[gmx-users] Electric field unit

[David van der Spoel]

On 14/01/16 16:50, mah maz wrote:
> Thank you David but it still seems conflicting to me. V/nm and V/m are 100
> times different! If it is in V/nm then force unit should be wrong!
Did you check it yourself?

 From units.h:

#define E_CHARGE (1.602176565e-19)
#define FARADAY  (E_CHARGE*AVOGADRO)  /* (96000 C/mol)      */
#define FIELDFAC (FARADAY/KILO)       /* ~ 96 C/mol */

 From sim_util.cpp
             Ext[m] *= Ex[m].a[0]*FIELDFAC;
             for (i = start; (i < start+homenr); i++)
             {
                 f[i][m] += charge[i]*Ext[m];
             }

In other words, if we specify the field (Ex[m].a[0]) in V/nm we get

(V/nm)*(96 C/mol)
fill in
1 V = 1 J/C
J/(nm C) * (96 C/mol) -> 96 J/mol nm = 0.096 kJ/mol nm
which is the MD force unit.

>
> On Thu, Jan 14, 2016 at 12:20 PM, mah maz <mahmaz71 at gmail.com> wrote:
>
>> Hi David,
>>
>> In the manual 4.6.7, Page8. It seems logical since the force unit is in KJ
>> /mol.nm. And it can roughly be deduced from ions movements.
>> What is the use of FIELDFAC in src code then?
>>
>> thank you.
>>
>> On Thu, Jan 14, 2016 at 10:54 AM, mah maz <mahmaz71 at gmail.com> wrote:
>>
>>> Dear users,
>>>
>>> What is the electric field unit in gromacs? In the manual it is KJ/mol.nm.e
>>> but in the source code I encountered V/nm which is about 100 times KJ/mol
>>> .nm.e.
>>>
>>> Thank you.
>>>
>>
>>


-- 
David van der Spoel, Ph.D., Professor of Biology
Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205.
spoel at xray.bmc.uu.se    http://folding.bmc.uu.se