David Yarkony is supported by an award from the Theory, Models and Computational Chemistry program in the Chemistry Division to develop a novel method of treating conical intersections on potential energy surfaces. Characterization of the electronic potential energy surfaces, including conical intersections and inter-state interactions is essential to the successful simulation of nonadiabatic processes. The incorporation of conical intersections into "fit" potential energy surfaces is a challenging task. In the commonly used nonadiabatic dynamics method known as ab initio direct dynamics or dynamics on the fly, this problem is circumvented by obtaining the electronic structure information directly from ab initio electronic structure calculations as required. At present, given the intrinsically high cost of these electronic structure calculations, in ab initio nonadiabatic direct dynamics, one is routinely forced to trade off accuracy of the electronic wave functions for speed of evaluation. The approach developed in the PI's research group avoids this bottleneck, obtaining the electronic structure data from a non-local quasi-diabatic Hamiltonian Hd (the 'fit' Hamiltonian) that reliably approximates the high quality ab initio electronic structure data used to construct it. This Hd correctly locates conical intersection seams, local minima, saddle points, and dissociation asymptotes, and reproduces the local topography of a conical intersection.
Nonadiabatic processes in which molecules change their electronic states radiationlessly, that is, without absorbing or emitting a photon, are intimately involved in such essential areas as energy storage, vision and photochemistry. Once routinely dismissed as an arcane theoretical notion, conical intersections, i.e. points of intersection of two (or more), electronic potential energy surfaces with the topography of a double (or higher order) cone, are now understood to play a key role in nonadiabatic processes.
The Hd based electronic structure apparatus is being interfaced into existing direct dynamics procedures and made available to a broad scientific community. It is of particular interest to photoelectron spectroscopists. Photoelectron spectroscopy is a valuable tool in accessing nonadiabatic interactions in molecules. Both undergraduate and graduate students take part in this research project, gaining valuable training in developing and implementing sophosticated methods in computational chemistry.
Chemical processes are often pictured as marbles moving on a 2-dimensional surface in 3-dimensional space. For big energy processes, including photosynthesis , vision, solar energy conversion, and DNA base stability, this notion must be generalized. In this case, as the result of absorbing light, the marbles start on a higher surface. A key issue is how they get back to the lower surface and where they end up on that surface, that is, what molecule is made. This is the NUCLEAR MOTION PROBLEM. If the light is provided by a laser, the task at hand may be to control the reaction product. If the light comes from the sun, energy conversion, photosynthesis or DNA damage may be the issue. In all these cases the passage from the upper surface to the lower surface involves funneling by a double cone-like structure ( two cones joined at their vertices) called a conical intersection. See attached figure The mathematical description of these so-called NONADIABATIC processes requires detailed knowledge of the surfaces and the connecting cones. The more accurate the representation, the more likely the simulation will correspond to reality. One avenue of my NSF funded research focuses on the representation of the surfaces and connecting cones. Our approach provides much more accurate representations of the surfaces than had previously been available and enables use of more accurate methods to solve the nuclear motion problem. Initial studies of vibrationally mediated photodissociation – an example of laser control of chemical reactions- for the simple molecule ammonia (NH3) document the utility of our approach.
