+ /* constants */
+ if(num==aj->bnum) {
+ S=params->S[num];
+ R=params->R[num];
+ A=params->A[num];
+ B=params->B[num];
+ lambda=params->lambda[num];
+ mu=params->mu[num];
+ params->exchange.chi=1.0;
+ }
+ else {
+ S=params->Smixed;
+ R=params->Rmixed;
+ A=params->Amixed;
+ B=params->Bmixed;
+ lambda=params->lambda_m;
+ mu=params->mu_m;
+ params->exchange.chi=params->chi;
+ }
+
+ if(d_ij>S)
+ return 0;
+
+ f_r=A*exp(-lambda*d_ij);
+ df_r=-lambda*f_r/d_ij;
+
+ /* f_a, df_a calc + save for later use */
+ exchange->f_a=-B*exp(-mu*d_ij);
+ exchange->df_a=-mu*exchange->f_a/d_ij;
+
+ 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);
+ }
+ 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);
+ }
+
+ /* add forces */
+ v3_add(&(ai->f),&(ai->f),&force);
+ /* energy 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 and 2bp post processing */
+ exchange->run3bp=1;
+ exchange->run2bp_post=1;
+
+ /* reset 3bp sums */
+ exchange->sum1_3bp=0.0;
+ exchange->sum2_3bp=0.0;
+ v3_zero(&(exchange->db_ij));
+
+ 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 */
+
+ t_tersoff_mult_params *params;
+ t_tersoff_exchange *exchange;
+
+ t_3dvec force,temp,*db_ij,*dist_ij;
+ double db_ij_scale1,db_ij_scale2;
+ double b_ij;
+ double f_c,df_c,f_a,df_a;
+ double chi,betan;
+ double help;
+ double n;
+
+ params=moldyn->pot2b_params;
+ exchange=&(params->exchange);
+
+ /* we do not run if f_c_ij was dtected to be 0! */
+ if(!(exchange->run2bp_post))
+ return 0;
+
+ db_ij=&(exchange->db_ij);
+ f_c=exchange->f_c;
+ df_c=exchange->df_c;
+ f_a=exchange->f_a;
+ df_a=exchange->df_a;
+ betan=exchange->betan;
+ n=*(exchange->n);
+ chi=exchange->chi;
+ dist_ij=&(exchange->dist_ij);
+
+ db_ij_scale1=(1+betan*exchange->sum1_3bp);
+ db_ij_scale2=(exchange->n_betan*exchange->sum2_3bp);
+ help=pow(db_ij_scale1,-1.0/(2*n)-1);
+ b_ij=chi*db_ij_scale1*help;
+ db_ij_scale1=-chi/(2*n)*help;
+
+ /* db_ij part */
+ v3_scale(db_ij,db_ij,(db_ij_scale1*db_ij_scale2));
+ v3_scale(db_ij,db_ij,f_a);
+
+ /* df_a part */
+ v3_scale(&temp,dist_ij,b_ij*df_a);
+
+ /* db_ij + df_a part */
+ v3_add(&force,&temp,db_ij);
+ v3_scale(&force,&force,f_c);
+
+ /* df_c part */
+ v3_scale(&temp,dist_ij,f_a*b_ij*df_c);
+
+ /* add energy of 3bp sum */
+ moldyn->energy+=(0.5*f_c*b_ij*f_a);
+
+ /* add force of 3bp calculation (all three parts) */
+ v3_add(&(ai->f),&temp,&force);
+
+ return 0;
+}
+
+/* tersoff 3 body part */
+
+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;
+ double R,S,s_r;
+ double d_ij,d_ij2,d_ik,d_jk;
+ double f_c,df_c,f_a,df_a;
+ double f_c_ik,df_c_ik,arg;
+ 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,d2_h_cos2;
+ double frac,bracket,bracket_n_1,bracket_n;
+ double g;
+ 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
+ *
+ * we got f_c, df_c, f_a, df_a from 2bp calculation
+ */
+
+ d_ij=exchange->d_ij;
+ d_ij2=exchange->d_ij2;
+ dist_ij=exchange->dist_ij;
+
+ f_a=params->exchange.f_a;
+ df_a=params->exchange.df_a;
+
+ f_c=exchange->f_c;
+ df_c=exchange->df_c;
+
+ /* d_ij is <= S, as we didn't return so far! */
+
+ /*
+ * 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,
+ *
+ */
+
+ v3_sub(&dist_ik,&(ai->r),&(ak->r));
+ if(bc) check_per_bound(moldyn,&dist_ik);
+ d_ik=v3_norm(&dist_ik);
+
+ /* constants for f_c_ik calc */
+ if(num==ak->bnum) {
+ R=params->R[num];
+ S=params->S[num];
+ }
+ else {
+ R=params->Rmixed;
+ S=params->Smixed;