[Pw_forum] problems calculating large numbers of empty states

[Paolo Giannozzi]

On Oct 22, 2009, at 21:29 , Brad Malone wrote:

> Hi, I'm trying to calculate a large number of conduction band
> states for cubic Si (to be used in a subsequent GW calculation).
> However, when doing this I sometimes run into errors like
> "problems computing cholesky decomposition"  or "too many
> bands are unconverged".

the algorithm used in PWscf is based on the assumption that
the number of Kohn-Sham states (aka bands) is much smaller
than the number of basis functions (i.e. plane waves). If this
assumption doesn't hold (and it doesn't if you calculate 500
bands in fcc Si unless you use a very large cutoff) you may
run into trouble. Moreover iterative diagonalization becomes
slower and more memory-consuming than conventional
diagonalization. If you really need to do that, you need to
modify the code

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