[CP2K:4359] possible bug in Ewald routine ?

[hut... at pci.uzh.ch]

Hi

the input to the EWALD routines is for historical reasons
rather complicated. There are three parameters that define
an Ewald calculation:
- alpha (width of the Gaussian compensation charge)
- the real space cutoff (equal to the cutoff of nonbonded forces in CP2K!)
- the reciprocal space cutoff

Given a desired accuracy and anyone of the above three parameters
determines the possible range of the other two parameters.
However, in CP2K you have to define all three parameters independently
in the input. In addition, the reciprocal cutoff is given by
the number of grid points, this means depends on the box size!

And finally, the regression tests are not 'real' inputs. You
cannot rely on the parameters used there to give you converged
results. You have to find them yourself.

regards

Juerg Hutter 

--------------------------------------------------------------
Juerg Hutter                         Phone : ++41 44 635 4491
Physical Chemistry Institute   FAX   : ++41 44 635 6838
University of Zurich               E-mail:  hut... at pci.uzh.ch
Winterthurerstrasse 190
CH-8057 Zurich, Switzerland
---------------------------------------------------------------

-----cp... at googlegroups.com wrote: -----
To: cp... at googlegroups.com
From: vish 
Sent by: cp... at googlegroups.com
Date: 03/26/2013 01:17AM
Subject: [CP2K:4359] possible bug in Ewald routine ?

Dear CP2K users,

EWALD section requires one to input ALPHA (ang -1) parameter. In general, I would expect changing this value should not have any affect on the total potential energy of the system. However, when I change ALPHA in the tests files in CP2K tests files, the total potential energy changes a lot (for example water512.inp in tests/Fist folder). I was wondering if there is a bug in ewald routine or I am understanding this wrongly?

Any help on this is greatly appreciated. 

Thanks in advance,
Vishal  
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