[molpro-user] singlet-triplet excitation energy in diradicals

[Hans Jakob Woerner]

Dear all,

I'm running calculations on c5h5+ (cyclopentadienyl cation) to determine the 
singlet-triplet interval at D5h geometry (the geometry of the triplet ground 
state).
The lowest singlet state has symmetry 1E2' in D5h. When I run a CASSCF(4,5) 
calculation of the 1A1 component (in C2v) I find it 6.1 mHartree lower than 
if I run a state-averaged (1A1+1B1) calculation.  Is this expected? Input 
file is attached.
I am eventually interested in the stabilization energy of the singlet state 
upon distortion. Here again the excitation energies of a single vs. 
state-averaged calculation differ by 6 mHartree. Has anyone had such a 
problem?

Looking forward to hearing from you,
Hans Jakob

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Hans Jakob Wörner
ETH Zürich
Laboratorium für Physikalische Chemie
ETH-Hönggerberg, HCI E207
CH-8093 Zürich    
Switzerland

Telephone: +41-1-633-4327               
Fax: +41-1-632-1021
E-mail: woerner at xuv.phys.chem.ethz.ch    
http://www.xuv.ethz.ch