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Triple excitations in coupled‐cluster theory: Energies and analytical derivatives
Author(s) -
Watts John D.,
Bartlett Rodney J.
Publication year - 1993
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560480809
Subject(s) - coupled cluster , open shell , excitation , dipole , basis (linear algebra) , physics , cluster (spacecraft) , quantum mechanics , computer science , molecule , mathematics , geometry , programming language
Abstract A review is presented of our recent work which deals with some aspects of the inclusion of connected triple excitations in single reference coupled–cluster theory. We first consider basic aspects of the theory and computational cost, noting the need for accurate and economical approximations to the complete coupled–cluster single, double and triple excitation method ( CCSDT ). Several such approximations are now available, including the CCSD + T ( CCSD ) and CCSD ( T ) methods. We have now efficiently implemented analytical derivative methods for these and related methods for open‐shell reference determinants, including restricted open‐shell Hartree–Fock for CCSD ( T ). Some results with large basis sets are presented for the HCO radical which further demonstrate the reliability of these methods for the spectroscopic characterization of small open‐shell species. The remainder of this article deals with several applications of the complete CCSDT method, a new implementation of which we have recently completed. Like our previous work, this is for general single reference determinants, although we have now incorporated Abelian symmetry. We present a comparison with full configuration interaction data for CH 3 ; a study of the N 2 potential curve using restricted and unrestricted Hartree–Fock references; a comparison of the dipole moments and polarizabilities of BeO; and a study of the harmonic force field of 2 Σ u +C 3 + . These studies also provide assessments of the accuracy of the approximate schemes for including T 3 . Finally, we present some timing data for some CCSDT calculations with out new implementation. © 1993 John Wiley & Sons, Inc.