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Re: How should one treat ET between singlet and doublet species?

Pedro Silva

On Sun Jul 5 '15 7:14pm, olgerdovich wrote

>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?

RHF for a singlet normally yields the same result as a UHF (except in pathological cases).
ROHF does not suffer from spin-contaminaiton problemas, but it cannot (unlike UHF) describe situations where excess alpha and beta electrons are in spatially distinct regions of the molecule (e.g. the situtation observed in cytochrome P450 compound I)

>Is ROHF for singlet species exactly the same as RHF?

I believe so.

I usually perform 4 UHF computations in this instance:
x1) A-
x2) B
x3) A
x4) B-

x4+x3 - ( x2+x1) then affords the reaction energy

If you then perform 4 extra UHF computations:

x5) A- at the optimized geometry of A
x6) A at the optimized geometry of A-
x7) B at the optimized geometry of B-
x8) B- at the optimized geometry of B

you may obtain the reorganization energies of each species:

x5-x1 is the reorganization energy A-
x6-x3 is the reorganization energy A
x7-x2 is the reorganization energy B
x8-x4 is the reorganization energy B-

These reorganization energies may be used to draw Marcus parabolas and to obtain an estimate of the activation energy of the electron-transfer process
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