[molpro-user] Number of excited states: multi vs single reference methods

[Joseph Lane]

Could anyone please explain why multi reference methods like  
CASSCF/MRCI/CASPT2 calculate far fewer electronic excited states in a  
given region as compared with single reference methods like TDDFT or  
CCSD-EOM?

For example, MRCI calculations of H2SO4 shows that there are 6 excited  
states less than 120nm whereas CCSD-EOM or TDDFT methods calculate 12 and  
13 different states, respectively. Do multi reference methods include  
contributions of several single reference excited states to form each  
multi reference excited state?

Any help would be greatly appreciated


Kind regards



-- 
Joseph Lane
Department of Chemistry
University of Otago
P.O. Box 56
Dunedin, New Zealand
ph +64 3 479 5214
email : jlane at alkali.otago.ac.nz