On the universality of the long-/short-range separation in multiconfigurational density-functional theory. II. Investigating f0 actinide species.

In a previous paper [Fromager et al., J. Chem. Phys. 126, 074111 (2007)], some of the authors proposed a recipe for choosing the optimal value of the mu parameter that controls the long-range/short-range separation of the two-electron interaction in hybrid multiconfigurational self-consistent field short-range density-functional theory (MC-srDFT) methods. For general modeling with MC-srDFT methods, it is clearly desirable that the same universal value of mu can be used for any molecule. Their calculations on neutral light element compounds all yielded mu(opt)=0.4 a.u. In this work the authors investigate the universality of this value by considering "extreme" study cases, namely, neutral and charged isoelectronic f(0) actinide compounds (ThO(2), PaO(2)(+), UO(2)(2+), UN(2), CUO, and NpO(2)(3+)). We find for these compounds that mu(opt)=0.3 a.u. but show that 0.4 a.u. is still acceptable. This is a promising result in the investigation of a universal range separation. The accuracy of the currently best MC-srDFT (mu=0.3 a.u.) approach has also been tested for equilibrium geometries. Though it performs as well as wave function theory and DFT for static-correlation-free systems, it fails in describing the neptunyl (VII) ion NpO(2)(3+) where static correlation is significant; bending is preferred at the MC-srDFT (mu=0.3 a.u.) level, whereas the molecule is known to be linear. This clearly shows the need for better short-range functionals, especially for the description of the short-range exchange. It also suggests that the bending tendencies observed in DFT for NpO(2)(3+) cannot be fully explained by the bad description of static correlation effects by standard functionals. A better description of the exchange seems to be essential too.