Ceilidh ... Re: Minimum number of NSTATE required in MCSCF calculation

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Re: Minimum number of NSTATE required in MCSCF calculation

If you set CISTEP=ALDET (rather than CISTEP=GUGA), the program generates determinants, which do not correspond to any spin number. Spin eigenfunctions are constructed from the determinants; some combinations correspond to S=0 (singlet), some to S=1 (triplet), etc., with the same Sz=0 (spin multipicity=1). So, ALDET procedure generates both triplets and singlets, and you require only singlets. According to the Hund's rules, for a given electron configuration, the term with maximum multiplicity has the lowest energy. That is, for the configuration ...2110... (numbers denote orbital populations), the triplet state (S=1) is always lower than the singlet state (S=0). So, the first state is singlet ...2200..., the second state is triplet ...2110..., and the third state is singlet ...2110...

When you set CISTEP=GUGA, the program generates configuration state functions, that is the combinations of determinants, which are spin eigenfunctions. So, with CISTEP=GUGA the first state will be singlet ...2200..., and the second state will be singlet ...2110...

However, at present, ALDET algoritm is more efficient and less memory-consuming for large active spaces.

On Mon Oct 8 '12 6:46pm, Patrick SK Pang wrote
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>Dear all,

>When I am interested in the ground state of a molecule (charge=0 and spin multipicity=1), I specify ISTATE=1 and NSTATE=1, and the calculation can successfully completed.
>However, when I want to calculate the 1st excited state a molecule (charge=0 and spin multipicity=1), I specify ISTATE=2, NSTATE=2, WSTATE=0,1, and an error appears due to the wrong number of NSTATE. Then, I change to NSTATE=3, and the calculation can proceed.
>Why?

>Regards,

>Patrick

Tue Oct 9 '12 1:14am

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