[Pw_forum] variable-cell optimizations

[Paolo Giannozzi]

On Wednesday 04 January 2006 17:32, Sergey Lisenkov wrote:

>  So, does it mean that it is not necessary to increase energy cutoff
>  (like at VASP, ABINIT - they use constant plane wave basis in variable
>  cell calculations)  during vc-relax to get correct stress tensor?

if you use a constant plane-wave basis set, you have smooth results,
but bad convergence with cutoff (due to "Pulay stress", i.e. the dependence
of the basis set upon the volume). If you use a constant cutoff, you have
better convergence with cutoff but "rough" stress (because the number 
of plane waves changes when the volume changes). With a suitable choice of
parameters for the modified functional, you get smooth results with constant
cutoff. More details are in the paper that was mentioned earlier.

Paolo

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Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
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