A riddle for CASPT2 gurus!

Subject:

if B is 1s22s22p1, what is Li?

Passfield:

Comments:
: Dear All, : I have encountered an extraordinary occurrence in the CASPT2 energies for two different configurations of the same system. The data is the following: : 1st configuration: : CASSCF energy: : S0 (ground) state: -1083.32199351 : S1 (lowest excited) state: -1083.16358216 : Multistate CASPT2 energy: : S0 (ground) state: -1085.56591537 : S1 (lowest excited) state: -1085.45365823 : 2nd configuration: : CASSCF energy: : S0 (ground) state: -1083.25860324 : S1 (lowest excited) state: -1083.19263076 : Multistate CASPT2 energy: : S0 (ground) state: -1085.52232322 : S1 (lowest excited) state: -1085.45281629 : As you can see CASSCF predicts that the S1 state at the 2nd configuration is lower in energy than at the 1st configuration. Surprisingly CASPT2 does not agree with CASSCF and finds the S1 state higher in energy at the 2nd configuration. It is a priori known that S1 is lower in energy at the 2nd configuration! : The active space and all things are the same in all calculations, only the geometry is different. : I wonder why CASPT2 gives obviously a wrong result in this case. Has someone encountered the same behaviour? : Thanks! : Best, : Evgeniy