My research program is centered around understanding the electronic structure of molecules and solids and presently we are studying three broad areas.
The Electronic and Geometric Structure of Diatomic and Small Polyatomic Molecules Containing a First Transition Series Element. In these studies we use MCSCF and multireference CI techniques to investigate the bonding between a transition metal (either Mdeg., M+ or M++) and a main group atom or group of atoms. In particular we construct our wavefunctions to incorporate 1) the atomic correlation and spin couplings necessary to insure the asymptotically correct metal atoms/ions limits and 2) the molecular correlation necessary to describe continuously and accurately the formation of the bond to the metal atom/ion. Considerable effort is expended in extracting from the calculation qualitative insights into the nature of the bonding in these materials. We believe that these insights are essential to assigning (reliable) geometric and electronic structures to the intermediates and products of the reactions of metal atoms and ions with organic molecules.
Electronic and Magnetic Structure of Transition Metal Ions in Oxide Superconductors and other Complex Layered Oxides. In these studies we are interested in the crystal field splitting, ordered antiferromagnetic moment and magnetic form factor of La2CuO4, and YBa2Cu3O6, the insulating parents of the high-Tc superconducting oxides, as well as other complex layered oxides such as La2NiO4 and some related fluorides. We are using ab-initio MCSCF as well as CI calculations of various clusters to simulate the electronic structure of the extended solid. Madelung effects as well as Pauli repulsion effects on the cluster are incorporated via structureless point charges and effective core potentials respectively. We have recently completed applying this model to the NiF6-4 cluster in KNiF3 using a very large basis set and incorporating electron correlation via a MCSCF calculation. Our results suggest that calculations at the MCSCF level can adequately describe these systems.
Electronic Structure of Electrides. The goal of this research is to model the electronic structure of the Mott-insulating solid electrides Cs(18C6)2 and Cs(15C5)2. Initially we have focused on the electronic structure of the basic molecular units in these crystals and have studied Li(9C3)2 in considerable detail. As the two 9C3 crown molecules encapsulate the neutral Li atom, the 2s electron on Li becomes compressed and eventually redistributes itself into a Rydberg like orbital whose spatial extension is primarily external to the encapsulating crown molecules. This is a remarkable structure for a ground state molecule and strongly suggests that when a crystal of these molecules is constructed, this very diffuse, polarizable outer electron will undergo large distortion and may be forced into trap sites as proposed by Dye et. al. We are now exploring various models of the electronic structure of crystalline electrides.