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This work demonstrates the performance of the recently proposed explicitly correlated coupled-cluster method CCSD(F12*) and a new method using explicitly correlated triple excitations, CC3(F12*), in the calculation of the static ESHG hyperpolarizability of the Ne atom. [ABSTRACT FROM AUTHOR]

Many-body correlations play a central role in the structure of nuclei. The mean-field methods, such as Skyrme Hartree-Fock and its extensions to Energy Density Functionals, induce correlations dynamically at the functional level, leaving the explicit wave functions uncorrelated. The Configuration Interaction (CI) method treats the many-body correlations exactly, but in a small configurations space. It is therefore desirable to include as many correlations as possible in larger model spaces. In this contribution I compare the results of Coupled Cluster techniques and of extrapolations methods, such as the Exponential Convergence Method, with the full CI results for nuclei around 56Ni. [ABSTRACT FROM AUTHOR]

Coupled-cluster theory with singles, doubles and perturbative triples [CCSD(T)] is a high-accuracy reference to which other methods are measured; however, away from equilibrium RHF based CCSD(T) often fails. To remedy this failure, the ΛCCSD(T) method was introduced and implemented for both energies and gradients within the ACESII program system. Like CCSD(T), ΛCCSD(T) is extensive, orbitally invariant, and scales O(N7). Applying ΛCCSD(T) to molecules at equilibrium shows little degradation of the CCSD(T) results. For bond-breaking ΛCCSD(T) is shown to be a significant improvement over CCSD(T). [ABSTRACT FROM AUTHOR]

The equation-of-motion coupled-cluster (EOM-CC) method with full inclusion of triples is applied to evaluate the vertical ionization potentials (IP) of carbon dioxide, carbon disulphide and carbonyl sulfide for relatively large basis sets. The theoretical results are accurate enough to define a consistent set of vertical ionization potentials. IP-EOM-CCSDT (IP-EOM-CC Singles, Doubles, Triples) improves the IP-EOM-CCSD values by up to 0.4 eV for CO2, 0.5 eV for CS2 and OCS. [ABSTRACT FROM AUTHOR]