Kohn-Sham Density in a Slater Orbital Basis Set.

Finite, atom-centered Slater basis sets are used to determine approximate Kohn-Sham molecular orbitals. This is achieved by minimizing the kinetic energy plus the sum-squared difference between the Kohn-Sham density and the full configuration interaction density. As a result of the finite basis, a weight factor is introduced to balance the two minimization components. Results herein show that this can be done systematically, without sensitive dependence on the choice of scaling factor. In addition, the algorithm is applied to the LiH diatomic for fractional electron counts, where stretching the bond introduces significant reorganization of the electron density. The analysis will show the correct KS orbital structure and reveal the effects of correlation and electron locality on the KS solutions.

Toward Full Configuration Interaction for Transition-Metal Complexes.

An efficacious approximation to full configuration interaction (FCI) is adapted to calculate singlet-triplet gaps for transition-metal complexes. This strategy, incremental FCI (iFCI), uses a many-body expansion to systematically add correlation to a simple reference wave function and therefore achieves greatly reduced computational costs compared to FCI. iFCI through the 3-body expansion is demonstrated on four model transition-metal complexes involving the metals Zn, V, and Cu. Screening techniques to increase the computational efficiency of iFCI are proposed and tested, showing reduction in the number of 3-body terms by more than 90% with controlled errors. The largest complex treated herein by iFCI has 142 valence electrons, all of which are correlated among the full set of 444 active orbitals. Computed spin gaps approach experimental results for the four complexes, though room for improvement remains.

Mercury Magnetic Isotope Effect: A Plausible Photochemical Mechanism.

Large mass-independent fractionation signatures in Hg have been observed in the laboratory and the environment, prompting deep questions about the chemical reasons behind these signatures. Since the relative lack of mechanistic information about Hg chemistry in the environment has precluded explanations of these isotope effects, the present study uses high-level electronic structure methods to evaluate the possible photochemical mechanisms of mass-independent isotope effects (MIEs) in HgX2 and CH3HgX (X = Cl, Br, I, and SCH3). The results show that spin-orbit coupling wipes out the potential of MIEs for Hg bound to Br or I, but that complexes involving lighter elements, HgX2 and CH3HgX (X = Cl and SCH3), have relatively small spin-orbit couplings upon photolysis. This unexpected finding shows that magnetic isotope fractionation due to hyperfine coupling is possible, depending on the identity of the Hg complex. By examination of the photolysis potential energy profiles, this study shows that HgX2 complexes can have a positive or a negative MIE (depending on reaction conditions), while CH3HgX complexes exclusively result in a positive MIE. These findings agree with MIE recorded in natural samples, demonstrating a plausible mechanism for the surprising mass-independent fractionation of Hg in the environment.

Evaluation of full valence correlation energies and gradients.

Complete-active-space self-consistent field (CASSCF) wave functions are central to understanding strongly correlated molecules as they capture the entirety of electronic interactions within a subset of the orbital space. The most interesting case for CASSCF is the full valence limit, where all bonding and an equal number of virtual orbitals are included in the active space, and no approximation is made in selecting the important valence orbitals or electrons. While conventional algorithms require exponential computational time to evaluate full valence CASSCF, this article shows that the method of increments can do the same with polynomial effort, in a new method denoted iCASSCF. The method of increments can also provide density matrices and other necessary ingredients for the construction of the nuclear gradient. These goals are met through a many-body expansion that breaks the problem into smaller pieces that are subsequently reassembled to form close approximations of conventional CAS results. Practical demonstrations on a number of medium-sized molecules, with up to 116 valence electrons correlated in 116 orbitals, show the power of this methodology.