Subject: UMP2 on coaxial O2 U(r)

From: Harald Svedung <[email-masked]>
Date: Wed, 12 Jan 2000 12:00:59 +0100 (MET)

Hi,

When I scan the total energy over the closest intermolecular atom-atom 
distance in e.g. coaxial  N-N - N-N, I get a nice potential using RMP2.

I thought I might get something similar using UMP2/AUG-cc-pVTZ for the 
axial O-O - O-O interaction. Trying this, I get a well at apr. 2 
Angstrom outside which the total energy that is decaying (with some 
strange jumps at e.g. apr. 4 Angstroms) at least out to 30 Angstrom.

For O-O - Ar there is a strangely deep well (erroneously (?)) indicating 
complex binding at ambient temperatures.

Is there some spin-related dificulty, common to theese systems, in the 
UMP2 method?

Are there cut-offs giving the jumps in energy?   (Gaussian98)

Will a BSSE-correction change the picture dramatically? 

Atomic charges indicate a long range coloumbic repulsion for the O2-O2 
system. Is it physically reasonable to assume that there is a deep 
energy minima inside a repulsive domain in the coaxial O2-O2 interaction?

This is all likely to be much known. Any suggestions are very welcome.

:-)
/Harald

Harald Svedung (Ph.Lic.)                phone:          +46-31-7722816
Department of Chemistry                 fax:            +46-31-167194
Physical Chemistry                      home phone:     +46-31-240897, +46-709223206	
Goeteborg University                    home e-mail:    [email-masked]
SE-412 96 Goeteborg, Sweden             www.che.chalmers.se/~svedung/