期刊
JOURNAL OF CHEMICAL PHYSICS
卷 128, 期 12, 页码 -出版社
AMER INST PHYSICS
DOI: 10.1063/1.2834927
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资金
- Direct For Mathematical & Physical Scien
- Division Of Chemistry [749868] Funding Source: National Science Foundation
We report the first implementation with correct scaling of the Mukherjee multireference coupled cluster method with singles, doubles, and approximate iterative triples (Mk-MRCCSDT-n, n = 1a, 1b, 2,3) as well as full triples (Mk-MRCCSDT). These methods were applied to the classic H4, P4, BeH2, and H8 model systems to assess the ability of the Mk-MRCCSDT-n schemes to accurately account for triple excitations. In all model systems the inclusion of triples via the various Mk-MRCCSDT-n approaches greatly reduces the nonparallelism error (NPE) and the mean nonparallelism derivative diagnostics for the potential energy curves, recovering between 59% and 73% of the full triples effect on average. The most complete triples approximation, Mk- MRCCSDT-3, exhibits the best average performance, reducing the mean NPE to below 0.6 mE(h), compared to 1.4 mE(h) for Mk-MRCCSD. Both linear and quadratic truncations of the Mk-MRCC triples coupling terms are viable simplifications producing no significant errors. If the off-diagonal parts of the occupied-occupied and virtual-virtual blocks of the Fock matrices are ignored, the storage of the triples amplitudes is no longer required for the Mk-MRCCSDT-n methods introduced here. This proves to be an effective approximation that gives results almost indistinguishable from those derived from full consideration of the Fock matrices. (c) 2008 American Institute of Physics.
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