| 概 要 |
Coupled-cluster (CC) theory has become the de facto standard for high-accuracy molecular calculations, but as all electronic structure approaches that aim at the accurate description of many-electron correlation effects, it faces a number of challenges. Among them are the prohibitive costs of CC calculations for larger molecular systems.
To address this challenge, we have recently extended a number of CC methods, including the size-extensive, left-eigenstate, completely renormalized CC method with singles, doubles, and non-iterative triples, abbreviated as CR-CC(2,3), which is known to provide an accurate description of chemical reaction profiles involving single bond breaking and biradicals, to larger systems with dozens or hundreds of atoms through the use of the local correlation, cluster-in-molecule (CIM) ansatz. The resulting CIM-CR-CC(2,3) and other CIM-CC methods are characterized by (i) the linear scaling of the CPU time with the system size when the same level of theory is applied to all CIM subsystems, (ii) the use of orthonormal orbitals in subsystem calculations, (iii) the natural coarse-grain parallelism, which can be further enhanced by the additional fine-grain parallelism of each subsystem calculation, (iv) high computational efficiency and relatively low memory requirements, enabling calculations for large molecular systems at higher levels of CC theory, (v) the purely non-iterative character of the local triples (CR-CC(2,3), CCSD(T), etc.) and other perturbative (e.g., MPn) corrections to the correlation energy, and (vi) the applicability to the covalently and weakly bound molecular systems. In addition, one can use the flexi-bility of the CIM local correlation ansatz to mix different CC or CC and non-CC methods within a single calculation, enabling the rigorous formulation of multi-level local cor-relation theories that combine higher CC levels, such as CR-CC(2,3), to treat, for example, the reactive part of the large molecular system with the lower-order local or (if affordable) canonical ab initio approaches (e.g., MP2 or CCSD) to handle chemically inactive regions without splitting it into ad hoc fragments and saturating dangling bonds. The performance of the CIM-CR-CC(2,3) methodology and its multi-level extensions in applications involving chemical reaction profiles will be illustrated by examining bond dissociation curves in normal alkanes and alkyl radicals, diffusion of atomic oxygen on the silicon surface, proton transfer in the aggregates of dithiophosphinic acids with the water mole-cules, and cobalt-methyl bond dissociation in the methylcobalamin cofactor, which is one of the biologically active forms of vitamin B12, where the sophisticated multi-reference ab initio theories and the widely used hybrid density functional theory (DFT) approaches, particularly those with a significant fraction of the Hartree-Fock exchange, and the DFT methods without dispersion corrections face serious challenges.
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