[Pw_forum] question on the pressure in molecular dynamics

[Eduardo Ariel Menendez P]

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?


Thanks
Eduardo