[Pw_forum] Projection, convergence and imaginary phonon frequencies

[Nicola Marzari]

Dear John,


if it is a crystal, the most critical parameter should be k-point
sampling. Increase that, maybe using shifted meshes of the form
2n 2n 3n 1 1 1 , with n=1,2,3...

Norm-conserving and ultrasoft should matter little (unless
there is a problem with one pseudo), while GGAs should be a bit
tougher to converge than LDA.

Do "crystal" and "simple" really lead to a different number of
modes (10 vs 13) ??

			nicola


John Kendrick wrote:
> Hi,
> 
> I have been doing some calculations recently on molecular crystals. 
> Because of our interest in the lowest phonon frequencies I have been 
> concerned that translational invariance is maintained as far as 
> possible.  To this end I have increased the accuracy of the optimisation 
> and the scf convergence, to a point where I cannot see how to converge 
> the calculation any tighter.  I noticed that the norm conserving 
> pseudopotentials were having greater difficulties than the others.  In 
> particular for one molecule I have seen significant differences between 
> the different projection schemes that ensure translational invariance. 
> Particularly the intensities in the infrared a very different.  Below is 
> some output from a representative calculation.

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Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu