# [molpro-user] How to read the CSFs in a MRCI calculation?

Gerald Knizia knizia at theochem.uni-stuttgart.de
Wed Aug 12 18:15:30 BST 2015

Dear José,
you are right about \ and / indicating spin couplings in the
geneological coupling scheme. Regarding the doubly external
configurations: These are not individual CSFs, but they indicate
internally contracted configurations, in which spin-free excitation
operators are applied to the entire reference function:

|Phi^ij_ab> = E^{ab}_{ij} |Phi>

with |Phi> being the full MCSCF-type reference function (not individual
CSFs), i and j being occupied orbital labels, and a and b being external
orbitals.

The operator E^{ab}_{ij} denotes explicitly

E^{ab}_{ij} = \sum_{\sigma \in A,B} \sum_{\tau \in A,B} c^{a \sigma}
c^{b \tau} c_{j \tau} c_{i \sigma}

where \sigma and \tau sum over spin labels.

Best wishes,
Gerald

On Sun, 2015-08-09 at 21:51 -0500, José Cortés wrote:
> Dear molpro users
>
> I have a question about the output of the CSFs in a mrci calculation.
> As an example suppose the He2 molecule with the 6-311G basis,
> where an active space of 4e,4o was chosen in the part of the casscf
> calculation.
> Such election leaves only two external orbitals. As I understand the
> symbols
> "/" and "\" concerning to relative spin couplings, which are related
> to the t vectors
> in the genealogical coupling scheme.
>
> For example for the following configurations (which are in the output
> of the MRCI calculation)
>
>          Reference coefficients greater than 0.0000000
>          ================================
>          2200           0.9959538
>          /\/\           0.0600897
>
>          Coefficients of singly external configurations greater than
> 0.0000000
>          ================================================
>          /\\0   5.1    -0.0062116
>          20\0   6.1    -0.0055316
>
> the t vectors specified in the complete basis set would be like:
>          220000
>          /\/\00
>          /\\0/0
>          20\00/
>
> However, my question arises in the doubly external configurations
>
>         Coefficients of doubly external configurations greater than
> 0.0000
>          ===========================================
>          PAIR    I     J   ->   K     L     NP  SYM  REF
> COEFFICIENTS
>            4    2.1   2.1      5.1   5.1     1   1    1   -0.02645858
>            1    1.1   1.1      5.1   5.1     1   1    1   -0.02035037
>            4    2.1   2.1      6.1   6.1     1   1    1   -0.01367284
>            9    3.1   3.1      5.1   5.1     1   1    1   -0.01342122
>
> In this case, how the t-vectors should be specified for each one of
> the configurations in the complete basis set?
> How do I get in this case each one of the configurations?
> I would appreciate any information you could give me about it.
>
> Regards
> José Jara