Abstract
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The coupled-cluster (CC) singles and doubles with perturbative
triples [CCSD(T)] method is frequently referred to as the “gold
standard” of modern computational chemistry. However, the high
computational cost of CCSD(T) [O(N7)], where N is the number of basis functions,
limits its applications to small-sized chemical systems. To address
this problem, efficient implementations of linear-scaling coupled-cluster
methods, which employ the systematic molecular fragmentation (SMF)
approach, are reported. In this study, we aim to do the following:
(1) To achieve exact linear scaling and to obtain a pure ab
initio approach, we revise the handling of nonbonded interactions
in the SMF approach, denoted by LSSMF. (2) A new fragmentation algorithm,
which yields smaller-sized fragments, that better fits high-level
CC methods is introduced. (3) A modified nonbonded fragmentation scheme
is proposed to enhance the existent algorithm. Performances of the
LSSMF-CC approaches, such as LSSMF-CCSD(T), are compared with their
canonical versions for a set of alkane molecules, CnH2n+2 (n = 6–10),
which includes 142 molecules. Our results demonstrate that the LSSMF
approach introduces negligible errors compared with the canonical
methods; mean absolute errors (MAEs) are between 0.20 and 0.59 kcal
mol–1 for LSSMF(3,1)-CCSD(T). For a larger alkanes
set (L12), CnH2n+2 (n = 50–70), the performance of
LSSMF for the second-order perturbation theory (MP2) is investigated.
For the L12 set, various bonded and nonbonded levels are considered.
Our results demonstrate that the combination of bonded level 6 with
nonbonded level 2, LSSMF(6,2), provides very accurate results for
the MP2 method with a MAE value of 0.32 kcal mol–1. The LSSMF(6,2) approach yields more than a 26-fold reduction in
errors compared with LSSMF(3,1). Hence, we obtain substantial improvements
over the original SMF approach. To illustrate the efficiency and applicability
of the LSSMF-CCSD(T) approach, we consider an alkane molecule with
10,004 atoms. For this molecule, the LSSMF(3,1)-CCSD(T)/cc-pVTZ energy
computation, on a Linux cluster with 100 nodes, 4 cores, and 5 GB
of memory provided to each node, is performed just in ∼24 h.
As a second test, we consider a biomolecular complex (PDB code: 1GLA), which includes
10,488 atoms, to assess the efficiency of the LSSMF approach. The
LSSMF(3,1)-FNO–CCSD(T)/cc-pVTZ energy computation is completed
in ∼7 days for the biomolecular complex. Hence, our results
demonstrate that the LSSMF-CC approaches are very efficient. Overall,
we conclude the following: (1) The LSSMF(m, n)-CCSD(T) methods can be reliably used for large-scale
chemical systems, where the canonical methods are not computationally
affordable. (2) The accuracy of bonded level 3 is not satisfactory
for large chemical systems. (3) For high-accuracy studies, bonded
level 5 (or higher) and nonbonded level 2 should be employed.
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