Subject: Problems with O2 - Carbon Sheet Interaction Energies

From: Jeffery Klauda <[email-masked]>
Date: Tue, 27 Aug 2002 11:10:36 -0400

Dear CCLers,

I have been having some problems with calculating interactions energies 
between diatomic oxygen and a carbon surface of six and seven member 
rings.  All of my calculations correct for BSSE and are at 
ROMP2/aug-cc-VDZ with a carbon surface that is a singlet in its ground 
state (hydrogens added to simulate C-C bonds).

The problem lies in Hartree-Fock, without changing the initial guess for 
the orbital occupations the initial INDO guess puts the unpaired 
electrons on the carbon surface when O2 should have two singly occupied 
orbitals.  Therefore the incorrect spin density is calculated and the 
interaction energies are extremely repulsive.

To alleviate this problem, I thought a better initial guess (one with 
the singly occupied pi* like orbitals on oxygen) would solve this 
problem. However, this works only for a limited amount of cases and 
seems to be more trial and error which orbital to choose than a 
systematic approach that can be used for other O2 positions and 
orientations.

The correct solution only occurs when at the converged HF energy the 
spin density is essentially zero on the carbon surface and two on O2.

I was wondering if any of you have come across this problem and have a 
systematic way to solve this.  I have tried other approaches to no avail 
(UMP2, initial guesses of correct wavefunction for other O2 orientations 
with respect to the carbon surface, other basis sets).

Any help in this matter will be greatly appreciated,

Jeff Klauda

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Jeffery Klauda
Ph.D. Candidate in Chemical Eng.
University of Delaware
119 Colburn Lab
Newark, DE 19702
Phone: (302) 831-6857
Fax: (302) 831-1048
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