Hi!
I'm using SIESTA to calculate different sulfuric acid clusters, As a
standard procedure, I used to leave SIESTA choose the unit cell
(minimum+10%) but then I realized that the cell was not large enough,
so I make some test increasing the cell size until I get the same
value for energy and frequencies in two sizes. The problem is that I'm
not able to find that limit, Here are some results:
For Sulfuric acid neutral molecule:
Cell Eo (kcal/mol) G(kcal/mol) (calculated using frequencies)
Automatic -50582.74383 4.846501
10 10 10 -50582.58026 5.317222
20 20 20 -50582.40954 5.345038
30 30 30 -50582.39213 5.754734
40 40 40 -50582.3914 5.348183
50 50 50 -50582.38766 5.60117
In this case the problem is G calculation (so frequencies) that seems
to fluctuate instead of converging to some value.
For the sulfuric acid dimmer the situation is even worst:
Cell Eo(kcal/mol) G(kcal/mol)
auto -101196.8922 25.892183
10 -101196.709 25.702404
20 -101197.1805 23.643095
30 -101197.1229 26.935893
40 -101197.1984 23.479595
50 -101197.1391 24.773543
Here I get differences in G of 3.3 kcal...
And for the Bisulfate ion even worst (just have electronic energies
for that one):
Cell Eo (kcal/mol)
auto -50218.69544
10 -50221.10954
20 -50224.1256
30 -50224.41819
40 -50224.50091
50 -50224.54037
Here the differences in electronic energy between the automatic cell
and >20 is around 6 kcal/mol (still running FC calculations)
Any way the main problem is that G is not converging to some value, is
jumping up and down, so I don't have any criteria to choose one cell
size.
Some idea of what is going wrong here??
Thank you in advance
I.K. Ortega
---------------------------------------------------------------------------
Dr. Ismael Kenneth Ortega Colomer
postdoc researcher
Atmospheric Sciences Division
Dynamicum 1D16b
University of Helsinki
PL 48
00014
ResearcherID: http://www.researcherid.com/rid/A-7930-2008
---------------------------------------------------------------------------
