[gmx-developers] virial symmetry

[Berk Hess]

Hi,

At the end of do_walls it says:

  /* Since the lower wall is at z=0 it does not contribute
   * to the single sum virial.
   */
  if (nwall == 2)
    fr->vir_wall_zz = -0.5*box_zz*Fwall[1];

This line should ensure that the virial only depends on the distance 
between the particles and the upper wall.

I don't understand what you mean with asymmetric.
The wall only acts on the z-component of the forces and both the force 
and distance have no x and y
components, therefore the wall forces only occur in the zz element of 
the virial tensor.

Have you modified the code such that this correction no longer holds?

Berk

Alexander Herz wrote:
> 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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