The intermediate Hamiltonian technique is applied to reformulate the Fock-space coupled-cluster (FSCC) method with singles and doubles. A version of FSCC designated to direct calculation of excitation energies is considered. The presented scheme offers several advantages over those used so far like simplification of the equations, smooth convergence, and the possibility of one-root calculation. Results of some illustrative numerical examples are also shown where comparison with other methods as well as with the full configuration interaction results is available.

No abstract

No abstract

Articles you may be interested inThe general-model-space state-universal coupled-cluster method exemplified by the LiH molecule Hilbert space multireference coupledcluster methods. II. A model study on H8The openshell coupledcluster method in general model space: Five states of LiH J. Chem. Phys. 89, 956 (1988); 10.1063/1.455164 Molecular applications of multireference coupledcluster methods using an incomplete model space: Direct calculation of excitation energiesThe first results from a Hilbert space, multireference coupled-cluster (CC) method in an incomplete model (active) space are reported for the five lowest states of LiH. The active space is spanned by several configurations at the level of single and double excitations, where the configuration(s) causing intruder state problems are excluded from the complete Hilbert reference space. Full inclusion of single-and double-excitation operators is considered in the expansion for the cluster operator, with all quadratic terms in the renormalization part. The multireference CC results for the ground (X I~ + ) and four low-lying excited states (a 3~ + , A I~ +, a 3n, A In) ofLiH are compared with the corresponding full configuration-interaction (FCl) energies. The agreement between FCI and CC values within a few hundredths of mH for the n states proves the feasibility of the present method to describe, quantitatively, the quasicomplete reference space problem. Deviations of the incomplete multireference results from the exact FCI energies for excited ~ states vary with respect to the quasidegeneracies involved from a few tenths ofmH for the nondegenerate (near-equilibrium) geometries to several mH in the dissociation region.