Coupled cluster

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

Accurate thermochemistry from a parameterized coupled-cluster singles and doubles model and a local pair natural orbital based implementation for applications to larger systems.

Abstract: We have recently introduced a parameterized coupled-cluster singles and doubles model (pCCSD(α, β)) that consists of a bivariate parameterization of the CCSD equations and is inspired by the coupled electron pair approximations. In our previous work, it was demonstrated that the pCCSD(−1, 1) method is an improvement over CCSD for the calculation of geometries, harmonic frequencies, and potential energy surfaces for single bond-breaking. In this paper, we find suitable pCCSD parameters for applications in reaction thermochemistry and thermochemical kinetics. The motivation is to develop an accurate and economical methodology that, when coupled with a robust local correlation framework based on localized pair natural orbitals, is suitable for large-scale thermochemical applications for sizeable molecular systems. It is demonstrated that the original pCCSD(−1, 1) method and several other pCCSD methods are a significant improvement upon the standard CCSD approach and that these methods often approach the accu...

INVITED ARTICLE 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 is recommended. This method employs the overlap of fragment localized molecular orbitals (LMO) within the spherical Gaussian approximation. The LMO overlap integrals are also used to determine the damping of dispersion. Gaussian polarization damping functions are recommended for use within the EFP framework. With this set of damping functions, the EFP binding energies are within 0.5 kcal/mol

Calculations of nonlinear response properties using the intermediate state representation and the algebraic-diagrammatic construction polarization propagator approach: two-photon absorption spectra.

Abstract: An earlier proposed approach to molecular response functions based on the intermediate state representation (ISR) of polarization propagator and algebraic-diagrammatic construction (ADC) approximations is for the first time employed for calculations of nonlinear response properties. The two-photon absorption (TPA) spectra are considered. The hierarchy of the first- and second-order ADC/ISR computational schemes, ADC(1), ADC(2), ADC(2)-x, and ADC(3/2), is tested in applications to H2O, HF, and C2H4 (ethylene). The calculated TPA spectra are compared with the results of coupled cluster (CC) models and time-dependent density-functional theory (TDDFT) calculations, using the results of the CC3 model as benchmarks. As a more realistic example, the TPA spectrum of C8H10 (octatetraene) is calculated using the ADC(2)-x and ADC(2) methods. The results are compared with the results of TDDFT method and earlier calculations, as well as to the available experimental data. A prominent feature of octatetraene and other polyene molecules is the existence of low-lying excited states with increased double excitation character. We demonstrate that the two-photon absorption involving such states can be adequately studied using the ADC(2)-x scheme, explicitly accounting for interaction of doubly excited configurations. Observed peaks in the experimental TPA spectrum of octatetraene are assigned based on our calculations.

CASCCD: Coupled-cluster method with double excitations and the CAS reference

Abstract: A new multireference coupled-cluster method which includes double excitations and is based on the complete active space (CAS) multiconfigurational reference wave function is proposed. By partitioning the CAS orbitals into active and nonactive sets a two-component, coupled-cluster wave function involving excitations into orbitals of the different sets was constructed. The first component includes all the CAS excitations and the second component, which has the exponential form, consists of double external and semi-external excitations. The coupled-cluster equations for the energy and for the amplitudes involved in the two components of the wave function were derived and illustrated using a diagrammatic formalism. Several numerical tests were performed, and the results demonstrate a very good performance of the method as compared to the full configuration interaction results.

Multireference Brillouin-Wigner coupled clusters method with noniterative perturbative connected triples.

Abstract: We developed and implemented the state-specific Brillouin-Wigner coupled cluster method with singles, doubles, and noniterative perturbative triples, called MR BWCCSD(T), for a general number of closed- and open-shell reference configurations. To assess the accuracy of the method, we performed calculations of the three lowest electronic states of the oxygen molecule and of the automerization barrier of cyclobutadiene. For the oxygen molecule, the results were in a good agreement in comparison with those of the iterative MR BWCCSDTalpha method. For cyclobutadiene, the effect of connected triples was found to be minor, which is in agreement with the previous study by and Balkova and Bartlett [J. Chem. Phys. 101, 8972 (1994)].