CI-Singles Summary

 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
 Dear Demetrio!
 The Gaussian keyword CIS implies a CI-Singles calculation with inclusion of
 "all" singly excited configurations (however, core levels are usually
 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 >--<
 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).
 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:grimmes %! at !%
 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.
 Ast. Pr. Xavier Assfeld             Xavier.Assfeld %! at !%
 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      
 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.
 T. Daniel Crawford                           Department of Chemistry
 crawdad %! at !%                          Virginia Polytechnic Institute
 and              State University
                 Demetrio A. da Silva Filho
                    UNICAMP - IFGW
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