Subject: Re: CCL:CIS np* minimization

From: Wibke Sudholt <[email-masked]>
Date: Fri, 6 Sep 2002 11:54:24 -0700

Hi,

this sounds like the usual "root flipping" problem to me. If you are not
very lucky (meaning the exited state minima are quite separated
electronically and geometrically), you will only be able to optimize the
energetically lowest excited state in each given symmetry. This is due to
the fact that the system relaxes simultaneously electronically and
geometrically, finally mostly leading to the roots "flipping" (meaning the
state characters interchange) so that the overall lowest excited state is
represented (states of one symmetry mix when they are close). So if you
have Cs symmetry and you are only interested in states of one
multiplicity, you will only be able to optimize the 2A' and 1A'' states
(when 1A' is the ground state). When you have no (C1) symmetry, however,
you can usually only optimize the 2A excited state (1A is the ground
state). In my experience, this holds for CIS as well as MCSCF, although
their respective "overall lowest" excited state may be a different one due
to the influence of electron correlation.

I hope this helps,

Wibke Sudholt
University of California, San Diego
[email-masked]

On Fri, 6 Sep 2002, DanMajor wrote:

> Hi,
> I'm minimizing the geometry of np* excited states of heterocycles using
> CIS in Gaussian98 (I'm not using MCSCF due to the size of
> the molecules investigated).
> In Cs symmetry the geometry minimization of these states converge
> nicely, but they are not local minima.
> When I remove symmetry restrictions the nature of the CIS root changes
> to pp* for most molecules.
> I've tried to restart the minimization from the previous steps with the
> new geometry and correct root, but this doesn't always help. I've
> also tried adding the normal mode (corresponding to the imaginary freq.)
>
> of the symmetric np* excited states as a perturbation to the
> initial structure and this does help in some cases, but not all.
> Does anyone have any other ideas how to minimize excited np* states ?
> Why does the excited states minimization of np* states often
> collapse to pp* states ?
>
> Sincerely,
> Dan
>
>
>
>
>
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