Showing papers on "Coupled cluster published in 1983"

Ab initio analytical molecular gradients and Hessians

Abstract: Molecular gradients and Hessians have been derived for MCSCF, CI, coupled cluster, and Mo/ller–Plesset wave functions. In deriving the gradients and Hessians, atomic orbital basis set effects have been incorporated into the finite basis Hamiltonian, and unitary exponential operators have been used to determine the wave function’s configuration and orbital responses. The gradients and Hessians are expressed in terms of products of configuration and orbital responses and matrices of the same form as the gradient and Hessian matrices appearing in energy and wave function optimizations. The molecular gradients and Hessians have also been cast into forms that are computationally very tractable.

C2V Insertion pathway for BeH2: A test problem for the coupled‐cluster single and double excitation model

Abstract: The coupled-cluster single and double excitation model (CCSD) is applied to an energy path for the insertion of Be into H2 and compared to the full configuration interaction (FCI) and full valence–multiconfiguration self-consistent field (FV–MCSCF) results. This model problem is a severe test of a single-reference-function correlated method since two configurations are heavily weighted in the FCI description. CCSD is demonstrated to describe the FCI results using a single reference function which, however, changes orbital characteristics along the sampling path. In this case CCSD gives excellent agreement with the FCI results.

Coupled Cluster Approaches to Many-Electron Correlation Problem

Abstract: The origins, basic ideas and methodological developments of the coupled cluster approach (CCA) to the atomic and molecular many-electron correlation problem are briefly outlined. A rudimentary derivation of basic concepts of the CCA is attempted exploiting only very general algebraic concepts and the idea of an effective Hamiltonian. The relationship with other approaches to the correlation problem is pointed out. An extension of the CCA to quasi-degenerate cases is briefly contemplated and a generalization of the Davidson correction to a multi-reference case is suggested.

A size consistent unitary group approach Multi-reference linear coupled cluster theory

Abstract: A formulation of multi-reference linear coupled cluster theory (MR-LCC) within the context of the Unitary Group Approach (UGA) is presented. This size consistent unitary group method has been organized in a manner which is well suited to vector computers. Some examples are presented which illustrate the applicability of the method to potential surface calculations.

Coupled-cluster summation of the particle-particle ladder diagrams for the two-dimensional electron gas

Abstract: The correlation energy and antiparallel spin pair correlation function of the two-dimensional electron gas are studied within the particle-particle ladder approximation of the coupled-cluster equations. Unlike the three-dimensional electron gas the correlation energy is shown to be convergent in the particle-particle ladder approximation for the two-dimensional system. The coupled-cluster particle-particle ladder equations are cast into the form of singular integral equations. Using a product integration technique the integral equations are solved numerically. The correlation energy and r=0 value of the antiparallel spin pair correlation function are calculated from the numerical coupled-cluster coefficients for a range of electron densities from rs=0 to 16. The antiparallel spin pair correlation function at r=0 is shown to be positive for all electron densities when a wavefunction accurate to first order in the coupled-cluster particle-particle ladder coefficients is used. The calculated correlation energy is compared with dielectric results of Jonson (1976) and ring results of Freeman (1977, 1978). Good agreement between the dielectric results and the particle-particle ladder results are found for intermediate to low electron densities. The particle-particle ladder coupled-cluster correlation energy is shown to be exact in the high-density limit, although the approach to the limit is not correct owing to the neglect of ring contributions. It is suggested that simultaneous inclusion of particle-particle ladder and ring diagrams will give correlation energies accurate over the entire range of calculated densities.

Quasidegenerate coupled-cluster approach with hermitian model Hamiltonian

Abstract: The quasidegenerate coupled-cluster approach with a hermitian model Hamiltonian is constructed This theory provides an efficient method for simultaneous construction and evaluation of the hermitian diagrammatic model Hamiltonian up to an arbitrary high order of perturbation theory and prescribed form of topology

New horizons of quantum chemistry : proceedings of the Fourth International Congress of Quantum Chemistry, held at Uppsala, Sweden, June 14-19, 1982

Abstract: Symposium I. Quantum Mechanical Methods.- to the First Symposium.- Classical Implications of the Quantum Statistical Interpretation.- Propagators, Scattering Theory and Chemical Reactions.- Degenerate Perturbation Theory.- Coupled Cluster Approaches to Many-Electron Correlation Problem.- Symposium II. Time Independent Phenomena.- Chairman's Introduction to Second Symposium.- Energy Transfer along Alpha-Helical Proteins.- Collective and Independent-Particle Motion in Simple Atoms and Molecules: A Unification?.- Are CLi6, NLi5, OLi4, etc Hypervalent?.- Symposium III. Time Dependent Phenomena.- Introductory Remarks on the Time-Dependent Phenomena and Related Topics.- Nonadiabatic Effects in Slow Atomic Collisions - Many-Trajectory Semiclassical Approximation.- Time Dependent Constants of the Motion.- Electron Molecule Scattering at Low Energies: Relations Between Dynamics and Structure.- Symposium IV. Computational Quantum Chemistry.- Computational Quantum Chemistry: Introductory Remarks.- AB Initio Calculations Close to the Full CI Level of Accuracy and their Use for the Interpretation of Molecular Spectra.- Potential Energy Surfaces of Chemical Reactions.- AB-Initio Methods in Calculations of Intermolecular Interaction Energies.- The Unitary Group and the Electron Correlation Problem.- Symposium V. A. Surfaces and Catalysis.- From Heterogeneous Catalysis to Surface Chemistry and Chemisorption.- Molecular Ingredients of Heterogeneous Catalysis.- New Perspectives in Surface Crystallography.- Symposium V. B. Theory of Large Molecules and Solids.- Introductory Remarks to Symposium 5B: Large Molecules and Solid State Theory.- Insights in Many-Electron Theory from the Charge Density Wave Structure of Potassium.- Molecular Semiconductors, Metals and Superconductors.- Symposium VI. Quantum Biology and Quantum Pharmacology.- Membrane Proteins as Ion Channels.- Evolution of Molecules.- The MCM (Most Complicated Molecule) in Today's Quantum Chemistry: tRNAPhe. Molecular Potential Versus Molecular Field..- Index of Subjects.

Indirect exchange interaction between two doubly bridged paramagnetic atoms. A coupled-cluster model study

Abstract: A model analysis is made of the influence of the ligands and the geometrical structure on the exchange-coupling constant, ecc, between two paramagnetic cations doubly bridged by two diamagnetic ligands. The model is based on a four-center, six-electron description of the superexchange unit, symmetry D 2h , with 1s slater type orbitals. A coupled-cluster method is employed. First, SCF orbitals are constructed for the unpaired electrons neglecting the interactions between the cations. Then, using these nonorthogonal orbitals, the direct exchange is evaluated. In the latter step, the influence of the ligands is gradually diminished and the results are compared with those from complete configuration interaction. It is found that the Coulomb and exchange ligand potentials are important in the evaluation of the ecc. Further, the cation-ligand-cation angle for which the ecc is equal to zero is not sensitive to variations of the ligand-admixture coefficient in the cation orbitals. The values of the ecc, however, do change essentially. Finally, the usual application of the Heitler-London treatment is revised. It is shown that the off-diagonal Lagrange multipliers in the SCF procedures introduce non-negligible contributions of the ligands to the ecc.