# [gmx-users] Question about Electrostatic field in Gromacs

Tom dnaafm at gmail.com
Thu Jul 7 18:22:15 CEST 2011

``` Dear Gromacs developers or users:

I have a question about the implementation of the electrostatic field in
Gromacs (gromacs-4.5.3)

On the menu of this version, it looks that the functionality of
time-dependent has not been implemented yet.
--------
E_x ; E_y ; E_z:
If you want to use an electric field in a direction, enter 3 numbers after
the appropriate E_*,
the first number: the number of cosines, only 1 is implemented (with
frequency 0) so enter
1, the second number: the strength of the electric field in V nm-1, the
third number: the
phase of the cosine, you can enter any number here since a cosine of
frequency zero has no
phase.
E_xt; E_yt; E_zt:
not implemented yet
---------
But in the source code of sim_util.c for the part of calc_f_el :
I notice there is a parameter of Ext[m] as cos function of time together
with exp part. I am so confused about it.
Can anyone help explain this : *Ext[m] = cos(Et[m].a*(t-t0))*
exp(-sqr(t-t0)/(2.0*sqr(Et[m].a)))
What is meaning of the Ext[m] and its part of exp part?
I am want to make sure whether gromacs-4.5.3 support the time-dependent E
field calculation?*
And how to control these parameters in mdp file if it supports?
*
*------------------------
if (Et[m].n > 0)
{
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);
}
}

-----------------------------

Thanks,

Tom,

PS: the source code of of sim_util.c for the part of calc_f_el is attached
here:

* 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.
*
* WARNING:
* There can be problems with the virial.
* Since the field is not self-consistent this is unavoidable.
* For neutral molecules the virial is correct within this approximation.
* For neutral systems with many charged molecules the error is small.
* But for systems with a net charge or a few charged molecules
* the error can be significant when the field is high.
* Solution: implement a self-consitent electric field into PME.
*/
static void calc_f_el(FILE *fp,int  start,int homenr,
real charge[],rvec x[],rvec f[],
t_cosines Ex[],t_cosines Et[],double t)
{
rvec Ext;
real t0;
int  i,m;

for(m=0; (m<DIM); m++)
{
if (Et[m].n > 0)
{
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 > 0)
{
/* 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 != NULL)
{
fprintf(fp,"%10g  %10g  %10g  %10g #FIELD\n",t,
Ext[XX]/FIELDFAC,Ext[YY]/FIELDFAC,Ext[ZZ]/FIELDFAC);
}
}
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