(&force,&force,&tmp);
 601         v3_scale(&force,&force,pre_dzeta);
 602
 603         /* force contribution */
 604         v3_add(&(ai->f),&(ai->f),&force);
 605
 606 #ifdef DEBUG
 607         if(ai==&(moldyn->atom[0])) {
 608                 printf("force 3bp (k2): [%d %d %d]\n",ai->tag,aj->tag,ak->tag);
 609                 printf("adding %f %f %f\n",force.x,force.y,force.z);
 610                 printf("total i: %f %f %f\n",ai->f.x,ai->f.y,ai->f.z);
 611         }
 612 #endif
 613
 614         /* virial */
 615         //virial_calc(ai,&force,&dist_ij);
 616
 617         /* derivative wrt j */
 618         v3_scale(&force,&dcosdrj,fcdg*pre_dzeta);
 619
 620         /* force contribution */
 621         v3_add(&(aj->f),&(aj->f),&force);
 622
 623 #ifdef DEBUG
 624         if(aj==&(moldyn->atom[0])) {
 625                 printf("force 3bp (k2): [%d %d %d]\n",ai->tag,aj->tag,ak->tag);
 626                 printf("adding %f %f %f\n",force.x,force.y,force.z);
 627                 printf("total j: %f %f %f\n",aj->f.x,aj->f.y,aj->f.z);
 628         }
 629 #endif
 630
 631         /* virial */
 632         //v3_scale(&force,&force,-1.0);
 633         if(aj<ai)
 634                 virial_calc(ai,&force,&dist_ij);
 635
 636         /* derivative wrt k */
 637         v3_scale(&force,&dist_ik,-1.0*dfcg); // dri rik = - drk rik
 638         v3_scale(&tmp,&dcosdrk,fcdg);
 639         v3_add(&force,&force,&tmp);
 640         v3_scale(&force,&force,pre_dzeta);
 641
 642         /* force contribution */
 643         v3_add(&(ak->f),&(ak->f),&force);
 644
 645 #ifdef DEBUG
 646         if(ak==&(moldyn->atom[0])) {
 647                 printf("force 3bp (k2): [%d %d %d]\n",ai->tag,aj->tag,ak->tag);
 648                 printf("adding %f %f %f\n",force.x,force.y,force.z);
 649                 printf("total k: %f %f %f\n",ak->f.x,ak->f.y,ak->f.z);
 650         }
 651 #endif
 652
 653         /* virial */
 654         //v3_scale(&force,&force,-1.0);
 655         if(aj<ai)
 656                 virial_calc(ai,&force,&dist_ik);
 657
 658         /* increase k counter */
 659         exchange->kcount++;
 660
 661         return 0;
 662
 663 }