[Pw_forum] question on the pressure in molecular dynamics

[Davide Ceresoli]

Dear Eduardo,
     you are totally right regarding the pressure. At finite temperature,
even at constant volume, you must add the kinetic contribution to the
pressure. This contribution always tends to "expand" the solid and for
high enough temperature, it may become important.

I've looked at the Andersen and Parrinello-Rahman papers, and the
kinetic term is always present in the formulas. The "external" pressure
you apply on the system must to balance both the kinetic term and the
virial term.

So, the kinetic term is correctly taken into account in the P-R
equations of motion, in the code. Could be that this contribution
(which averages to P_kin = (N/V) k_B T) is not printed in output.

Regards,
     Davide


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
> 
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum