# [gmx-users] C6-C12 table potential

Alex alexanderwien2k at gmail.com
Wed May 16 20:59:28 CEST 2018

```Hi,
Suppose that the C6 and C12 for the nonbonded interaction between two beads
of P4 and SNa in the Martini force fields are as following:
C6                   C12
P4    SNda    1       0.17246E-00     0.18590E-02 ; semi attractive

Now, I was wondering how I can generate a table potential
(table_P4_SNda.xvg) using the above C6 and C12 parameters?

I guess the general below C code provided in the gromacs tabulated
potential does the job, but I am not sure if the columns of -1/r6(-1/r7)
and 1/r12(1/r13) should be multiplied by the above C6 and C12 parameters!?

#include <stdio.h>
#include <math.h>

main()
{
FILE    *fout;
double  r;

fout = fopen("table_example.xvg", "w");
fprintf(fout, "#\n# Example LJ 6-12 Potential\n#\n");

for (r=0; r<=2.4; r+=0.002) {

double f = 1/r;
double fprime = 1/(pow(r,2));

double g = -1/(pow(r,6));
/*   *C6   ? */
double gprime = -6/(pow(r,7));                                /*
*C6   ? */

double h = 1/(pow(r,12));
/*   *C12   ? */
double hprime = 12/(pow(r,13));                             /*
*C12   ? */

/* print output */
if (r<0.001) {
fprintf(fout, "%12.10e   %12.10e %12.10e   %12.10e %12.10e
%12.10e %12.10e\n", r,0.0,0.0,0.0,0.0,0.0,0.0);
} else {
fprintf(fout, "%12.10e   %12.10e %12.10e   %12.10e %12.10e
%12.10e %12.10e\n", r,f,fprime,g,gprime,h,hprime);
}
}

fclose(fout);
return(0);
}

Regards,
Alex
```

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