Natural Linear-Scaled Coupled-Cluster Theory with Local Transferable Triple Excitations: Applications to Peptides

Abstract

The natural linear-scaled coupled-cluster (NLSCC) method ( Flocke, N.; Bartlett, R. J. J. Chem. Phys. 2004, 121, 10935 ) is extended to include approximate triple excitations via a coupled-cluster with single, double, and triple excitation method (CCSDT-3). The triples contribution can potentially be embedded in a larger singles and doubles region. NLSCC exploits the extensivity of the CC wave function to represent it in terms of transferable natural localized molecular orbitals (NLMOs) or functional groups thereof that are obtained from small quantum mechanical (QM) regions. Both occupied and virtual NLMOs are local because they derive from the single-particle density matrix. Noncanonical triples amplitudes are avoided by applying the unitary localization matrix to the canonical CC wave function for a QM region. A generalized NLMO code interfaced to the ACES II quantum chemistry software package provides NLMOs for the relevant number of atoms in a given functional group. Applications include linear polyglycine and the pentapeptide met-enkephalin, which was chosen as a more realistic three-dimensional system with nontrivial side chains. The results show that the triples contributions are quite large for aromatic bonds suggesting an interesting active space method for triples in which different bonds require different excitation levels. The NLSCC approach recovers a very large percentage (>99%) of the CCSD or CCSDT-3 correlation energy.

An efficient local coupled cluster method for accurate thermochemistry of large systems

“…26 This is based on tensor factorizations of the MP2 amplitudes. 27 There are also many other methods that exploit the locality of electron correlation, [28][29][30][31][32][33][34][35][36][37][38][39][40] but a review of all these methods is beyond the scope of the current article.…”

Communication: An improved linear scaling perturbative triples correction for the domain based local pair-natural orbital based singles and doubles coupled cluster method [DLPNO-CCSD(T)]

“…7 Many groups have worked on new developments in order to be able to apply the CCSD(T) method to medium-and large-sized systems. [8][9][10][11][12][13][14][15][16][17][18][19][20] The large and fast growing literature about low order or linear scaling CCSD algorithms has been extensively reviewed in earlier publications. [11][12][13] In this communication, we only a) Author to whom correspondence should be addressed: Frank.Neese@ cec.mpg.de discuss the studies that are focused on the (T) correction.…”

“…10 Besides the above mentioned direct local correlation methods, fragment-based local CCSD(T) method development is a very active area of research. [14][15][16][17][18][19][20] Popular approaches include the cluster-in-molecule (CIM), 15 the natural linear-scaled coupled cluster, 16 the divide-expandconsolidate (DEC), 20 the incremental, 14 and the local natural orbital (LNO) 18 methods, all of which have led to low-order or linear scaling CCSD(T) methods. 17,19,[27][28][29] Most recently, Nagy and Kállay combined their LNO approach with LT CCSD(T).…”

Communication: Improved pair approximations in local coupled-cluster methods

“…Pair approximations are also implicit in fragmentation approaches, [27][28][29][30][31][32][33][34][35][36][37] in which (overlapping) groups of orbitals are correlated independently. These methods rely on the assumption that the contribution of amplitudes that are not included in a fragment has a negligible effect on the computed pair energies.…”

“…Furthermore, it is not sufficient to use LMP2 weak pair amplitudes to avoid this error. This should also affect the accuracy of fragmentation approaches [27][28][29][30][31][32][33][34][35][36][37] and implies that the fragments must be very large in order to obtain converged results.…”