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How should one treat ET between singlet and doublet species?


Dear collegues,

please, help me understand the problem in the title.

Let's consider an electron transfer between an anion-radical A(.-) and a neutral singlet molecule B as the following equilibrium:

A(.-) + B = A + B(.-)

To calculate relative electron affinity of two neutral species A and B, one should found the energy of the above process. However, methods for calculation of the wave function differs principally for singlet and doublet species except for UHF, which has it own drawbacks such as spin contamination and slow convergence. So, which methods should one use (i.e. which method for geom optimization and which for energy calculation) to treat the above equilibrium in a reasonable way ?

Should one uniformly use UHF or ROHF for both singlet and doublet species?
May one use RHF for singlet species and ROHF or UHF for doublet?
Is ROHF for singlet species exactly the same as RHF?

Thanks in advance for your kind attention

Vladimir Smirnov

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