Subject: CCL:INDO/S with double excitations - SUMMARY

From: <gustavo(at)mercury.chem.pitt.edu>
Date: Sun, 21 Sep 2003 19:34:00 -0400 (EDT)

Dear CCL readers,
     here you have the answers I received for my question
about including double excitations in a INDO/S CI. I Thank
Deepak Singh and Shahar Keinan for answering my question.
      Sincerely yours,
                 Gustavo L.C. Moura
             gustavo(at)mercury.chem.pitt.edu

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Original question:

Dear CCL readers,
      I have been reading papers dealing with the Sum Over
States (SOS) calculation of two-photon absorption cross
sections of organic molecules with the INDO/S methodology.
In these papers the authors include single and double
excitations in the CI.
      As far as I know, the INDO/S methodology was
parameterized for single excitations only. The authors of
these papers don't say why they need to include double
excitations in the calculations. Does someone on this list
know the reason for including these double excitations in
these INDO/S calculations?
      Thank you very much in advance. I will summarize.
      Sincerely yours,
                      Gustavo L.C. Moura
                  gustavo(at)mercury.chem.pitt.edu

Answer #1:

Dear Gustavo,

The INDO/S was parameterized for single excitation (actually, for a window
of CIS 10 10), but the double excitations can be used, with caution. You
have to make sure, by comparing to experimental known data, what is better
to use. An example can be found at:

Optical limiting chromophores. Correlation effects in computing
triplet-triplet absorption energies of organic molecules
Albert IDL, Marks TJ, Ratner MA, Rauh RD
JOURNAL OF PHYSICAL CHEMISTRY A
104 (4): 837-844 FEB 3 2000

Also, if you're using the old ZINDO code, there are some known problems
with using larger window of excitation than CIS 10 10 and CID 5 5 - mainly
memory allocations, so make sure that you code does this without writing on
data and getting gibberish.

Hope it helps,
Shahar

Answer #2:

Gustavo,

It has been some time since I worked in this area, but for many systems,
e.g. polyenes, it is necessary to include double CI, as the low-lying
energy state are forbidden for one-photon transitions.  To get a more
accurate representation of the energy states, it is essential to use
single and double CI.  Please look at the MNDO-PSDCI methods which my
former research group has used to study polyene, and retinal-like
molecules

Stuart, J. A., B. W. Vought, C. F. Zhang, and R. R. Birge. 1995. The
active site of bacteriorhodopsin. Two-photon spectroscopic evidence for a
positively charged chromophore binding site mediated by calcium.
Biospectroscopy. 1:9-28.

Martin, C. H., and R. R. Birge. 1998. Reparameterizing MNDO for excited
state calculations using ab initio effective Hamiltonian theory:
application to the 2,4-pentadien-1-iminium cation. J. Phys. Chem. A.
102:852-860.

Sincerely,

-- Deepak

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Deepak Singh, Ph.D.
Product Specialist, Simulations
Accelrys
9685 Scranton Road
San Diego, CA 92121-3752
Tel: 858-799-5207
Cell: 858-449-8790
Email: dsingh(at)accelrys.com
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