Coupled cluster

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

Energy landscapes of nucleophilic substitution reactions: A comparison of density functional theory and coupled cluster methods

Abstract: We have carried out a detailed evaluation of the performance of all classes of density functional theory (DFT) for describing the potential energy surface (PES) of a wide range of nucleophilic substitution (SN2) reactions involving, amongst others, nucleophilic attack at carbon, nitrogen, silicon, and sulfur. In particular, we investigate the ability of the local density approximation (LDA), generalized gradient approximation (GGA), meta-GGA as well as hybrid DFT to reproduce high-level coupled cluster (CCSD(T)) benchmarks that are close to the basis set limit. The most accurate GGA, meta-GGA, and hybrid functionals yield mean absolute deviations of about 2 kcal/mol relative to the coupled cluster data, for reactant complexation, central barriers, overall barriers as well as reaction energies. For the three nonlocal DFT classes, the best functionals are found to be OPBE (GGA), OLAP3 (meta-GGA), and mPBE0KCIS (hybrid DFT). The popular B3LYP functional is not bad but performs significantly worse than the best GGA functionals. Furthermore, we have compared the geometries from several density functionals with the reference CCSD(T) data. The same GGA functionals that perform best for the energies (OPBE, OLYP), also perform best for the geometries with average absolute deviations in bond lengths of 0.06 A and 0.6 degrees, even better than the best meta-GGA and hybrid functionals. In view of the reduced computational effort of GGAs with respect to meta-GGAs and hybrid functionals, let alone coupled cluster, we recommend the use of accurate GGAs such as OPBE or OLYP for the study of SN2 reactions.

Polarizable interaction potential for water from coupled cluster calculations. II. Applications to dimer spectra, virial coefficients, and simulations of liquid water.

Abstract: The six-dimensional CC-pol interaction potential for the water dimer was used to predict properties of the dimer and of liquid water, in the latter case after being supplemented by a nonadditive potential. All the results were obtained purely from first principles, i.e., without any fitting to experimental data. Calculations of the vibration-rotation-tunneling levels of (H2O)2 and (D2O)2, a very sensitive test of the potential surface, gave results in good agreement with experimental high-resolution spectra. Also the virial coefficients and properties of liquid water agree well with measured values. The present model performs better than published force fields for water in a simultaneous reproduction of experimental data for dimer spectra, virials, and properties of the liquid.

Reduced multireference coupled cluster method: Ro-vibrational spectra of N2

Abstract: The reduced multireference (RMR) coupled cluster method with singles and doubles (CCSD), relying on four- and eight-dimensional reference spaces, is employed to generate the potential energy curve for the nitrogen molecule, using both the correlation-consistent polarized valence-triple-zeta (cc-pVTZ) and atomic natural orbital basis sets. The resulting potentials are then used to compute the ro-vibrational term values, which are compared with the available experimentally determined values. Moreover, a direct comparison with the measured spectral line frequencies, for both the fundamental rotational Raman band and the vibrationally excited Q branches, is also carried out. Comparisons with the single reference CCSD, as well as with the corresponding multireference configuration interaction results that serve as the external source for RMR CCSD, are also made. An excellent performance of RMR CCSD, and its systematic improvement with the increasing dimension of the reference space employed, is demonstrated fo...

Damping functions in the effective fragment potential method

Abstract: This work presents the implementation and analysis of several damping functions for Coulomb, induction, and dispersion interactions within the framework of the general effective fragment potential (EFP) method. Damping is necessary to obtain the correct asymptotic short-range behavior of these interactions. Correctly chosen damping functions allow a balanced description of different parts of intermolecular potential energy surfaces and improve the accuracy of predicted intermolecular distances and binding energies. The performance of different damping functions is tested by comparing the EFP energy terms with the symmetry adapted perturbation theory (SAPT) energy terms in a range of intermolecular separations for ten molecular dimers. The total EFP binding energies in these dimers were compared with the binding energies obtained from SAPT and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. A formula for electrostatic damping that was derived from first principles...

Basis set limit coupled cluster study of h-bonded systems and assessment of more approximate methods.

Abstract: Hydrogen bonds are of utmost importance in both chemistry and biology. As the applicability of density functional theory and ab initio methods extends to ever larger systems and to liquids, an accurate description of such interactions is desirable. However, reference data are often lacking, and ab initio calculations are only possible and done in very small basis sets. Here, we present high level [CCSD(T)] ab initio reference calculations at the basis set limit on a large set of hydrogen-bonded systems and assess the accuracy of second-order perturbation theory (MP2). The possibilities of using basis set extrapolations for geometries and dissociation energies are discussed as well as the results of R12 methods and density functional and local correlation methods.