Subject: A problem with O(1D)

From: "Alexei Khalizov" <[email-masked]>
Date: Tue, 23 Jul 2002 17:11:36 -0400

Dear CCLers,

Does anyone know how to calculate correctly the energy of an oxygen 
atom in the singlet state O(1D)? I am having a problem of evaluating 
the singlet-triplet energy difference E(S-T) = E[O(1D)] - E[O(3P)] 
whose experimental value is 190 kJ/mol. Using DFT methods (B3LYP and 
PW91) with small basis sets (6-31G*) significantly overestimates this 
energy splitting, giving E(S-T) = 270-280 kJ/mol. Calculation at 
CCSD(T)/6-311+G(3df) level of theory leads to a better result, 
however, it is still far from being acceptable, E(S-T) = 214 kJ/mol. 
So far I have found that it is the energy of O(1D) that is incorrect 
while the energy of O(3P) is reproduced properly. What goes wrong? 
The wavefunction for O(1D) seems to be correct - population analysis 
shows that it is symmetrical (2Px alpha) (2Px beta) (2Py alpha) (2Py 
beta).

Thank you in advance,
Alexei

-------------------------
Dr. Alexei Khalizov
Department of Chemistry
McGill University,
801 Sherbrooke St. W.,
Montreal, Quebec, CANADA,
H3A 2K6

Phone: (514) 398-6920
Fax: (514) 398-2382