[Pw_forum] about the fft grid

[xianghjun]

Dear all:
    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?
If not,how can I get the smallest but accurate fft dimension without  
doing  a  lot of test?
Thanks a lot.

Best regards,
xianghjun