Analytic evaluation of energy gradients for the single, double and linearized triple excitation coupled-cluster CCSDT-1 wavefunction: Theory and applications
✍ Scribed by Gustavo E. Scuseria; Henry F. Schaefer III
- Publisher
- Elsevier Science
- Year
- 1988
- Tongue
- English
- Weight
- 677 KB
- Volume
- 146
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
The analytic energy gradient of the coupled cluster single, double and linearized triple excitation method (CCSDT-I ) is formulated and computationally implemented. Explicit expressions are given for the closed-shell restricted Hartree-Fock reference case. The analytic CCSDT-1 gradient method scales as N ', the same magnitude in computational effort as the evaluation of the CCSDT-I energy itself. Optimized geometries, dipole moments, vibrational frequencies and infrared intensities predicted for Hz0 and NH3 are compared to corresponding configuration interaction including all single, double, triple, and quadruple excitations (CISDTQ) values. For these two molecules, results obtained show that the inclusion ofthe connected triple (r,) excitations at the linearized level accounts for most of the difference between CCSD and CISDTQ. We conclude that CCSDT-1 is a costeffective, size-extensive, one-reference method giving results approaching full Cl quality in favorable cases.
' Contribution CCQC No. 14.