Comparison of perturbatively corrected energy results from multiple reference double‐excitation configuration‐interaction method calculations with exact full configuration‐interaction benchmark values

1990 ◽  
Vol 92 (1) ◽  
pp. 585-596 ◽  
Author(s):  
David B. Knowles ◽  
José Ramon Alvarez‐Collado ◽  
Gerhard Hirsch ◽  
Robert J. Buenker
Keyword(s):  
Configuration Interaction ◽  
Interaction Method ◽  
Configuration Interaction Method ◽  
Full Configuration Interaction ◽  
Double Excitation ◽  
Multiple Reference

Composite natural orbitals for configuration interaction method

1988 ◽  
Vol 53 (10) ◽  
pp. 2266-2278 ◽  
Author(s):  
Yuichi Yamamoto ◽  
Takeshi Noro ◽  
Kimio Ohno
Keyword(s):  
Configuration Interaction ◽  
Ground States ◽  
New Method ◽  
Entire System ◽  
Interaction Method ◽  
Configuration Interaction Method ◽  
Excitation Energies ◽  
Natural Orbitals ◽  
Double Excitation ◽  
Ci Calculations

A new method of constructing approximate natural orbitals (NO's) is proposed. A system can usually be regarded as consisting of fragments. The approximate NO's of the entire system are constructed from the NO's of fragments in their ground states. These orbitals are expected to be useful in configuration interaction (CI) calculations of a large system where orbital truncation is necessary. The usefulness of these approximate NO's, which are called composite NO's (CNO's), is demonstrated on C4H6. This molecule may be regarded as a combination of two fragments i.e., C2H4's. Excitation energies calculated by single and double excitation CI with the truncated CNO's (14σ, 7π correlating orbitals) agree reasonably well with those calculated with a full orbital set (37σ, 12π correlating orbitals), while the truncated SCFMO's of the same size give disastrous excitation energies.

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