Thursday, 17 May 2007

HF-SCF bases

I found some time to try out GAMESS a little bit more. What I was interested in is how a geometry optimisation changes with different bases.

I tried out most of the standard bases that GAMESS offers without any extra functions. The input looks like this (enter the base instead of ***). After evaluation I extracted the information with Visual Basic.

! Ethane staggered
!
$CONTRL SCFTYP=RHF RUNTYP=Optimize $END
$SYSTEM TIMLIM=30 $END
$BASIS *** $END
$GUESS GUESS=HUCKEL $END
$DATA
Ethane...staggered...***
S2n 3

C 6.0 0.0000000000 0.0000000000 -0.7688320347
H 1.0 -1.0157199281 0.0000000000 -1.1532895468
$END

This is the result. You see the C-C and C-H bond lengths (in Angstrom) and the C-C-H angle. The table is arranged according to the complexity of the base (information). In the first line are the literature values [1], in the other lines relative deviations.

BasisC-CC-HC-C-H
Experimental1.53511.0940111.17°
MNDO-0.01430.01520.0221°
AM1-0.03480.0231-0.4514°
PM3-0.03060.00380.4605°
STO-2G0.0029-0.0020-0.0537°
STO-3G0.0026-0.0080-0.4380°
STO-4G0.0002-0.0116-0.4738°
STO-5G-0.0001-0.0128-0.4628°
STO-6G-0.0004-0.0125-0.4612°
3-21G0.0072-0.0099-0.3732°
6-21G0.0064-0.0099-0.3084°
4-31G-0.0062-0.0106-0.0081°
5-31G-0.0054-0.00980.0183°
6-31G-0.0052-0.00960.0236°
6-311G-0.0070-0.01130.0494°
DZV0.0024-0.0102-0.1002°
TZV-0.0047-0.0109-0.0096°

The first three are semiemperical methods. They show the worst results (as it is supposed to be).

It's more difficult to say things about the ab-initio calculations. It can be seen that bond lengths never differ much more than 0.01 (which is nice). Another interesting fact is that with a better basis, bond lengths tend to be shorter [2]. At the HF-SCF limit (self-consistent eigenfunctions of the Fock operator) bond lengths are usually too short (which is not nice). This is just shows a limitation of the method. Sometimes medium sized bases give better results than large bases. To overcome the problem, you need correlated calculations.

[1]Lide DR. Handbook of Chemistry and Physics 86th Ed. 2005-2006. CRC
[2]Kutzelnigg W. Pure appl. Chem. 49, 981 (1977)

2 comments:

baoilleach said...

You may find it useful to extract information with http://cclib.sf.net, a Python library.

Felix said...

thanks for the link. it's added to the list programs I should check out. but right now I am happy with that visual basic program of mine