[Pw_forum] big calculations II

[Paolo Giannozzi]

On Jan 20, 2011, at 3:26 , Ricardo Faccio wrote:

> i need more k-points, and thus I will need more RAM  memory.
> Would you recommend anything else for getting a good DOS for
> my quad core 8 GB RAM machine?

"more k-points" does not imply more memory:  the code needs to
keep in memory the Kohn-Sham orbitals for a single k-point only.
This implies a large amount of I/O, though. Note that the size of KS
orbitals for Gamma-only calculations is half the size of the same KS
orbitals for a generic k-point, though, so if you are already at the  
limit
with Gamma only, you may run out of memory with generic k-point.

There are a few indications in the user guide on how to reduce the
memory usage to the strict minimum. You could use conjugate-gradient
diagonalization ('cg': slow). If you use Davidson (derault), reduce
diago_david_ndim to 2. For scf calculations, reduce mixing_ndim
to 4 or so.

P.
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222