[gmx-users] virial calculation in the presence of electric field

[David van der Spoel]

Sanghyun Park wrote:
> Thank you David.
> 
> I believe virial is typically calculated in terms of pairwise 
> interparticle coordinates and forces (r_{ij} and f_{ij}).  If that is 
> the case in gromacs, how is external force incorporated into 
> interparticle forces?
> 

this is not how it's done in gromacs, see appendix of the manual. quite 
the opposite it is computed as sum_i f_i r_i
therefore the electric field is taken into account.
> Sanghyun
> 
> How is the external force included in virial?
> 
>> Sanghyun Park wrote:
>> > Hi,
>> >
>> > When electric field is applied (using E_x, E_y, E_z in mdp), are the
>> > forces due to the field included in the virial calculation?  I found
>> > some previous messages that seem to indicate that they are included, 
>> but
>> > would like to double-check.
>> >
>> > Thank you,
>> > Sanghyun
>> >
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> 
>> yes they are, but they should cancel for a neutral system.
>>
>> -- 
>> David.
> 
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-- 
David.
________________________________________________________________________
David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,  	75124 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se
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