Author Topic: RICC2 excited state multiplicity  (Read 1711 times)

djelenfi

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RICC2 excited state multiplicity
« on: September 24, 2019, 08:26:53 PM »
Dear all,

I calculate excited state with ricc2 for triplet ground state molecule, but I don't get multilicity information from the excited state.

 +================================================================================+
 | sym | multi | state |          CC2 excitation energies       |  %t1   |  %t2   |
 |     |       |       +----------------------------------------+--------+--------+
 |     |       |       |   Hartree    |    eV      |    cm-1    |    %   |    %   |
 +================================================================================+
 | a   |   -   |   1   |    0.1797394 |    4.89096 |  39448.249 |  95.27 |   4.73 |
 | a   |   -   |   2   |    0.2074392 |    5.64471 |  45527.644 |  94.43 |   5.57 |
 | a   |   -   |   3   |    0.2284917 |    6.21757 |  50148.121 |  96.84 |   3.16 |
 +================================================================================+

The input is:
$ricc2
  cc2
$excitations
  irrep=a nexc=5 npre=0 nstart=0
 
or:

$excitations
  irrep=a multiplicity=1 nexc=5 npre=0 nstart=0

or:

$excitations
  irrep=a multiplicity=3 nexc=5 npre=0 nstart=0

Their outputs are same, without multiplicity information.
How can I get multiplicity?

Thanks,
David
« Last Edit: September 25, 2019, 06:50:44 AM by djelenfi »

christof.haettig

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Re: RICC2 excited state multiplicity
« Reply #1 on: November 18, 2019, 11:44:21 AM »
If you run for a triplet ground state it means you run an open-shell calculation. This uses in the ricc2 part spin-orbitals (because it needs diagonal occupied/occupied and virtual/virtual blocks of the Fock matrix), i.e. different MOs for alpha and beta spin.
With that  you don't get pure singlet and pure triplet states but states with some spin contamination.

So, the multiplicity input has no meaning for these calculations and is ignored.

To determine the approximate spin multiplicity of the states you can compute the expection values for S^2 with the s2expect keyword in the $excitations and $response data groups