[gmx-developers] virial symmetry

[Alexander Herz]

Having looked a bit closer into this, it seams that the definition of
the virial used in gromacs
(using absolute particle coords) is obviously only translation invariant
as long newtons 3rd law
is obeyed inside the system and no external forces act on the system (so
all forces sum to zero).
Apparently this condition is violated when using 2 walls with different
parameters like density???

Alex

Alexander Herz wrote:
> Hi guys,
>
> I'm having some problems with the walls feature (new in gromacs 4.0).
>
> If I understand correctly then I do get a symmetric virial tensor even for
> systems which are inhomogenous as long only pair potentials are used
>
> (Theta=-0.5*sum(r_ij*F_ij) where r_ij is r_j-r_i (so the distance
> between to particles acting on each other)
> and F_ij is the force acting on them;with F_ij=-F_ji the contributions
> cancel out and the viral is symmetric )
>
> Now if I use 2 walls with different density I get an asymmetric virial
> and looking at the wall's source
> it seams that for walls not r_ij is used but absolute wall coordinate
> (box_zz for wall2 and 0 for wall 1
> giving a zero contribution to the viral for wall1). Also for the
> individual atoms it seams the absolute position
> is used rather than r_ij (looking at calc_vir() in calcvir.c).
>
> So why do the walls use the absolute z coordinate instead of
> particle_z-wall_z and why
> do I get an asymmetric virial for walls with different density but not
> for the same
> system without walls which is still asymmetric in z (some crystal
> occupies the lower z half off the system, the upper
> half is filled with water).
>
> Thx,
> Alex
>
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