[Pw_forum] question on the pressure in molecular dynamics

[degironc]

If you want the real pressure you should do a real (so to speak) 
dynamics in a variable cell.
In such a case the system will be evoluted according to a 
Parrinello-Raman or Wentzcovitch
extended Lagrangian and the system (after equilibration) will fluctuate 
around the pressure
specified in the input [variable press in the &CEL namelist].
Hope it helps,
   Stefano de Gironcoli

Eduardo Ariel Menendez P wrote:

>Hi,
>Let me insist in this question, as I have had no answer yet.
>
> >
> > Hi everybody,
> > I have a doubt concernig the pressure reported by pw or cp in a molecular
> > dynamics run. The stress tensor and its trace (the pressure) reported in
> > molecular
> > dynamics by  pwscf and other
> > quantum codes is a static pressure. In a
> > molecular dinamics simulation,
> > if I want the "real" pressure, I must add
> > dp=(N/V) k_B T, N being the number of atoms and V the volume of the
> > cell, and T the instantaneous temperature or its meand value.
> >
> > In other words, the pressure of a non ideal gas (or liquid) is
> >
> > P=(N/V) k_B T + virial term  (do not worry about the exact expression),
> >
> > then I understand that the pressure given by the codes is equivalent to
> > the virial term.
> > Is this correct? I found strange that nobody mentions it.
> >
> > Therefore, I understand that to have the "real" pressure I must add
> > (N/V)k_B T to the pressure reported by pw or cp. Please, correct me if I
> > am
> > wrong.
> >
> > Next level of question. If I do Parrinello-Rahman dynamics with a Nose
> > thermostast, what is the pressure specified by the keyword  press ? Does
> > it include the thermal pressure?
> > I see that there is also a Nose thermostast for the cell. What is its
> > importance? Is it neccessary to use this cell thermostat for the NPT
> > ensemble?
> >
>Later I saw in the CP source that there is a subroutine
>'add_thermal_stress', (which seems better than just 'add_thermal_press'),
>
>cpr_mod.f90:   subroutine add_thermal_stress( stress, pmass, omega, h, vels, nsp, na )
>
>However,  I found  no CALL to this subroutine. Then my doubt increased.
>
>
>Eduardo
>
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