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Unreasonable energies in XMC-QDPT2

Bernhard Dick

I obtained a rather strange result in a CAS(10|10) calculation with subsequent XMC-QDPT2. I am interested in the electronic excitation energies of an organic dye molecule. The energies obtained with CIS or CASSCF are much too high (i.e. ca. 150 mH), the experimental value is ca. 85 mH. When I try XMC-QDPT2 on top of the CASSCF, I get an excitation energy of 2.036 H, which is competely beyond any reasonable value. (see table below) The first order result is 155 mH, which is too large, but apparently "normal" for a CASSCF on this molecule.

I attach the input and output - perhaps there is a an incompatible combination of keywords that I am not aware of?

Two further questions:
a) can a TRANSITN run also calculate oscillator strength for XMC-QDPT2 states?
b) Is there a 1:1 correspondence between the CASSCF states and the XMC-QDPT2 states? I.e., could a state with a large oscillator strength have a different place in the list in CASSCF and with XMC-QDPT2?

I used the NEVPT2 method in ORCA for comparison. It provides oscillator strengths, but the excitation energies are too high.  

  STATE                       1ST ORDER                       2ND ORDER
    1     E(MCSCF)=  -1099.890894827395     E(MP2)=  -1106.119584872618
    2     E(MCSCF)=  -1099.735990116105     E(MP2)=  -1104.083136633190
    3     E(MCSCF)=  -1099.733877626624     E(MP2)=  -1103.954679662041
    4     E(MCSCF)=  -1099.681143061018     E(MP2)=  -1103.936946076146
    5     E(MCSCF)=  -1099.674487604719     E(MP2)=  -1103.931452963207
    6     E(MCSCF)=  -1099.666980999407     E(MP2)=  -1103.895001735584
    7     E(MCSCF)=  -1099.666499513098     E(MP2)=  -1103.879138376253
    8     E(MCSCF)=  -1099.656218109184     E(MP2)=  -1103.872965665376
    9     E(MCSCF)=  -1099.633450909673     E(MP2)=  -1103.861755356159
   10     E(MCSCF)=  -1099.618913815137     E(MP2)=  -1102.811860797204

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