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Alex Granovsky
gran@classic.chem.msu.su

I'm afraid XMCQDPT2 with dimension of model space equal
to ca. 30-50 as you suggest will be prone to intruder
state problem. I'd not use more than ca. 20 states for
organic pi-systems. Unfortunately there are no general
recipe here. You can try to perform several calculations
with increasing dimension of H_eff and check
how stable the transition energies of interest are. Another
thing to check is the overlap matrix of non-orthogonal
eigenvectors corresponding to H_eff before symmetrization.
There should not be large off-diagonal values here, and
all eigenvalues of non-symmetric H_eff must be
real numbers.

Cheers,
Alex


On Sun May 29 '11 5:11pm, sanya wrote
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>How to estimate the size of H_eff in XMCQDPT (nstate or kstate keys)? I know that for 1 excited state H_eff of 10-12 states is enough, but typically I need 3-4 excited states. Does this proportion keeps for higher-lying states?