[molpro-user] Single reference CI

[Michael Stenrup]

Dear Molpro users,

I have recently downloaded the evaluation version of Molpro in order to 
evaluate its capabilities and limitations. I want to use the general MRCI 
program to calculate accurate energies and non-adiabatic couplings for 
small molecules. However, when I tried to do an FCI calculation (single 
and double excitations within all virtual orbitals obtained from a HF) on 
the H2 molecule, I immediately ran into truble. If I only ask for the 
ground state energy no problems are encountered, but if I request also 
excited states the program objects that more reference configurations need 
to be included. This is a bit puzzling to me since on the paper (as well 
as in other programs), a single reference CI should be capable of 
giving excited state energies. I've read the manual several times but my 
understanding of this problem is still lacking. I have only been doing 
quantum chemistry for a short time and I apologize if this is is a stupid 
question with a trivial answer. Still, this is a real mystery to me and 
any help would be much appreciated.

Best regards,

Michael Stenrup
Department of Theoretical Chemistry
Royal Institute of Technology
Stockholm, Sweden