Theoretical and Computational Chemistry
The interaction of light and matter gives rise to many fascinating phenomena. Some of these are very poorly understood at present but are exceedingly important in a wide variety of fields across chemistry, physics, biochemistry, new materials, and medicine. Our research is based on theoretical and computational investigations of electronically excited states, including both spectroscopy and photochemistry, from high accuracy studies of small molecules, to supramolecular sensors, and photoactive molecules of biological importance. Such research is vital to understand these processes at the molecular level.
1. Linear and Non-linear Photodynamic Therapy
Medical treatments involving light naturally fall into three classes: phototherapy (direct action of light), photochemotherapy (action of light on a drug molecule), and photodynamic therapy (PDT) (action of light on a sensitizer molecule, which in turn creates singlet molecular oxygen that can induce apoptopic cell death). PDT has emerged as a most promising medical technique in recent years. Much interest is in non-linear excitation schemes for PDT as they offer greater tissue transparency and spatial resolution. A schematic Jablonski diagram of the main steps involved in PDT is shown below. Computational modeling of these helps in the rational design of new advanced PDT agents.
Figure 1. Schematic Jablonski type diagram showing the main steps involved in PDT, from initial linear (one-photon) or non-linear (two-photon) excitation, internal conversion, intersystem crossing, intermolecular energy transfer, to the activation of singlet molecular oxygen.
2. Inorganic Photochemistry
Computational photochemistry of transition metal complexes is often much more demanding than their organic counterparts due in part to the larger density of states, greater degree of degenerate and quasi-degenerate states, and relativistic effects. We model the spectroscopy of such complexes with coupled cluster and density functional response theory, while we model the reactive potential energy surfaces via multi-reference wavefunction techniques. Shown below is a schematic of coupled potential energy surfaces, which are a common feature of this photochemistry, and allows for ultrafast processes (femtochemistry) involving a coupling of the nuclear and electronic degrees of freedom.
Figure 2. Schematic potential energy surfaces for an ultrafast inorganic photochemical reaction. The system is excited into a manifold of states of different electronic character; a photochemical reaction then occurs, followed by relaxation through a conical intersection.
3. New Methods for Electron Correlation
The correlated many-electron problem is at the heart of quantum chemistry. Further developments will lead to more accurate wavefunctions for larger systems from which we can obtain an array of molecular properties. We are interested in developing and applying new techniques that deal with both static (near-degeneracy), and dynamic (instantaneous repulsion) correlation in a consistent manner. We have particular interest in stochastic Monte-Carlo methods to generate high-accuracy wavefunctions without any a priori orbital partitioning required.
Figure 3. Convergence of the Monte-Carlo Configuration Interaction method for both the total energy and electric dipole moment of carbon monoxide. Essentially exact results are obtained at a fraction of the computational cost of more standard methods.