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Re^3: Spin density from CASSCF in a cube file

Alex Granovsky

Dear Pavlo,

rhoalpha-rhobeta is not an approximation for non-relativistic MCSCF and spin-pure states.
rhoalpha is computed based on the alpha spin one-particle density matrix.
Similarly, rhobeta is computed based on the beta density matrix.
Both GUGA and ALDET CI or MCSCF would produce identical results but
presently spin density matrices are only available for ALDET.

How large is the area of inverse spin polarization? I believe if it is not very large you can ignore it.

All the best,

On Wed Jul 23 '14 3:12pm, Pavlo Solntsev wrote
>Dear Alex.

>If i understood correctly, rhoalpha-rhobeta is an approximation in case of MCSCF wave function. Or this equation corresponds to determinant(CSF) part, where the two sets of orbitals available in a clear form. Is it safe to ignore negative sign for the analysis of spin-density? In my case, i have a doublet. And a picture i got, actually, perfect.
>On Wed Jul 23 '14 1:15pm, Alex Granovsky wrote
>>Dear Pavlo,

>>With Firefly, the computed spin density is always rhoalpha-rhobeta density.
>>It is identically zero for singlets, but is a nontrivial function
>>for other multiplicities. For instance, it does not necessary have
>>the constant sign despite an excess of alpha electrons. The chemical
>>meaning of this is in the beta electron density becomes polarized by
>>the alpha electrons so that there are areas with excess of beta spins.

>>Hope this helps.

>>All the best,
>>On Tue Jul 22 '14 6:01pm, Pavlo Solntsev wrote
>>>Dear Alex and all forum users.

>>>I have calculated spin-density using CASSCF with the following line
>>> $eldens spind=.t. ieden=1 where=cube  $end
>>> $cube $end
>>>I got my cube file, it looks good to me. But i realized it contains surface of two different signs: negative and positive. The positive sign in a spin density surface corresponding to a localization of electron(s). It makes total sense from the picture i got. But negative sign is not clear to me. Does it represent a hole? Does it have a chemical meaning? Do we need to worry about negative sign if we are interesting in location of the electron?

>>>I know, that spin density within UHF wavefunction is alpha-beta surface. But what about MCSCF wave function?

>>>Maybe someone from this forum has a reference related to the idea about spin density. It would be great.


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