doi:10.1016/j.cplett.2004.07.081 
Copyright © 2004 Published by Elsevier B.V.
Natural orbitals as substitutes for optimized orbitals in complete active space wavefunctions
Micah L. Abrams and C. David Sherrill
, 
Center for Computational Molecular Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, GA 30332-0400, USA
Received 24 March 2004;
revised 24 May 2004.
Available online 20 August 2004.
References and further reading may be available for this article. To view references and further reading you must
purchase this article.
Abstract
Complete-active-space self-consistent-field (CASSCF) orbitals are computationally expensive and are sometimes difficult to converge. We assess complete-active-space configuration interaction (CASCI) in a basis of natural orbitals as a less expensive alternative to CASSCF. Natural orbitals are generated from various single-reference wavefunctions. The approach is applied to bond breaking in methyl fluoride and ethylene. With natural orbitals from correlated wavefunctions, CASCI parallels CASSCF potential curves, and coupled-cluster singles and doubles natural orbitals give nonparallelity errors of only 1–3 kcal mol−1 even for a very large active space in methyl fluoride or double bond breaking in ethylene.
Fig. 1. CASCI potential energy curves for CH3F 
CH3 + F using a 6-31G** basis set.
Fig. 2. CASCI error curves versus CASSCF for CH3F CH3 + F using a 6-31G** basis set. The CASCI RHF and UHF–NO error curves are shifted by −70 and −20 kcal mol−1, respectively.
Fig. 3. CASCI potential energy curves for twisted ethylene using a DZP basis set. The minimum energy at each level of theory has been set to zero. The inset contains the points nearest the cusp at 90°.
Fig. 4. CASCI error curves versus CASSCF for twisted ethylene using a DZP basis set. The CASCI RHF and UHF–NO error curves are shifted by −57 and −6.5 kcal mol−1, respectively.
Fig. 5. CASCI potential energy curves for C2H4 2 CH2 using a DZP basis set.
Fig. 6. CASCI error curves versus CASSCF for C2H4 2 CH2 using a DZP basis set. The CASCI RHF error curve is shifted by −50 kcal mol−1.
Table 1.
Maximum, minimum, and nonparallelity errors (kcal mol−1) versus CASSCF for CH3F CH3 + F using the 6-31G** basis set
Values in parentheses indicate the corresponding bond distance (a.u.).
Table 2.
Maximum, minimum, nonparallelity errors (kcal mol−1), and barrier heights (eV) for CASCI versus CASSCF for twisted ethylene using the DZP basis set
Values in parentheses indicate the corresponding torsion angle (°).
Table 3.
Maximum, minimum, nonparallelity errors (kcal mol−1), for CASCI versus CASSCF for C2H4 2 CH2 using the DZP basis set
Values in parentheses indicate the corresponding bond distance (Å).
Abstract
Purchase PDF (330 K)
E-mail Article
Add to my Quick Links
Cited By in Scopus (4)
Purchase PDF (146 K)
