Re: [cclib-devel] etsecs conversions

[Noel O'B. <bao...@gm...>]

This code may be old and creaky, but the general idea was as follows:

(1) You multiply by two because for an excitation from a
doubly-occupied orbital there are two excitations in fact...one
involving the alpha electron and one involving the beta electron. For
an unrestricted calculation, you shouldnt multiple by two.

(2) You square because that is the definition of etsecs. It is a
somewhat arbitary definition so I am happy to change it. The idea is
that these sqr values somehow represent the fractional contribution of
a particular SEC to the whole transition. Unfortunately, at least in
TD-DFT, this is almost but not exactly true (i.e. these fractional
contributions dont add to 1). In fact, this is probably a good unit
test to have to make sure that all parsers are doing the same thing.

Noel

On 27/03/07, Karol Langner <kar...@kn...> wrote:
> Hi,
>
> The fragment  of the Gaussian parser that reads etsecs:
>
>                   percent = self.float(t[1])
>                   sqr = percent**2*2 # The fractional contribution of this CI
>                   if percent < 0:
>                       sqr = -sqr
>
>  Why are the excitation coefficients squared and multiplied by 2?
>
> Karol
>
> --
> written by Karol Langner
> Tue Mar 27 10:52:43 CEST 2007
>
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