The correct description of electron correlation is a major challenge in electronic structure theory. In mean-field theories, it is assumed that each electron interacts with an average charge distribution due to the other N - 1 electrons, and therefore such theories lack correlation. The static correlation arises when more than one configuration is required to give a good approximation to the wave function of a state, for example, in the case of degenerate or nearly degenerate states; this correlation can be is accounted for in multiconfigurational calculations, such as complete-active space self-consistent-field (CASSCF) theory. The remaining correlation can be recovered through perturbation theory, coupled cluster theory, or density functional methods. In multi-configuration pair-density functional theory (MC-PDFT), a multiconfigurational wave function and a density functional are combined for this purpose. This research shows how the MC-PDFT total energy, its components, and the ingredients (total density, on-top pair density, and the ratio of on-top pair density to the square of the electron density) that form independent variables for these components depend on the active space choice in the case of the H2 and N2 molecules. The active space dependence of the satisfaction of the virial theorem is also explored. We find that the density and on-top pair density do not change significantly with changes in the active space; however, the on-top ratio does change significantly with respect to active space change, and this affects the on-top energy. This study provides a foundation for designing on-top density functionals and automatizing the active space choice in MC-PDFT.

Active space dependence of energy components and ingredients in multiconfiguration pair-density functional theory

255th National Meeting (2018)

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