doi:10.1016/j.cplett.2005.06.107 
Copyright © 2005 Published by Elsevier B.V.
Important configurations in configuration interaction and coupled-cluster wave functions
Micah L. Abrams
, 
and C. David Sherrill
Center for Computational Molecular Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, 770 State Street, Atlanta, GA 30332-0400, United States
Received 5 February 2005.
Available online 18 July 2005.
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Abstract
Using a new general-order determinant-based program, we construct compact configuration interaction and coupled-cluster wave functions by selecting the most important configurations, by weight, from a full configuration interaction or full coupled-cluster wave function. Our results show that for the symmetric dissociation of water, chemical accuracy can be achieved across the surface with 
2% of the full coupled-cluster expansion compared to 10% of the full configuration interaction expansion.
Table 1.
Truncated CI total energy, error from FCI, and non-parallelity error (in hartree, kcal/mol in parentheses) using one of three thresholds for three O–H bond distances (in Å) corresponding to the symmetric dissociation of H2O
The FCI energy is −76.121174 at Re, −75.876474 at 2Re, and −75.836657 at 3Re.
Table 2.
Truncated CC total energy, error from FCI, and non-parallelity error (in hartree, kcal/mol in parentheses) using one of three thresholds for three O–H bond distances (in Å) corresponding to the symmetric dissociation of H2O
The FCI energy is −76.121174 at Re, −75.876474 at 2Re, and −75.836657 at 3Re.
Table 3.
Number of configurations according to excitation level for the truncated CI wave function using one of three thresholds for three O–H bond distances (in Å) corresponding to the symmetric dissociation of H2O
Also given are the total number of configurations and the percentage of the full configuration space. Total number of configurations in the full wave function according to excitation level: 24, 384, 2792, 10550, 19936, 18680, 7840, 1234; for a total of 61440.
Table 4.
Number of configurations according to excitation level for the truncated CC wave function using one of three thresholds for three O–H bond distances (in Å) corresponding to the symmetric dissociation of H2O
Also given are the total number of configurations and the percentage of the full configuration space. Total number of configurations in the full wave function according to excitation level: 24, 384, 2792, 10550, 19936, 18680, 7840, 1234; for a total of 61440.
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