Single-reference Coupled-cluster Methods Research Articles

Fock-space coupled-cluster method

A generalization of the single-reference coupled-cluster method, employing the algebraic properties of the fermionic Fock space, is presented. This Fock-space coupled-cluster (FSCC) method is capable of providing not only the ground-state energy of anN-electron system, but also an important fraction of system's excitation spectrum, including ionization potentials, electron affinities, and excitation energies corresponding toN-electron singlet and triplet states. The FSCC method is applied to study the electronic spectra corresponding to the Pariser-Parr-Pople model of butadiene, hexatriene, and benzene, with the full configuration-interaction results taken as the reference. The problem of intruder states is discussed.

Highly correlated single-reference studies of the O3 potential surface. I. Effects of high order excitations on the equilibrium structure and harmonic force field of ozone

The potential energy surface of ozone in the vicinity of the equilibrium geometry is investigated by single-reference many-body perturbation theory (MBPT) and coupled-cluster (CC) methods. As expected from the known inadequacies of the independent-particle picture of O3, analysis of the CCSDT-1 wave function reveals considerable mixing between the [core⋅⋅⋅]4b226a211a22 and [core⋅⋅⋅]4b226a212b21 configurations. Smaller, but still significant, contributions come from other configurations involving redistribution of electrons within the out-of-plane π orbital framework. As expected, the equilibrium structure and harmonic force field computed at the SCF level of theory are in considerable error. When allowance is made for electron correlation effects, the discrepancies between theory and experiment for the equilibrium structure and totally symmetric force field are significantly reduced, and the MBPT(4), CCSD, CCSD+T(CCSD) and CCSDT-1 results are in reasonable agreement with accepted values. Asymmetric stretching frequencies, however, are found to oscillate wildly [2373 cm−1 at MBPT(2), 1547 cm−1 at MBPT(4), and 680 cm−1 at CCSDT-1] about the experimental value of 1089 cm−1, suggesting that the Cs subspace of the potential surface poses a great challenge to theory. A possible cause of this behavior is discussed, along with implications for future studies of the global O3 potential surface.