+/*
+ * lennard_jones.c - lennard jones potential
+ *
+ * author: Frank Zirkelbach <frank.zirkelbach@physik.uni-augsburg.de>
+ *
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <math.h>
+
+#include "../moldyn.h"
+#inlcude "../math/math.h"
+//#include "lennard_jones.h"
+
+int lennard_jones(t_moldyn *moldyn,t_atom *ai,t_atom *aj,u8 bc) {
+
+ t_lj_params *params;
+ t_3dvec force,distance;
+ double d,h1,h2;
+ double eps,sig6,sig12;
+
+ params=moldyn->pot2b_params;
+ eps=params->epsilon4;
+ sig6=params->sigma6;
+ sig12=params->sigma12;
+
+ if(ai<aj) return 0;
+
+ v3_sub(&distance,&(aj->r),&(ai->r));
+ if(bc) check_per_bound(moldyn,&distance);
+ d=v3_absolute_square(&distance); /* 1/r^2 */
+ if(d<=moldyn->cutoff_square) {
+ d=1.0/d; /* 1/r^2 */
+ h2=d*d; /* 1/r^4 */
+ h1=h2*h2; /* 1/r^12 */
+ moldyn->energy+=(eps*(sig12*h1-sig6*h2)-params->uc);
+ h2*=d; /* 1/r^8 */
+ h1*=d; /* 1/r^14 */
+ h2*=6*sig6;
+ h1*=12*sig12;
+ d=+h1-h2;
+ d*=eps;
+ v3_scale(&force,&distance,d);
+ v3_add(&(aj->f),&(aj->f),&force);
+ v3_scale(&force,&force,-1.0); /* f = - grad E */
+ v3_add(&(ai->f),&(ai->f),&force);
+ virial_calc(ai,&force,&distance);
+ virial_calc(aj,&force,&distance); /* f and d signe switched */
+ }
+
+ return 0;
+}
+