- /* d_costheta contrib to db_ij (needed in all remaining cases) */
- v3_scale(&temp,&dist_ij,d_costheta1);
- v3_scale(&force,&dist_ik,d_costheta2);
- v3_add(&force,&force,&temp);
- v3_scale(&force,&force,-2.0*frac*h_cos/d2_h_cos2); /* f_c_ik missing */
+ /* 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);