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Re^2: Differences between MCSCF and XMCQDPT2 results

Panwang Zhou

Dear Prof. Alex,

Thanks for you reply.

>You can verify your results including more states into model space.
>For instance, you can average over four states, while include ten states or so.

I have done the test calculations including ten states into model space and the results are slightly different from that only including four states in model space.

The state order of calculations are:
i) MCSCF_S0_OPT:      s0, pipi*, npi*,  pipi*
  MCSCF_PIPI*_OPT:   s0, pipi*, npi*,  pipi*
  MCSCF_CI_OPT:      s0, pipi*, npi*,  pipi*
  MCSCF_NPI*_OPT:    s0, npi*,  pipi*, pipi*
ii)PT2_4_S0_OPT:      s0, pipi*, pipi*, npi*
  PT2_4_PIPI_OPT:    s0, pipi*, pipi*, npi*
  PT2_4_CI_OPT:      s0, pipi*, npi*,  pipi*
  PT2_4_NPI*_OPT:    s0, npi*,  pipi*, pipi*
iii)PT2_10_S0_OPT:    s0, pipi*, pipi*, npi*
   PT2_10_PIPI_OPT:  s0, pipi*, pipi*, pipi*, npi*
   PT2_10_CI_OPT:    s0, pipi*, pipi*, npi*
   PT2_10_NPI*_OPT:  s0, npi*,  pipi*, pipi*

It can be seen that, when the states number of model space is increased to ten, for pipi_opt and ci_opt, the npi* state now become to the third and fourth excited state, respectively. That is to say, a more pipi* appears below the npi* state. Now I'm not sure how to explain the new appeared pipi* state in the XMCQDPT2 calculations with nstates=10.
And which pipi* state in the XMCQDPT2 results is corresponding to the first pipi* state in MCSCF results? Thanks.

Kind Regards,

Panwang Zhou

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