Showing papers on "Coupled cluster published in 1979"

The quartic force field of H2O determined by many‐body methods that include quadruple excitation effects

Abstract: Many‐body perturbation theory (MBPT) and coupled cluster methods are employed in an investigation of the potential energy surface of H2O in the vicinity of its equilibrium geometry. The basis set of 39 Slater‐type orbitals is the same as that previously used in a configuration interaction study (limited to all single and double excitations, SD‐CI) of this surface, and is capable of accounting for 80% of the total correlation energy of the molecule. Detailed comparisons among the results of the various methods are presented, with particular reference to the role of size extensivity in providing a reliable model for the prediction of the shape of the surface. The predicted quartic force field obtained by the coupled cluster doubles (CCD) and by several MBPT models is in very good agreement with experiment. The inclusion of quadruple excitations, which account for about 5% of the correlation energy, is found to have a significant effect on the shape of the surface, bringing the predicted force field into sub...

Orthogonally-spin-adapted coupled-cluster theory for closed-shell systems including triexcited clusters

Abstract: The orthogonally-spin-adapted form of the nonlinear system of equations for the extended coupled-pair many-electron theory, which involves the monoexcited, biexcited, and triexcited states and cluster components, is derived. A diagrammatic approach, based on second quantization, the time-independent Wick theorem, and the graphical methods of spin algebras, is employed. The advantages of using the orthogonally-spin-adapted states and cluster components, rather than the nonorthogonally-spin-adapted ones which have been previously employed, particularly in the triexcited case, are also discussed. It is also shown that the formulas for the direct configuration-interaction method can easily be obtained in compact form from the linear part of the system of extended coupled-pair equations.

Correlation problems in atomic and molecular systems. VII. Application of the open‐shell coupled‐cluster approach to simple π‐electron model systems

Abstract: The coupled-cluster variational-like direct approach to the calculation of ionization and electron attachment energies and of excitation energies is applied to several π-electron model systems, using the PPP Hamiltonian with various parametrizations. A simple approximation, which represents the triexcited clusters in terms of disconnected W1T2 terms, is employed. All the necessary diagrams for both excitation energy and ionization potential (electron affinity) calculations are given in the compact Hugenholtz nonoriented form. The results of the calculations for benzene, trans-butadiene, all-trans-hexatriene, and fulvene are compared with the corresponding full CI results, and the conclusions about the validity and efficiency of this approach are drawn.

Time-dependent coupled-cluster approximation to nuclear dynamics. II. General formulation

Abstract: In the first paper of this series, ' hereafter denoted as I, the ti.me-dependent coupled-cluster theory was shown to provide a convenient systematic hierarchy of corrections to the intuitively appealing time-dependent mean-field approximation. This theory, which is formulated in terms of multi-particle-hole amplitudes, offers the significant advantage of representing many-particle correlations much more economically than correspondingg