[Pw_forum] 'relax' and 'scf'

[Stefano Baroni]

Dear Roberto: a "relax" calculation implies the calculation of the 
forces acting on each atoms for many different geometries. Forces are 
calculated using the Hellmann-Feynman theorem which is based on the 
assumption that the total energies satisfy a variational principle (in 
the present case, the Hohenberg-Kohn-Sham principle). Of course, in 
order for the energy to be variational, the Kohn-Sham orbitals have to 
be self-consistent *for each individual geometry*. There would be no 
point, of course, in calculating separately the SCF ground state only 
for one of them (the first).

Structure optimization based on some variant of the Car-Parrinello 
method (in which electronic and ionic degrees of freedom are treated on 
a same footing) may require a pre-optimization of the electronic 
structure for the first geometry, but this would not be a sensible 
procedure when moving the ionic degrees of freedom on the 
Born-Oppenheimer surface. Hence, Paolo's answer.

Hope this helps.
Stefano


On Dec 30, 2004, at 8:49 PM, Paolo Giannozzi wrote:

> On Thursday 30 December 2004 20:06, Roberto G. A. Veiga wrote:
>
>> Must I run a 'scf' calculation before a 'relax' one?
>
> no
>
>> Or does 'relax'  calculation also perform a scf 'one'?
>
> yes
>
> -- 
> 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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---
Stefano Baroni    ---  SISSA  &  DEMOCRITOS National Simulation Center
via Beirut 2-4 34014 Trieste Grignano / [+39] 040 3787 406 (tel) -528 
(fax)

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