Coupled-cluster singles, doubles, and triples calculations with Hartree-Fock and Brueckner orbital reference determinants: A comparative study

Conference · Sat Dec 31 04:00:00 EST 1994

OSTI ID:107043

Watts, J D; Bartlett, R J ^[1]

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1. Univ. of Florida, Gainesville, FL (United States)

A series of complete coupled-cluster singles, doubles and triples (CCSDT) calculations have been performed with Hartree-Fock (HF) and Bruechner (B) orbitals. Calculations have been performed with a double-zeta plus polarization basis set on the H{sub 2}O, SiH{sub 2}, NH{sub 2}, BeO, C{sub 2}, CN{sup +}, and BN molecules. Calculations on H{sub 2}O and SiH{sub 2} at equilibrium and stretched geometries show negligible difference between HF- and B-CCSDT energies. This is also true for NH{sub 2}, except when the bonds have been stretched to twice their equilibrium values, at which point there is about a 2.5 milli-Hartree (mE{sub h}) difference. Calculations on the isoelectronic systems BeO, C{sub 2}, CN{sup +}, and Bn were performed at equilibrium geometries. Even though these systems have large T{sub 1} amplitudes, the difference between HF-and B-CCSDT energies is only about 1 mE{sub 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{sub e} and w{sub e}, although the difference for {mu} is unaffected.

• OSTI ID: 107043
• Report Number(s): CONF-9402143--; CNN: Grant F49620-93-1-0127
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
66 PHYSICS
AMIDES
BERYLLIUM OXIDES
BORON NITRIDES
CARBON COMPOUNDS
CARBON NITRIDES
CATIONS
ELECTRONIC STRUCTURE
HARTREE-FOCK METHOD
MATHEMATICAL MODELS
QUANTUM MECHANICS
SILANES
WATER