[CP2K:8695] Isolated system Geometry Optimizations

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

Hi

the problem is connected to the fact that the plane wave basis
depends on the box size. The same is true for the XC integration grid.
Now with a very small cutoff for the PW this will show the dramatic effects
you have.
To solve this proble you have to use a much higher cutoff, especially the 
relative cutoff. I would suggest values of 80 and 400, respectively.
The smoothing you use for the XC will also have an effect. I don't know
how bad this is in your case. I would start without the smoothing and
make later some tests to see if it helps.

In general, it is a good idea for such tests to increase accuracy in all
parts of the calculation.

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: Babgen Manookian 
Sent by: cp... at googlegroups.com
Date: 02/14/2017 07:21PM
Subject: [CP2K:8695] Isolated system Geometry Optimizations

Hey Everyone,
I am a new cp2k user and have been working on a test system in order to get a feel for how the program works. My test system is a cyclohexane molecule and I am interested in determining the energy difference between boat and chair conformations. I am using a wavelet poisson solver with periodicity turned off and the molecule centered in the cell. I attached the template input file which I used to create the different inputs with varying cells size.
What I have noticed is that as I increase my cell size the energy does not converge on a single value, instead it seems to oscillate. The graphs below show the energies of each conformation and their differences as functions of cell size. I am very curious as to what is causing these oscillations. If I have a single cyclohexane molecule centered in my cell, as I increase my cell size, I would think that the extra vacuum space around the molecule will not affect the energy. 
If there is anyone out there who could provide any sort of input on this it would be much appreciated.

Thanks,Bobby



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[attachment "cychex_boat_temp.inp" removed by Jürg Hutter/at/UZH]