Re: CCL:A question on DFT

On Mon, 31 Jan 2000, Armando Navarro wrote:
 >
 > Dear members:
 > I have a doubt about the Kohn-Sham implementation of DFT theory.
 > Provide we would have the "true" functional, we can in principle
 =
 > determine the "true" electronic density. But, it could be always
 =
 > represented as a single determinant of Kohn-Sham molecular orbitals?
 > Thanks in Advance
 > Armando Navarro, Ph-D student
 > Facultade de quimica
 > Departamento de quimica Organica
 > Universidade de Santiago de Compostela
 >
 > e-mail:qoajnv at.at usc.es
 At the same time, you do not have to do restricted calculations. You
 can do unrestricted UKS calculations even for singlets (why not?...).
 The result may be spin contaminated (from autopsy, it usually is
 when you have a multiconfigurational beast like a polyradical -- you have
 to remember that you cannot start SCF with the same alpha and beta
 charge densities, coefficients, etc., since there would be nothing
 in the SCF method to kick you out from the local solution: F(alpha) = F(beta)).
 The contaminated spin state is by definition a combination or pure parent and
 higher spin states (all of them may be formally multideterminant).
 Hence, the "pure" DFT function (state) derived from the unrestricted
 calculations need not be a single determinant but can be a multideterminant.
 While we do not necessarily know what is the form of these functions (states)
 (though there is a way to find out, since formally the highest possible
 multiplet is when all electrons are unpaired, which will be a single
 determinant, and we can back substitute), we can guess that they are
 multideterminant.
 Hence, the pure spin parent function will be most likely also a combination
 of determinants. And, while we are doing a single determinant DFT, and
 do not count twice the correlation by introducing MCSCF, we actually
 work on the multiconfigurational states.
 And by the way, purists will tell you that KS orbitals are not MOs...
 (I am joking here, I will not get involved in religious wars... I just
 do not have the ammo... I am not a DFT guru, but it is a damn interesting
 problem. I wish I knew this stuff better...).
 Jan
 jkl at.at ccl.net
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