# [gmx-users] David, still confused with the alternating current electric field.

xieyh at hkusua.hku.hk xieyh at hkusua.hku.hk
Sun Nov 21 20:54:32 CET 2004

```Dear David:

As you suggested, I have downloaded and installed the GMX CVS version for the
applying of alternating current (AC) electric field (EF).However, my purpose
can not yet be realized. Maybe, you can give me much more help.

Compared with GMX3.1 version, the source code about the electric field has been
modified as shown in /src/mdlib/sim_util.c, also listed in the following.

I think this part of code is fully considering the alternating current electric
field. Howerver, I made some tries, then the AC electric field could not be
applied. In other words, although the source code is modified, what is mdp
options about AC electric field? As known, for direct current (DC) electric
field: "E_z = 1.0   0.2   0.0" denotes an electric field with frequency=0 and
strength=0.2.

If applying the time-dependent AC electric field, some items need to be added
into E_zt options. So can you give me a light about what is the E_zt option?
For example, I want to apply a AC electric field: E = E0*cos (omega*t + phi),
what is the corresponding definition in E_zt option?

be greatly appreciated.

/*
* calc_f_el calculates forces due to an electric field.
*
* force is kJ mol^-1 nm^-1 = e * kJ mol^-1 nm^-1 / e
*
* Et[] contains the parameters for the time dependent
* part of the field (not yet used).
* Ex[] contains the parameters for
* the spatial dependent part of the field. You can have cool periodic
* fields in principle, but only a constant field is supported
* now.
* The function should return the energy due to the electric field
* (if any) but for now returns 0.
*/
static void calc_f_el(FILE *fp,int  start,int homenr,
real charge[],rvec x[],rvec f[],
t_cosines Ex[],t_cosines Et[],real t)
{
rvec Ext;
real t0;
int  i,m;

for(m=0; (m<DIM); m++) {
if (Et[m].n) {
if (Et[m].n == 3) {
t0 = Et[m].a;
Ext[m] = cos(Et[m].a*(t-t0))*exp(-sqr(t-t0)/(2.0*sqr(Et[m].a)));
}
else
Ext[m] = cos(Et[m].a*t);
}
else
Ext[m] = 1.0;
if (Ex[m].n) {
/* Convert the field strength from V/nm to MD-units */
Ext[m] *= Ex[m].a*FIELDFAC;
for(i=start; (i<start+homenr); i++)
f[i][m] += charge[i]*Ext[m];
}
else
Ext[m] = 0;
}
if (fp)
fprintf(fp,"%10g  %10g  %10g  %10g #FIELD\n",t,
Ext[XX]/FIELDFAC,Ext[YY]/FIELDFAC,Ext[ZZ]/FIELDFAC);

Yours,
Xie YH
The Univ. of Hong Kong

On Fri, 2004-11-19 at 17:33 +0800, xieyh at hkusua.hku.hk wrote:
> Dear gmx users:
>
>
>
> I would like to apply an alternating current (AC) electric field to my
> simulation box. However, nowadays, only direct current (DC) electric
> field is used in Gromacs. Maybe, the application of AC electric field
> can be realized through the modification in src/mdlib/sim_util.c
> file.
>
>
>
> Anyone knows how to make such a modification? At least, Is that
> promising for do this modification?

>It's implemented in cvs, not sure about 3.2.1. Try fiddling with mdp options.

```