[Pw_forum] question on the pressure in molecular dynamics

[Nicola Marzari]

Another way of saying this: two supercells containing the same
partincles in the same positions, but with different velocities
(e.g. in one case zero, in the other very large) will have the same 
instantaneous stress, but will have different average pressures, and
in a constant pressure molecular dynamics will lead to different
average volumes.

			nicola


degironc wrote:
> 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
-----------------------------------------------
Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu