[Pw_forum] Default values used in Quantum espresso manuals

[Paolo Giannozzi]

On Aug 31, 2009, at 11:49 , Shaptrishi Sharma wrote:

> forc_conv_thr =1.0D-3
> conv_thr =1.0D-6
> etot_conv_thr=1.0D-4
> How did the values calculated?


they were not "calculated" but estimated so that for a
typical system they yield sufficiently small errors for
most purposes, without forcing excessive computation.
No warranty they fit your specific case. This is why you
can change them. They are provided to give at least a
rough guide and an order of magnitude.

conv_thr has a precise meaning: the error on the energy
due to incomplete self-consistency is smaller than conv_thr,
in Ry.

The two other criteria, for structural optimization, do not
have a direct relation with the error on atomic positions
due to incomplete structural optimization, but unless you
have a pathologic system (with floppy modes, saddle
points, almost flat energy surface) you will get accurate
positions.

Paolo
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Paolo Giannozzi, Dept of Physics, University of Udine
via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222