Analytic evaluation of energy gradients for the single, double and linearized triple excitation coupled-cluster CCSDT-1 wavefunction: Theory and applications

• Published in: Chemical physics letters Vol. 146; no. 1; pp. 23 - 31
• Main Authors: Scuseria, Gustavo E., Schaefer, Henry F.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 29.04.1988; Elsevier Science
• Subjects: Atomic and molecular physics; Electron correlation calculations for atoms and molecules; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; Physics; Water; Total energy; Restricted Hartree Fock theory; Theoretical study; Configuration interaction; Contracted gaussian orbital; Inorganic compound; Vibrational transition; Ammonia; Transition energy; Polarization function; Analytical method; Small molecule; Triatomic molecule; Coupled cluster method; Gradient method; Comparative study; Dipole moment; Conformation
• ISSN: 0009-2614; 1873-4448
• DOI: 10.1016/0009-2614(88)85042-5

The analytic energy gradient of the coupled cluster single, double and linearized triple excitation method (CCSDT-1) 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 N7, the same magnitude in computational effort as the evaluation of the CCSDT-1 energy itself. Optimized geometries, dipole moments, vibrational frequencies and infrared intensities predicted for H 2O and NH 3 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 of the connected triple ( T 3) excitations at the linearized level accounts for most of the difference between CCSD and CISDTQ. We conclude that CCSDT-1 is a cost-effective, size-extensive, one-reference method giving results approaching full CI quality in favorable cases.