A method previously suggested to calculate the correlation energy at the complete one-electron basis set limit by reassignment of the basis hierarchical numbers and use of the unified singlet- and triplet-pair extrapolation scheme is applied to a test set of 106 systems, some with up to 48 electrons. The approach is utilized to obtain extrapolated correlation energies from raw values calculated with second-order Møller-Plesset perturbation theory and the coupled-cluster singles and doubles excitations method, some of the latter also with the perturbative triples corrections. The calculated correlation energies have also been used to predict atomization energies within an additive scheme. Good agreement is obtained with the best available estimates even when the (d, t) pair of hierarchical numbers is utilized to perform the extrapolations. This conceivably justifies that there is no strong reason to exclude double-zeta energies in extrapolations, especially if the basis is calibrated to comply with the theoretical model.

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We seek correlation consistent double- and triple-zeta basis sets that perform optimally for extrapolating the correlation energy to the one-electron complete basis set limit. Since the methods used are approximate, the novel basis sets become method specific in the sense of performing best for the chosen level of theory. Such basis sets are also shown to perform accurately for tensorial properties and do not significantly alter the Hartree-Fock energy. Quantitatively, the extrapolated correlation energies from (oVdZ, oVtZ) outperform typically by three- to fivefold those obtained from traditional ansatzes with similar flexibility, thus being (VtZ, VqZ) type or even better. They may even outperform explicitly correlated ones. Not surprisingly, the outperformance in relative energies (e.g., atomization and dissociation energies, and ionization potential) is somewhat downscaled, albeit consistently better than with traditional basis sets. As a case study, we also consider the polarizability of p-nitroaniline, a sizeable system for which complete basis set (CBS)(oVdZ, oVtZ) calculations are shown to outperform equally expensive CBS(VdZ, VtZ) results.

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A method previously suggested to calculate the correlation energy at the complete one-electron basis set limit by reassigning the basis hierarchical numbers and using the unified singlet- and triplet-pair extrapolation scheme is here utilized to extrapolate tensorial properties, with specific use for the polarizabilities of eight molecules whose raw values are obtained with second-order Møller-Plesset perturbation theory and coupled-cluster singles and doubles excitation methods, both without and with inclusion of the perturbative triples correction. Good agreement is obtained with the best available estimates even when the (d, t) pair of hierarchical numbers is utilized to perform the extrapolations. This conceivably reinforces our previous finding that there is no good reason to exclude double-ζ results in extrapolations, especially if the basis is calibrated to comply with the theoretical model.

By using density functional theory, spin states, geometries, and mean static dipole polarizabilities of group VIIIA metallocenes M(C5H5)2 (M = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) are examined. For all metallocenes studied, comparison of the polarizability of the accessible spin states reveals that the lowest polarizability was found for the spin ground state. Therefore, our findings indicate that the minimum polarizability principle might be useful in determining the ground state multiplicity for transition metal metallocenes. The metallocenes from the group 8 and group 9 possess the same multiplicity, singlet and doublet, respectively. Additionally, one observes that the polarizability increases monotonically with the atomic number of the central metal atom for metallocenes from the same column. The B3LYP/ADZP is one of the most reliable procedure tested so far to predict the static dipole polarizability of the complexes studied here, with mean absolute deviation from the experimental data smaller than 1.8 au.

A detailed analysis of the electronic structure of the ground and first excited spin state of three diatomic molecules ( N2, BH and CO) under static applied electric field is performed at CCSD(T), DFT, MRCI and MRCI(Q) levels of theory. Our findings have revealed that by boosting the applied field one induces changes in the occupation numbers of molecular orbitals, giving rise to changes in the equilibrium geometry and in the HOMO-LUMO energy gap. Specifically, singlet to triplet spin transition can be induced by increasing the applied electric field beyond a critical value. Accordingly, affecting the accuracy of the widely used expression of energy expanded in Taylor series with respect to the applied electric field. © 2018 Wiley Periodicals, Inc.