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Coupled cluster
About: Coupled cluster is a research topic. Over the lifetime, 6280 publications have been published within this topic receiving 301055 citations.
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TL;DR: In this article, structural, vibrational frequencies, atomization energies at 0 K, and heats of formation at 298 K were obtained for four oxyfluoride molecules, several of which are known to present difficulties for single reference ab initio methods.
Abstract: Structures, vibrational frequencies, atomization energies at 0 K, and heats of formation at 298 K were obtained for four oxyfluoride molecules, several of which are known to present difficulties for single reference ab initio methods. Whereas much of this work was carried out with coupled cluster theory, multireference configuration interaction calculations were also performed, as an independent check on the reliability of the former. The use of large basis sets (up through augmented sextuple zeta quality in some cases) and a simple basis set extrapolation formula enabled us to accurately estimate the complete basis set limit. However, to achieve near chemical accuracy (±1 kcal/mol) in the thermodynamic properties, it was necessary to include three corrections to the frozen core atomization energies, in addition to the zero-point vibrational energy: (1) a core/valence correction; (2) a Douglas−Kroll−Hess scalar relativistic correction; and (3) a first-order atomic spin−orbit correction. Several approache...
81 citations
TL;DR: The results demonstrate that the ansatz significantly enhances the basis set convergence of the noniterative triple excitation correction and thus improves upon previous approaches to explicitly correlated CCSD(T).
Abstract: A way to incorporate explicit electron correlation into connected triple excitations in coupled-cluster theory is proposed. The new ansatz is applied to the coupled-cluster singles and doubles model with noniterative triple excitations [CCSD(T)] and does not introduce any further sets of equations to be solved. A first implementation using automated generation and string-based evaluation of the explicit expressions is reported. The results demonstrate that the ansatz significantly enhances the basis set convergence of the noniterative triple excitation correction and thus improves upon previous approaches to explicitly correlated CCSD(T).
81 citations
TL;DR: In this paper, the triples correction is calculated from a reduced space of virtual orbitals, while the full space is employed for the coupled-cluster singles-and-doubles model.
Abstract: Multiple basis sets are used in calculations of perturbational corrections for triples replacements in the framework of single-reference coupled-cluster theory. We investigate a computational procedure, where the triples correction is calculated from a reduced space of virtual orbitals, while the full space is employed for the coupled-cluster singles-and-doubles model. The reduced space is either constructed from a prescribed unitary transformation of the virtual orbitals (for example into natural orbitals) with subsequent truncation, or from a reduced set of atomic basis functions. After the selection of a reduced space of virtual orbitals, the singles and doubles amplitudes obtained from a calculation in the full space are projected onto the reduced space, the remaining set of virtual orbitals is brought into canonical form by diagonalizing the representation of the Fock operator in the reduced space, and the triples corrections are evaluated as usual. The case studies include the determination of the spectroscopic constants of N2, F2, and CO, the geometry of O3, the electric dipole moment of CO, the static dipole polarizability of F−, and the Ne⋯Ne interatomic potential.
81 citations
TL;DR: In this article, the transition state for cyclo-O4 dissociation to two O2 molecules has been located at a number of levels of theory using a double zeta plus polarization (DZP) basis set.
Abstract: The cyclo‐O4 molecule is isoelectronic with cyclobutane and has been mentioned as a potential new high energy density material (HEDM). The important unresolved question has been whether or not cyclo‐O4 is a genuine minimum on the O4 potential energy hypersurface. Here the transition state for cyclo‐O4 dissociation to two O2 molecules has been located at a number of levels of theory using a double zeta plus polarization (DZP) basis set. The theoretical methods with which the transition state geometry was optimized include two‐configuration self‐consistent‐field (TCSCF), configuration interaction including all single and double excitations (CISD), coupled cluster including all single and double excitations (CCSD), and the latter with a noniterative correction for connected triple excitations, CCSD(T). The equilibrium geometry of O4 has D2d symmetry, while a stationary point of D2 symmetry is of Hessian index two (i.e., two imaginary vibrational frequencies) at the highest level of theory, DZP CCSD(T). The t...
81 citations
TL;DR: In this paper, an equation-of-motion coupled cluster method was used to describe the electronic excitation spectra of the model carbonyl compounds, formaldehyde, acetaldehyde, and acetone.
80 citations