[CP2K:9763] MGRID vs KPOINTS for small systems

[hut... at chem.uzh.ch]

Hi

the actual grid sizes is calculated from the plane wave energy cutoff
and optimal sizes for 3-d FFTs. Defining grid spacings would result
in similar results, but the actual grid spacing in the calculation would 
most likely differ from the input.
Energy cutoffs are the standard units for accuracy in electronic structure
calculations.

regards

Juerg
--------------------------------------------------------------
Juerg Hutter                         Phone : ++41 44 635 4491
Institut für Chemie C                FAX   : ++41 44 635 6838
Universität Zürich                   E-mail: hut... at chem.uzh.ch
Winterthurerstrasse 190
CH-8057 Zürich, Switzerland
---------------------------------------------------------------

-----cp... at googlegroups.com wrote: -----To: cp2k <cp... at googlegroups.com>
From: Sebastian Hütter 
Sent by: cp... at googlegroups.com
Date: 12/02/2017 07:06PM
Subject: Re: [CP2K:9763] MGRID vs KPOINTS for small systems

Hi,

thank you. The way I understood it is that to get better 
results, I can either increase the volume of real-space 
the integration happens over (MULTIPLE_UNIT_CELLS), or can move to a model in the reciprocal lattice and sample the 1st BZ at high(er) 
resolution? That means that for such a small system, what I effectively did in my test was the same as if I used a too coarse k-point grid? The thing that confused me was that most energies were very similar, just that one term wildly different.

For my test system, I need 27 supercells to get the kpt
 energies, but even more for correct stresses - but at about 1/10th the  computation
time and a little less memory/proc that could be worth it. But now I 
have all quantities output for the replicated atoms as well, making 
things a bit harder to use... guess I'll stick with k-point sampling. Is
 there a break-even point regarding volume or atom/electron count where 
one method is generally faster than the other?

As a related question, is there a way to specify an M-P grid using point spacing instead of point counts?

Best regards,
Sebastian




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