/* force contribution to atom i */
v3_scale(&force,&force,-1.0);
v3_add(&(ai->f),&(ai->f),&force);
/* force contribution to atom i */
v3_scale(&force,&force,-1.0);
v3_add(&(ai->f),&(ai->f),&force);
/* derivative wrt k */
v3_scale(&force,&dist_ik,-1.0*dfcg); // dri rik = - drk rik
v3_scale(&tmp,&dcosdrk,fcdg);
/* derivative wrt k */
v3_scale(&force,&dist_ik,-1.0*dfcg); // dri rik = - drk rik
v3_scale(&tmp,&dcosdrk,fcdg);
/* force contribution to atom i */
v3_scale(&force,&force,-1.0);
v3_add(&(ai->f),&(ai->f),&force);
/* force contribution to atom i */
v3_scale(&force,&force,-1.0);
v3_add(&(ai->f),&(ai->f),&force);