[Pw_forum] more on variable-cell optimizations

[Paolo Giannozzi]

On Thursday 05 January 2006 16:28, Lilia Boeri wrote:

> I was wondering whether it is possible with PWSCF to optimize the unit
> cell while keeping the internal degrees of freedom (i.e. the reduced
> coordinates in the unit cell) fixed.

you may try to set the force on atoms to 0: in variable-cell dynamics,
atomic positions are automatically scaled with the crystal axis. Not
sure at all it will work, and even less that it will do what you hope for.

> if I have, for example, an hexagonal system and I want to find how the c/a
> ratio evolves with pressure, I suppose I should calculate, for different
> volumes, the total energy as a function of c/a keeping the volume fixed.
> Then I would take the minimum point for each volume and with this build
> a E(V) curve, differentiate and calculate the P(V) curve.
> [...] is the above procedure correct?

I hope so: this is how I calculated the equilibrium lattice  parameter of
MgB_2, once upon a time... see attached script and fortran code. And
yes, at equilibrium the stress is isotropic (it should be 0 but its diagonal
part, i.e. the pressure, won't be 0 until you go to very high cutoffs)

Paolo
-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy
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