Showing papers on "Coupled cluster published in 1977"

Correlation problems in atomic and molecular systems. V. Spin‐adapted coupled cluster many‐electron theory

Abstract: The graphical techniques of spin algebras are combined with a diagrammatic approach based on the time independent Wick theorem to yield the spin‐adapted form of the coupled cluster theory. The general rules for the implementation of this formalism are formulated and illustrated on the basic coupled‐pair many‐electron theory pertaining to closed shell ground states, for which case the explicit spin‐adapted equations are derived. The advantages of the spin‐adapted form of the theory are discussed along with the new insights it affords.

Coupled-cluster expansion applied to the electron gas: Inclusion of ring and exchange effects

Abstract: The coupled-cluster expansion (or the Coester-K\"ummel-\ifmmode \check{C}\else \v{C}\fi{}i\ifmmode \check{z}\else \v{z}\fi{}ek method) is applied to the correlation problem in the uniform electron gas Coupled nonlinear integral equations are developed for ring summations and the analytic structure of the expansion coefficients is examined To facilitate the solution of the equations a technique is introduced to reduce the dimensionality of the problem Numerical solution of the equations enable the evaluation of both ring and exchange effects The direct random-phase-approximation (RPA) energy agrees with other work to the accuracy of the calculation The screened exchange energy is evaluated for the first time and contributes about 30% of the RPA energy The resulting correlation energy compares favorably with recent calculations using a dielectric formulation over the range of metallic electron densities