[Pw_forum] question on the pressure in molecular dynamics

[Eduardo Ariel Menendez P]

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