[Pw_forum] about the fft grid

Paolo Giannozzi giannozz at nest.sns.it
Tue Apr 13 15:29:38 CEST 2004

On Friday 09 April 2004 13:58, xianghjun wrote:

> When I used ultrasoft pseudopotential, I often found the default
> fft grid is not enought to get the almost zero Gamma acoustic  
> frequency. Does the fft grid avoid the wrap around error?

yes it does, but it cannot avoid the loss of translational invariance
caused by the exchange-correlation functional. Larger cutoffs 
(ecutrho) can reduce such error, but you should be able to get
almost zero Gamma acoustic frequencies by simply imposing
the Acustic Sum Rule 

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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