No abstract

It is shown that spin-component-scaled second order Møller-Plesset theory proposed by Grimme [J. Chem. Phys. 118, 9095 (2003)] can be interpreted as a two-parameter scaling of the zero order Hamiltonian, a generalization of the approach reported by Feenberg [Phys. Rev. 103, 1116 (1956)].

Recent nuclear-magnetic-resonance measurements on isotope engineered double walled carbon nanotubes ͑DWCNTs͒ surprisingly suggest a uniformly metallic character of all nanotubes, which can only be explained by the interaction between the layers. Here we study the intershell interaction in DWCNTs by densityfunctional theory and the intermolecular Hückel model. Both methods find charge transfer between the inner and outer tubes. We find that the charge transfer between the walls is on the order of 0.001 e − / atom and that the inner tube is always negatively charged. We also observe orbital mixing between the states of the layers. We find that these two effects combined can in some cases lead to a semiconductor-to-metal transition of the double walled tube, but not necessarily in all cases. We extend our study to multiwalled nanotubes as well, with up to six layers in total. We find similar behavior as in the case of DWCNTs: electrons tend to be transferred from the outermost layer toward the innermost one. We find a notable peculiarity in the charge transfer when the ͑5,0͒ tube is present as the innermost tube; we attribute this to themixing in such small diameter tubes.

A simple variant of perturbation theory is used to correct reference states of a general multiconfigurational character. The full solution of an active space is not required, and no iterative procedure is applied to construct the resolvent operator. The perturbed wave function is expanded in a complete set of determinants from which the reference function is projected out, and the overlap between projected determinants is handled by an explicit, analytic inversion of the overlap matrix.

A linearized Multireference Coupled Cluster (MR-LCC) theory is formulated based on the Antisymmetrized Product of Strongly Orthogonal Geminals (APSG) reference state. The role of dispersive interbond interactions is discussed. The presented theory has led to qualitatively correct potential curves for the case when both OH bonds dissociate in H2O, a result that cannot be achieved by adding only perturbative corrections to APSG. The potential curve obtained for the He···He problem practically coincides with the full CI (FCI) result, showing the unexpected accuracy of the MR-LCC approach in this case.