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Igor Polyakov
polyakoviv@gmail.com

I'm not Alex, but I'll try to help. First of all, CASSCF state order is not guaranteed to be the correct one=)
Larger effective Hamiltonian dimension should yield more correct results, however for big systems it is simply impractical (high computational effort) to calculate Heff for more than 10-20 states. 2x2 seems to be a bit extreme in the low end because higher states (up to 5 or even more sometimes) can be of influence to the lowest ones. I would recommend to conduct a single large calculation to see the "EIGENVECTORS OF THE EFFECTIVE HAMILTONIAN".
For example for one of my systems:
                     1          2          3          4          5

              *******************************************************

   1            -0.996755  -0.063006  -0.025667   0.000488   0.007272
   2             0.068939  -0.930901  -0.324050   0.023948   0.052243
   3             0.002769   0.257066  -0.672227   0.680566  -0.104661
   4             0.004256   0.225480  -0.647910  -0.704316   0.166305
   5             0.013621   0.100592   0.148076   0.039046   0.262885
   6            -0.004291  -0.005923  -0.014240  -0.189029  -0.930557
   7            -0.005277   0.019798   0.006718  -0.032537   0.111529
   8             0.036419  -0.043997  -0.013186  -0.042064  -0.102410
   9            -0.008000  -0.005780   0.016341   0.009414   0.020153
  10            -0.008890   0.003653   0.006373  -0.004601  -0.015154

This means that to correctly describe state 3 it is crucial to include state 4. This can also be seen in the CAS-CI states, states 3 and 4 share some large weighted CSF-S

Best regards, Igor

On Fri Dec 23 '16 5:37pm, Maksim Shundalau wrote
------------------------------------------------
>Dear Alex,
>
>
>Let's pretend that I calculated two lower states (triplet and singlet) of transient molecule (or radical) at the CASSCF level of theory. I get triplet as a ground state and singlet is only a few tens of cm-1 higher than the triplet.
>How many triplets and singlets should I take in the XMCQDPT2 procedure to get the same sequence of states and trust the results? If I take 2 and 2 (or 10 and 10), I have triplet higher than singlet. Should I take 3 and 2 (or 5 and 2, for example) for triplet and singlet, respectively?
>
>
>Kind regards,
>Maksim Shundalau.
>

[ This message was edited on Sat Dec 24 '16 at 9:00pm by the author ]