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Solntsev Pasha
solntsev@univ.kiev.ua

I have been trying to investigate an excited state of a molecule with the XMCQDPT. From CIS,TDDFT is known that formation of first excited state rises from HOMO->LUMO transition and 2nd excited state from HOMO-1->LUMO. The LUMO orbitals are double degenerated.

To start my work, i decided consider only HOMO-1, HOMO, 2xLUMO orbitals in my active space. So, i had done  SA-CASSCF(4,4)/6-311G(d,p) with nstate=6
wstate(1)=1,1,1,1,1,0
ntrack=5
and found after MCSCF minimization (only the biggest coefficients are shown)
1st state corresponds to my ground state
STATE #    1  ENERGY =   -1727.128479778


     CSF      COEF    OCCUPANCY (IGNORING CORE)

     ---      ----    --------- --------- -----

       1    0.964072  2200

.....
2nd state corresponds to single electron excitation (HOMO->LUMO)
STATE #    2  ENERGY =   -1726.991351175



     CSF      COEF    OCCUPANCY (IGNORING CORE)

     ---      ----    --------- --------- -----

       1    0.001215  2200

       2    0.942511  2101

..............................
       8    0.261968  1210

..............................
      13    0.136427  1012

.............................

3rd state also corresponds to single electron excitation (HOMO->LUMO)
STATE #    3  ENERGY =   -1726.991351174


     CSF      COEF    OCCUPANCY (IGNORING CORE)

     ---      ----    --------- --------- -----

.................................
       3    0.261719  1201

.......................................
       7   -0.942700  2110

......................................
      10    0.129259  2011

....................................
      18   -0.137203  1021

...............................
Rest of states don't have single electron excitation and corresponds to two electron excitation. The are not presented here.

It is of interest to note, there is no state corresponding to excitation from HOMO-1->LUMO. Such states exist only as a minor component of another transition (HOMO->LUMO). Is it problem with active space?

After that i did XMCQDPT and used
$xmcqdpt state(1)=1,1,1,1,0,0                                                
avecoe(1)=1,1,1,1,1,-0
wstate(1)=1,1,1,1,1,-0  $end  

I got following results:
*** MC-XQDPT2 ENERGIES ***

-----------------------------------------------------------------------

  STATE                       1ST ORDER                       2ND ORDER

    1     E(MCSCF)=    -1727.1284797779     E(MP2)=    -1731.6455874427

    2     E(MCSCF)=    -1726.9913511748     E(MP2)=    -1731.6292601411

    3     E(MCSCF)=    -1726.9913511741     E(MP2)=    -1731.5827124526

    4     E(MCSCF)=    -1726.9163689912     E(MP2)=    -1730.2704096821

EIGENVECTORS OF THE EFFECTIVE HAMILTONIAN



                     1          2          3          4



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



   1             0.990372   0.120637   0.025590   0.062892

   2             0.107653  -0.972246   0.185035   0.094396

   3             0.044631  -0.178233  -0.982213   0.038706

   4            -0.074714   0.091749   0.019079   0.992792



EIGENVALUES OF THE NON-SYMMETRIC EFFECTIVE HAMILTONIAN



    1    -1731.63989383779100000000 + I *        0.00000000000000000000

    2    -1731.62858390071600000000 + I *        0.00000000000000000000

    3    -1731.58269032210300000000 + I *        0.00000000000000000000

    4    -1730.27680165785000000000 + I *        0.00000000000000000000

As we can see from "EIGENVECTORS OF THE EFFECTIVE HAMILTONIAN
" part i steel have contribution from 2nd CASSCF state to the 1st XMCQDPT. And the same situation for 2nd and 3rd states for XMCQDPT.

Experimental value for 1st excited state is 564 nm or 17730 cm^-1 or 0.08078 a.u.

It looks i very close to finish, but maybe i missed something.

Also i have couple questions to clarify my understanding.

Q1:
If only 1-n states corresponds to single electron excitation, then we don't need to include another state for XMCQDPT. Or we should consider only state from CASSCF that corresponds to single electron excitation. Am i right?

Q2:
What is a difference between
kstate(1)=1,1,1,1,1,0
avecoe(1)=1,1,1,0,0,0
wstate(1)=1,1,1,0,0,0

and

kstate(1)=1,1,1,0,0,0
avecoe(1)=1,1,1,0,0,0
wstate(1)=1,1,1,0,0,0

In both cases we have same number of states but weight "0"
in second corresponding state in kstate array is "0".

Q3:
What should we look in an output file to check XMCQDPT results
"EIGENVECTORS OF THE EFFECTIVE HAMILTONIAN"
"EIGENVALUES OF THE NON-SYMMETRIC EFFECTIVE HAMILTONIAN"
anything else?

I will be very appreciate for you help.
If you need extra information about my current problem just let me know.

Best wishes.

Pavel.