Subject: computation of HOMO-LUMO separation

From: "Kiniu WONG Kin-Yiu" <[email-masked]>
Date: Wed, 11 Oct 2000 16:10:43 +0800

Dear all,

    I know there is a CIS option Gaussian to give the excitation energy,
however, this excitation energy involves configuration interactions (CI)
which would destroy the picture of a HOMO-LUMO gap (since in the CI, there
is no more orbitals could be defined).  So, I just want to know the energy
of the dominant (largest coefficient) Slater determinants of CI. Then we
treat it as the HOMO-LUMO gap. But from the Gaussian output, I only see the
coefficient and can't find the energy corresponds to this determinant, for
example:

 CIS wavefunction symmetry could not be determined.
 Eigenvector   1:   ?Spin  -?Sym  Eigenvalue= 0.0000000
      6A ->  8A        -.46865
      6A -> 11A        -.51043
      6A -> 15A        -.10310
      6B ->  8B         .46894
      6B -> 12B         .51278
      6B -> 16B         .10241

So the first question I would want to ask is: How to know the energy
corresponds to each determinant?

    There is a follow-up question.  HF ground energy is just the energy of a
self-consistent Slater determinant involving all occupied (lowest energy)
orbitals.  The second question is: Could Gaussian calculate the energy of a
self-consistent Slater determinant in which some occupied orbitals change to
some virtual orbital?  I know we could change the multiplicity to force one
of electrons to fill a virtual orbital, but could Gaussian do it
arbitrarily?  For example, just see again the above output, could directly
ask Gaussian calculate the energy of a self-consistent Slater determinant in
which 6A changes to 11A?

    Since I am a beginner to Gaussian, any advices are helpful. For example,
is there other softwares could do so? Or are there any literature about
computation of HOMO-LUMO separation?  Thanks in advance!

Regards,
Kiniu

----
Kiniu WONG Kin-Yiu
Undergraduate student
Department of Physics
The Chinese University of Hong Kong
Email: [email-masked]  or  [email-masked]