Subject: CI-Singles Summary

From: Demetrio Antonio da Silva Filho <[email-masked]>
Date: Wed, 19 Jul 2000 09:21:55 -0300

Original post:

Dear Netters,

I'm in doubt about the CI-Singles method and the number of  single 
substitutions that this method
uses on its calculation.

In Pople's paper (J. B. Foresman, M. Head-Gordon, J. A. Pople and M. J. 
Frisch, Toward a Systematic Molecular Orbital Theory for Excited States, J. 
Phys. Chem. 96, 135 (1992).) the CI-Singles method is
defined as configuration interaction among "all" singly substituted 
determinants. In Gaussian manual,
there is a keyword for this method (NStates) which sets the number of 
states that will be solved.

This keyword (Mstates) sets the number of states used in the CI expansion 
of the wavefunction or
independent of the number used in this keyword all singly substituted 
determinants will
be used?

Thanks in advance,
Demetrio FIlho

Replies!

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Dear Demetrio!

The Gaussian keyword CIS implies a CI-Singles calculation with inclusion of
"all" singly excited configurations (however, core levels are usually not
included in the CI).

The NStates option specifies how many excited states should be calculated; the
default value is 3.  The value of NStates has no effect on the number of
configurations included in the CI.

Yours, Jens >--<

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Dear Antonio,
This keyword (Mstates) sets the number of states to solve for
and this does not affect the number of determinants (precisely: CSF) in
the CI expansion (which is Nocc x Nvirt of course).
Stefan
_________________________________________________________
Prof. Dr. Stefan Grimme
Organisch-Chemisches Institut (Abt. Theoretische Chemie)
Westfaelische Wilhelms-Universitaet, Corrensstrasse 40
D-48149 Muenster, Tel (+49)-251-83 36512/33241/36515(Fax)
Email:[email-masked]
http://www.uni-muenster.de/Chemie/OC/research/grimme/
_________________________________________________________

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Hello,

I think that Gaussian does not diagonalize the complete CIS matrix
to get the eigenvectors (and eigenvalues), but rather solve the
problem iteratively for each eigenvector. Then the CIS matrix contains
all singly excited determiants, but Gaussian gives you only the first
"Nstates" eigenvectors.
I don't know if I am right, but that the way I understand it.
Hope this helps.

                                       ...Xav

Ast. Pr. Xavier Assfeld             [email-masked]
Laboratoire de Chimie theorique     (T) 33 3 83 91 21 49
Universite Henri Poincare           (F) 33 3 83 91 25 30
F-54506 Nancy BP 239                http://www.lctn.u-nancy.fr

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 Dear Demetrio,
	The Nstates keyword in gaussian determines the number of excited states
whose energies and other properties are solved for using CIS.  It doesn't
affect the number of single excitations used in the CI.
Best regards,
Sam Abrash

Dear Demetrio,

	The CIS method is indeed configuration interaction among all singly
substituted determinants.  That is, the electronic Hamiltonian is
represented in a basis consisting of only determinants which are single
orbital-substitutions away from a reference (usually the Hartree-Fock
determinant).  Once you have this CIS Hamiltonian, you can extract its
eigenvalues, which represent energies of electronically excited states.
The keyword "NStates" denotes the number of eigenvalues of the CIS
Hamiltonian you wish to obtain, not the number of singly excited determinants
used for the representation of H.

-Daniel

--
T. Daniel Crawford                           Department of Chemistry
[email-masked]                          Virginia Polytechnic Institute and
www.chem.vt.edu/chem-dept/crawford              State University

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_____________________________________

                 Demetrio A. da Silva Filho
                    UNICAMP - IFGW
                     Prédio D - Sala 17
             CEP 13083-970 C.Postal 6165
                    Campinas - SP - Brasil
_____________________________________
"Se não houver frutos, valeu a beleza das flores. Se não houver flores,
valeu a sombra das folhas. Se não houver folhas, valeu a intenção da
semente." Henfil