Coupled cluster

About: Coupled cluster is a research topic. Over the lifetime, 6280 publications have been published within this topic receiving 301055 citations.

Theory of Hypervalency: Recoupled Pair Bonding in SFn(n= 1-6)

Abstract: To gain new insight into the nature of hypervalency, we have characterized the bonding across the entire SFn sequence (n = 1−6) with high-level quantum chemical theory (multireference configuration interaction and coupled cluster calculations using correlation consistent basis sets). In contrast to most previous studies, this work examined both the stable equilibrium structures and the process of SFn−F bond formation. We conclude that two different types of bonding can occur in these species: normal polar covalent bonding and a new type that we call recoupled pair bonding. The two bonding processes can be seen in diatomic SF, where hypervalent behavior first occurs. In the covalently bonded 2Π ground state, the bond is formed by straightforward singlet coupling of electrons in the singly occupied S 3p and F 2p orbitals. But there is also a low-lying 4Σ− excited state where the S 3p2 pair of electrons must first be decoupled so that one of the electrons can singlet couple with the electron in the F 2p orbi...

Third-order Douglas–Kroll relativistic coupled-cluster theory through connected single, double, triple, and quadruple substitutions: Applications to diatomic and triatomic hydrides

Abstract: Coupled-cluster methods including through and up to the connected single, double, triple, and quadruple substitutions have been derived and implemented automatically for sequential and parallel executions by an algebraic and symbolic manipulation program TCE (TENSOR CONTRACTION ENGINE) for use in conjunction with a one-component third-order Douglas–Kroll approximation for relativistic corrections. A combination of the converging electron-correlation methods, the accurate relativistic reference wave functions, and the use of systematic basis sets tailored to the relativistic approximation has been shown to predict the experimental singlet–triplet separations within 0.02 eV (0.5 kcal/mol) for five triatomic hydrides (CH2, NH2+, SiH2, PH2+, and AsH2+), the experimental bond lengths (re or r0) within 0.002 A, rotational constants (Be or B0) within 0.02 cm−1, vibration–rotation constants (αe) within 0.01 cm−1, centrifugal distortion constants (De) within 2%, harmonic vibration frequencies (ωe) within 8 cm−1 (0...

Effect of Electron Correlation on Intermolecular Interactions: A Pair Natural Orbitals Coupled Cluster Based Local Energy Decomposition Study.

Abstract: The development of post-Hartree–Fock (post-HF) energy decomposition schemes that are able to decompose the HF and correlation components of the interaction energy into chemically meaningful contributions is a very active field of research. One of the challenges is to provide a clear-cut quantification to the elusive London dispersion component of the intermolecular interaction. London dispersion is well-known to be a pure correlation effect, and as such it is not properly described by mean field theories. In this context, we have recently developed the local energy decomposition (LED) analysis, which provides a chemically meaningful decomposition of the interaction energy between two or more fragments computed at the domain-based local pair natural orbitals coupled cluster (DLPNO-CCSD(T)) level of theory. In this work, this scheme is used in conjunction with other interpretation tools to study a series of molecular adducts held together by intermolecular interactions of different natures. The HF and corre...

Full configuration interaction benchmark calculations of first-order one-electron properties of BH and HF

Abstract: Full configuration interaction benchmark calculations have been carried out for the electric dipole and quadrupole moments and the electric field gradient at the nuclei of BH and HF. The accuracy of perturbation theory from second to fourth order and coupled cluster theory with up to triple excitations has been investigated. For all the properties the coupled cluster models outperform the perturbation series. The convergence of the series of coupled cluster models is significantly faster and more systematic than the one of the perturbation series, and only the coupled cluster series defines a hierarchy of models with well defined levels of accuracy. The CCSD(T) model is a good approximation to the full coupled cluster singles, doubles, and triples model. It recovers 80%–90% of the full effect of triple excitations, and the small error due to the approximate description of triple excitations is comparable in size to the error due to neglect of higher-order excitations. For accurate calculations, the CCSD(T...

Energy versus amplitude corrected coupled-cluster approaches. II. Breaking the triple bond

Abstract: We examine the effectiveness of various energy corrections to the standard CCSD and to the reduced multireference (RMR) CCSD methods. These corrections are based on the asymmetric energy formula, but instead of projecting onto the reference configuration, as in the standard CCSD method, we employ for this purpose either the MR CISD wave function that is based on a suitable model space of the kind used in RMR CCSD, or simply the zero-order wave function in that model space. Both full complete-active-space and severely-truncated model spaces are employed. The method is applied to the prototypical case of the triple-bond dissociation, namely, to the exactly solvable double-zeta model of the N2 molecule. It is shown that in this way we can eliminate the breakdown of the standard CCSD method in the region of highly stretched geometries and obtain reliable potential energy curves. The comparison with the recently proposed renormalized CCSD(T) and variational CCD methods is also briefly addressed.