Thank you for your suggestions. I finally have been able to obtain the desired 12,10 active space. The trick was to first create an 8,8 active space (consisting of the pi bonding and antibonding orbitals) with state averaging (wstate(1)=1.0,1.0). After that, I could expand the active space to 12,10, including the two p lone pairs.
This wasn't possible without the state averaging, so I'm thankful that you pointed out its importance.
On Wed Aug 31 '11 3:08am, sanya wrote
>It's really a difficult problem. This happens sometimes. Different situations are described by different active spaces. State-specific CASSCF for the ground state (as in your example) is described by sigma orbitals (natural orbitals #46 and #50, or optimized orbitals #49 and #50) and pi orbitals. Lowest excited states (mostly pi->pi* transitions) are described by pi orbitals. I can see two ways: include MOs #43 and #44 and/or use state-averaging (at least, wstate(1)=1.0,1.0,1.0). Probably, after converging CASSCF(16,12) you may try to use this solution as a start for the desired CASSCF(12,10).