We are interested in electronically nonadiabatic processes, processes in which the Born-Oppenheimer approximation fails and a molecular wave function can transition radiationlessly between potential energy surfaces. Although the Born-Oppenheimer approximation is fundamental to the way we think about and quantify virtually all of chemistry, its breakdown, rather than being a rare event is quite common, when an electronically excited state is involved. Since electronically excited states are routinely formed when visible and higher energy photons are absorbed nonadiabatic processes are of fundamental importance in such diverse areas as vision, photosynthesis, solar energy conversion, photochemistry and the photo stability of biomolecules.

The Born-Oppenheimer approximation fails in dramatic fashion in the vicinity of conical intersections, which represent the intersection of two (or more) Born-Oppenheimer potential energy surfaces with the topography of a double cone. Once thought of as an archane theoretical notion largely restricted to molecules with point group symmetry, as for example in the Jahn-Teller effect, due to work reported in the past two decades conical intersections that do not involve point group symmetry are now understood to be ubiquitous. Over that period we have developed a computationally oriented, formal description of conical intersections, including extensions to intersections of 3-states and intersections where the spin-orbit interaction cannot be neglected.

Matsunaga, Nikita and Yarkony, David R. Energies and derivative couplings in the vicinity of a conical intersection. 3. The ‘most’ diabatic basis. Molecular Physics. 1998, 93(1), 79-84.

Matsika, Spiridoula and Yarkony, David R. Accidental conical intersections of three states of the same symmetry. I. Location and relevance. Journal of Chemical Physics. 2002, 117(15), 6907-10.

Matsika, Spiridoula and Yarkony, David R. Spin-Orbit Coupling and Conical Intersections. IV. A Perturbative Determination of the Electronic Energies, Derivative Couplings, and a Rigorous Diabatic Representation near a Conical Intersection. The General Case. Journal of Physical Chemistry B. 2002, 106(33), 8108-16.

##### Some relevant review articles include:

Yarkony, David R. Diabolical conical intersections. Reviews of Modern Physics. 1996, 68(4), 985-1013.

Yarkony, David R. Conical Intersections: Diabolical and Often Misunderstood. Accounts of Chemical Research. 1998, 31(8), 511-8.

Our current research objectives focus on how nonadiabatic interactions induced by conical intersections are reflected in dynamical process and measured spectra. Another major area of interest is the electronic structure and dynamical aspects of nonadiabatic photodissociation mediated by conical intersections. We have considerable expertise in the electronic structure aspects of this problem. We have extended our method for determining quasi-diabatic coupled electronic state Hamiltonians for bound molecules to handle dissociating systems.

Zhu, Xiaolei and Yarkony, David R. Toward eliminating the electronic structure bottleneck in nonadiabatic dynamics on the fly: An algorithm to fit nonlocal, quasidiabatic, coupled electronic state Hamiltonians based on ab initio electronic structure data. Journal of Chemical Physics. 2010, 132(10), 104101.

Zhu, Xiaolei and Yarkony, David R. On the Representation of Coupled Adiabatic Potential Energy Surfaces using Quasi-Diabatic Hamiltonians: Description of Accidental Seams of Conical Intersection. Molec. Phys. 2010, 108(19-20), 2611-19.

We are currently preparing for publication full 6-dimensional coupled potential energy surfaces for the 1,21A states of NH3. One of the major goals for our research is to extend the range of molecules accessible to this approach and use it to construct highly efficient dynamics on the fly methodologies.

One of the uses of the methodological tools being developed in our group is the calculation of photodetachment spectra. In these calculations ab initio determination of the electron photodetachment cross sections, basically the transition moments to the individual diabatic electronic states for the continuum electron, is important. Despite its importance, this aspect of the problem, a challenging exercise in nonadiabatic electron scattering theory, has received little attention. We have developed a unique, fully nonadiabatic formalism to treat this electron scattering problem, which builds on our above described spectral simulation tools, takes full account of the nuclear kinetic energy, and exact account of electron exchange.