I would like to report some problems I have with MRMP2/XMCQDPT2 calculations on the four lowest excited states of pyrazine.
Pyrazine has D2h symmetry at its ground state equilibrium geometry and the four lowest singlet excited states are B3u(npi*) Au(npi*) B2u(pipi*) and B2g(npi*).
There has been a number of calculations of these states using various methods. Several CASPT2 studies have given good results compared to experiments and are all more or less consistent with each other in terms of accuracy. For instance, Woywod et al. Theor Chem. Acc. 125, 521 (2010) obtained 3.93, 4.58, 4.79 and 5.54 eV for the vertical excitation energies using a triple zeta basis set.
I performed MRMP2 calculations using the aug-cc-pVDZ basis set and using the D2h symmetry. I obtained 3.53, 4.13, 4.55 and 5.17 eV.
These values are well below the CASPT2 results. Note that already the CASPT2 results tend to underestimate the experimental results.
I then performed a XMCQDPT2 calculation ignoring symmetry and using a state averaged CASSCF wavefunction. The five states were included into averaging. And only those five state are included in the XMCQDPT2 model space. Since they are of different symmetry the CASSCF states are not mixed by the XMCQDPT2 procedure but it was just to obtain the five energies in one calculation. I obtained results in good agreement with CASPT2 results : 3.93, 4.45, 4.79 and 5.38.
This is surprising since the second approach makes no sense and the first approach would be expected to be more accurate. Inspection of the orbitals in each case show that they keep the same character in all the calculations.
The corresponding files are attached.
Can anyone tell me what I am doing or thinking wrong here ?
Any help or comment would be greatly appreciated.
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[ pyrazine_files.zip ] zipped folder containing the files of the pyrazine calculation