v3_add(&(ai->f),&(ai->f),&force);
virial_calc(ai,&force,&distance);
virial_calc(aj,&force,&distance); /* f and d signe switched */
v3_add(&(ai->f),&(ai->f),&force);
virial_calc(ai,&force,&distance);
virial_calc(aj,&force,&distance); /* f and d signe switched */