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