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Coupled‐Cluster singles, doubles, and triples calculations with hartree–fock and brueckner orbital reference determinants: A comparative study
Author(s) -
Watts John D.,
Bartlett Rodney J.
Publication year - 1994
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.560520820
Subject(s) - coupled cluster , atomic orbital , basis set , chemistry , atomic physics , hartree–fock method , amplitude , formalism (music) , physics , molecule , computational chemistry , quantum mechanics , density functional theory , electron , musical , visual arts , art
A series of complete coupled‐cluster singles, doubles, and triples ( CCSDT ) calculations have been performed with Hartree‐Fock ( HF ) and Brueckner ( B ) orbitals. Calculations have been performed with a double‐zeta plus polarization basis set on the H 2 O, SiH 2 , NH 2 , BeO, C 2 , CN + , and BN molecules. Calculations on H 2 O and SiH 2 at equilibrium and stretched geometries show negligible difference between HF ‐and B‐CCSDT energies. This is also true for NH 2 , except when the bonds have been stretched to twice their equilibrium values, at which point there is about a 2.5 milli‐Hartree (m E h ) difference. Calculations on the isoelectronic systems BeO, C 2 , CN + , and BN were performed at equilibrium geometries. Even though these systems have large T 1 amplitudes, the difference between HF ‐ and B‐ CCSDT energies is only about 1 m E h . For the CCSD method and the CCSD(T) method, which includes triple excitations in an approximate, noniterative manner, however, somewhat larger differences are observed between and HF ‐and B‐CC results. Finally, some properties of BN were computed using HF ‐ and B‐CC methods. There are quite small differences between the HF ‐ and B‐CCSDT results, but significantly larger ones for the more approximate CCSD and CCSD(T) methods. For this difficult system, where the CCSD(T) approximation seems to be inadequate for HF orbitals, the use of Brueckner orbitals improves the agreement of CCSD(T) with CCSD(T) substantially for r e and w e , although the difference for μ is unaffected. © 1994 John Wiley & Sons, Inc.