Subject: Instability

From: "Don H. Phillips" <[email-masked]>
Date: Thu, 6 Apr 95 09:08:46 -0400

In reply to <[email-masked]>
Gavin:

>...... We found that there is a
> triplet state that is lower in energy than the singlet at HF levels of
> theory, but singlet is lower for MP2 and DFT levels of theory.

I suspect that your "HF level theory" singlet is closed shell and that
the true ground state is an open shell singlet.  My guess is that the
orbitals involved are (essentially) p orbitals on the terminal oxygen
and the nitrogen, but I haven't done any work on this compound so that's
only a guess. 

Possible approaches to exploring this are:

(1) Carry out an open shell singlet HF calculation.  Use singles and doubles
CI from the open shell reference to improve the description (or open
shell MP4 if available).  Note that, depending upon the relative
importance of the single reference function, this may not yield an
accurate result but it will be improved over what you have now.
An MCSCF treatment to get orbitals and a multiple reference CI or
equivalent coupled cluster calculation would be required for accuracy
if the state isn't described well by a single reference function.

(2) An alternative approach would be to obtain natural orbitals for
the true ground state and base a CI treatment on these orbitals.  The
same caveats as above apply with regard to the suitability of a single
function reference.  The natural orbitals from a UHF treatment of the
lowest triplet state might be a good starting point.  INDO may not provide
a qualitatively correct description of the lowest states.  I would recommend
sticking with ab-initio treatments throughout.

I don't have any experience with Gaussian, which I assume you are using,
or with DFT treatments of open shell singlets, so I am of no help to you
there.  The GAMESS and COLUMBUS packages offer alternatives.  The latter
has a lot of flexibility, but was difficult to use when I last looked.
A recent posting to CCL indicated that there is a more user-friendly
version available somewhere on the net.

It sounds like you have discovered an "interesting" system.  Have fun.

Good luck,
Don