[Pw_forum] vc-md question

[Stefano de Gironcoli]

As Paolo say vc related things are far from optimal... however it is 
possible tu use it to perform structural relaxation
at finite pressure using it.

Check information about namelist &CELL in file Doc/INPUT_PW

calc='vc-md' performs variable cell  MD (no damping) at the pressure 
specified by the variable press (in Kbar) in namelist &CELL using either 
Parrinelllo-Raman or Wentzcovitch extended lagrangian ( specified as 
cell_dynamics='pr' or cell_dynamics='w' ).
BEWARE default value of cell_dynamics is NONE even if calc='vc-md'.

calc='vc-relax' performs relaxation by  damped dynamics using either 
Parrinello-Raman or Wentzcovitch extended lagrangian 
(cell_dynamics='damp-pr' or 'damp-w' ... againg default is NONE) at the 
pressure specified by press.

stefano


Miguel Martínez Canales wrote:

> Hi again, Paolo
>
>> I think that there is presently no check on convergence, so if you 
>> keep sending jobs, it will keep running. Does it converge or not?
>
>
> Ok, so there is no check. I should have known better. Use the source, 
> Luke.
> Regarding convergence, it starts closing (slowly if dt is not good,
> obviously) and then it oscillates. Or that is what it seems. Some 
> output at
> the end of the e-mail.
>
>> By the way, there is also a 'vc-relax' option that is supposed to 
>> perform structural optimization. I am not sure if vc-md can
>> actually do damped dynamics
>
> I understood that vc-relax fully relaxes the structure. I mean, the 
> target
> pressure of the relaxation is p=0. That is why I used vc-md. How do you
> compute equations of state for noncubic cells? Because maybe I am working
> in a very ineffective way.
>
> Oh! and vc-md not being damped might explain the following output:
>
> Input is the same as in first e-mail, but dt=100 (note: dt=150 leads to
> structure deestabilisation) and the following &system flags and 
> ATOMIC_POS:
>