[Wien] External pressure

[Stefaan Cottenier]

>     Can wien2k give the external pressure of the unit cell ? I know 
> min_lapw can optimize the internal force between atoms in the unit cell, 
> and we can get the internal forces by :FGL***. But I am more intersted 
> the external pressure(stress) of the whole unit cell when opimizing the 
> volumn of unit cell ? 

No, it can't -- unfortunately, it would make my life quite a bit
easier... You can determine the external pressure post factum, by the
derivatives of the total energy surface as a function of the different
cell degrees of freedom in your structure, but that is a lot of work (if
you know the angles and c/a and b/a ratios as a function of volume, it
is easier: then you only have to take the negative 1D derivative of E(V)
in a given volume).

>     By the way, I checked the userguide, and I found in case.in2, there 
> is a parameter 'FOR' which can calculate the Pulay force. Is this Pulay 
> force the exteranl pressure I am looking for ? ( if it is , userguide 
> said only need to calculate at the final scf, is this right enough ?)

No, this is a contribution to the forces on individual atoms inside the
unit cell.

Stefaan