Coupled cluster

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

Explicitly time-dependent coupled cluster singles doubles calculations of laser-driven many-electron dynamics

Abstract: We report explicitly time-dependent coupled cluster singles doubles (TD-CCSD) calculations, which simulate the laser-driven correlated many-electron dynamics in molecular systems. Small molecules, i.e., HF, H(2)O, NH(3), and CH(4), are treated mostly with polarized valence double zeta basis sets. We determine the coupled cluster ground states by imaginary time propagation for these molecules. Excited state energies are obtained from the Fourier transform of the time-dependent dipole moment after an ultrashort, broadband laser excitation. The time-dependent expectation values are calculated from the complex cluster amplitudes using the corresponding configuration interaction singles doubles wave functions. Also resonant laser excitations of these excited states are simulated, in order to explore the limits for the numerical stability of our current TD-CCSD implementation, which uses time-independent molecular orbitals to form excited configurations.

Spin-contamination of coupled-cluster wave functions

Abstract: The propensity of approximate solutions of the electronic Schrodinger equation to break spin-symmetry is directly related to the quality of the approximate wave function, and thus can be used as a diagnostic tool. The quasi-variational nature of the (valence) optimized orbitals coupled-cluster doubles methods, (V)OO-CCD, enables a discussion of the stability of coupled-cluster wave functions in terms of both spin-contamination and a corresponding energy lowering relative to the pure spin solutions. The spin-contamination of (V)OO-CCD models has been studied for bond-breaking processes and diradicals. The main findings are: (i) The OO-CCD method is stable for a relatively large range of nuclear distortions and is capable of eliminating even very large spin-contamination of the unrestricted Hartree–Fock solution given that the molecular electronic configuration remains essentially single-reference. When a spin-contaminated solution arises, the energy splitting rapidly becomes large and 〈Ŝ2〉 approaches the H...

Low-lying potential energy surfaces

Abstract: Preface 1. Overview: Accurate Description of Low-Lying Electronic States and Potential Energy Surfaces 2. Simultaneous Account of Dynamic and Nondynamic Correlations Based on Complementarity of CI and CC Approaches 3. Method of Moments of Coupled-Cluster Equations: A New Theoretical Framework for Designing "Black-Box" Approaches for Molecular Potential Energy Surfaces 4. The Photoelectron Spectrum of the NO3 Radical Revisited: A Theoretical Investigation of Potential Energy Surfaces and Conical Intersections 5. Coupled Cluster Methods for Bond-Breaking 6. State-Specific Multireference Coupled Cluster-Based Methods for Potential Energy Surfaces and Their Approximate Variants 7. The Excited and Ion States of Allene 8. Hamiltonian Matrix Elements for the Table-CI Method Using Genealogical Configuration State Functions 9. Jacob's Ladder for Time-Dependent Density-Functional Theory: Some Rungs on the Way to Photochemical Heaven 10. Spectral Theory of Chemical Bonding 11. Comparison of CaF, ZnF, CaO, and ZnO: Their Anions and Cations in Their Ground and Low-Lying Excited States 12. The X1(2(3/2) and X2(2(1/2) Potentials of the Halogen Monoxides: A Comparison of RKR and Ab Initio Results 13. Symmetry in Spin-Orbit Coupling 14. Non-Adiabatic Bending Dissociation of OCS Induced by Orbital Unlocking 15. Electronic Structure and Dynamics in the O4 System 16. Model Studies of Intersystem Crossing Effects in the O + H2 Reaction 17. The Challenge of High-Resolution Dynamics: Rotationally Mediated Unimolecular Dissociation of HOCl 18. The Electronic Adiabatic-to-Diabatic Transformation Matrix and the Irreducible Representation of the Rotation Group 19. Spectroscopic Determination of Potential Energy Surfaces for the Out-of-Plane Ring Vibrations of Indan and Related Molecules in Their S0 and S1 ((, (*) States 20. Semiclassical Time Evolution in the Forward-Backward Stationary-Phase Limit 21. Development and Application of an Ab Initio Methane-Water Potential for the Study of Phase Equilibria of Methane Hydrates Indexes Author Index Subject Index

Externally corrected singles and doubles coupled cluster methods for open-shell systems

Abstract: The idea of an externally corrected singles and doubles coupled cluster (CCSD) method using an independent source for connected triples and quadruples is generalized to open-shell high-spin states and implemented within the framework of the unitary group based CCSD method. New and more efficient programs are designed to perform cluster analysis of ab initio wave functions and to calculate corrections to standard CCSD equations. The method is applied to describe a single bond breaking of OH in the 2Π state, using both double zeta (DZ) and DZ plus polarization (DZP) basis sets. Both full or limited configuration interaction (CI) within small and carefully chosen active spaces (AS) and complete AS self-consistent-field (CASSCF) wave functions are employed as external sources for triple and quadruple corrections. It is shown that the CI or CASSCF corrected CCSD method can effectively account for higher than pair clusters while requiring only a small additional computational effort over that of the standard CC...

Comparisons of results from parametrized configuration interaction (PCI‐80) and from hybrid density functional theory with experiments for first row transition metal compounds

Abstract: Different methods and schemes have been tested for the difficult class of first row transition metal complexes. The systems investigated are the M+, MH+, MCH+3, and MCH+2 systems for the entire row and the Ni(CO)x systems with x=1–4. In general quite satisfactory results are obtained both at the PCI‐80 and hybrid density functional levels. In particular, for the MH+ and MCH+3 systems the PCI‐80 average deviation to experiments is of the same size as the uncertainty in the experiments. The MCH+2 systems are somewhat more difficult to describe and a rather large error is found for chromium at the PCI‐80 level, due to a large coupling of exchange energy loss and change of correlation energy resulting from the formation of two covalent d‐bonds. Scaling at the modified coupled pair functional (MCPF) level is also compared to scaling at the coupled cluster singles and doubles (CCSD) level. In most cases very similar results are obtained, but classes of systems can be identified where scaling works better at the...