Coupled cluster

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

Energy-Specific Equation-of-Motion Coupled-Cluster Methods for High-Energy Excited States: Application to K-edge X-ray Absorption Spectroscopy.

Abstract: Single-reference techniques based on coupled-cluster (CC) theory, in the forms of linear response (LR) or equation of motion (EOM), are highly accurate and widely used approaches for modeling valence absorption spectra. Unfortunately, these equations with singles and doubles (LR-CCSD and EOM-CCSD) scale as O(N6), which may be prohibitively expensive for the study of high-energy excited states using a conventional eigensolver. In this paper, we present an energy-specific non-Hermitian eigensolver that is able to obtain high-energy excited states (e.g., XAS K-edge spectrum) at low computational cost. In addition, we also introduce an improved trial vector for iteratively solving the EOM-CCSD equation with a focus on high-energy eigenstates. The energy-specific EOM-CCSD approach and its low-scaling alternatives are applied to calculations of carbon, nitrogen, oxygen, and sulfur K-edge excitations. The results are compared to other implementations of CCSD for excited states, energy-specific linear response ti...

Many-body effects in nonadiabatic molecular theory for simultaneous determination of nuclear and electronic wave functions: Ab initio NOMO/MBPT and CC methods

Abstract: We have investigated the many-body effects in a molecular theory to determine simultaneously nuclear and electronic wave functions without the Born–Oppenheimer (BO) approximation. We first apply the many-body perturbation theory using the electron–nucleus and nucleus–nucleus interactions to the non-BO theory and show the importance of the electron–nucleus correlation rather than the nucleus–nucleus one. We next combine the non-BO theory with the coupled cluster double and Brueckner double methods using the one-electron plus one-nucleus excitation operators.

Chapter One - A Practical Guide to Reliable First Principles Computational Thermochemistry Predictions Across the Periodic Table

Abstract: The Feller–Peterson–Dixon approach to the reliable prediction of thermochemical properties to chemical accuracy is described. The method calculates the total atomization energy of a molecule and then uses experimental atomic heats of formation to calculate the heat of formation. The method is based on extrapolating coupled cluster CCSD(T) calculations of the valence electronic energy to the complete basis set limit followed by additive corrections to the electronic energy including: core–valence interactions, scalar relativistic, spin–orbit, and higher order corrections beyond CCSD(T). Corrections for the nuclear motion in terms of the zero-point energy need to be included and for high-accuracy, Born–Oppenheimer corrections may be added. Issues with these terms and the experimental atomic data are described. A summary of the reliability of the approach is presented.

A coupled‐cluster based effective Hamiltonian method for dynamic electric polarizabilities

Abstract: A coupled‐cluster based approach for calculating dynamic polarizabilities is described. In this procedure, the polarizability is calculated by a strategy that is formally equivalent to a sum over states corresponding to the diagonal representation of a similarity transformed Hamiltonian operator. However, the explicit evaluation of excited state wave functions and energies is avoided. The present treatment is closely related to the equation of motion coupled‐cluster approximation for excited states and offers an accurate approximation to the second derivative of the energy with respect to an applied electric field; the two approaches are equivalent in the limit that the spectrum of states corresponding to the effective Hamiltonian is exact within the basis set. Terms contributing to the second derivative, but neglected in the proposed approach are shown to be insignificant for a representative set of small molecules. The method is applied to calculate the polarizability of benzene at the wavelength of the...

Multireference Character for 4d Transition Metal-Containing Molecules

Abstract: Four diagnostic criteria have been examined to identify the suitability of single-reference wave function-based quantum chemistry methods for a set of 118 4d transition metal species. These diagnostics include the weight of the leading configuration of the CASSCF wave function, C02; the Frobenius norm of the coupled cluster amplitude vector related to single excitations, T1; the matrix 2-norm of the coupled cluster T1 amplitude vector arising from coupled cluster calculations, D1; and the percent total atomization energy, %TAE, corresponding to a relationship between energies determined with CCSD and CCSD(T) calculations. New criteria, namely, T1 ≥ 0.045, D1 ≥ 0.120, and %TAE ≥ 10%, are herein proposed as a gauge for 4d transition metal-containing molecules to predict the possible need to employ multireference (MR) wave function-based methods to describe energetic and spectroscopic properties.