The Yang Group

On the unifying basis of quantum mechanics for almost all things, the Yang research group uses theoretical and computational quantum chemistry tools for studying problems in diverse chemistry disciplines where we are interested in discovering how electrons move around and cooperate with surroundings and light in molecules and materials. We look at phenomena in inorganic (transition metal compounds) and organic chemistry (conjugated polymer), solar materials (photovoltaic or optoelectronic materials), as well as pharmaceutical sciences (drug molecular crystals). An important challenge common to many of these systems is the quantum mechanical nature of electronic and nuclear many-body motions across vast length and time scales. To this end, we therefore develop and use scalable emerging electronic structure and dynamics computational methods to simulate molecular and materials systems in complex environment. Specific research themes include various topics in the following.

New scalable electronic structure methods for excited states of complex system

A major difficulty for treating excited complex systems arises from the different nature of the various competing excited electronic states, for example, localized neutral and delocalized charge transfer excitons, as a result of the relatively large length scale. Many theoretical methods aimed at targeting excited states can “in principle” deal with excitons (electron-hole pairs): density functional theory (DFT, with hybrid functionals), quantum Monte Carlo, Green’s Function methods, Coupled-Cluster, and so on, in a time independent or time-dependent framework. However, due to self-interaction error, DFT does not capture the correct 1/r asymptotic behavior of the electron-hole pair and thus requires special ad hoc corrections to properly treat excitonic structure and migration. Other many-body approaches are typically limited to small systems since the computational cost increases so rapidly with the system size. Reduced scaling local correlation methods require the mean-field (e.g., Hartree-Fock) calculations of the whole system as prerequisite, and the determination of appropriate orbital domains for excited states is not as trivial as that for the ground state.

Our group develops and implements novel electronic structure algorithms and scalable computational methods for excited states of complex system. Here we employ the bottom-up strategy by taking advantage of high level molecular electronic structure theories, with new idea drawn from other fields of fragment-based quantum chemistry, quantum embedding, renormalization group, statistical physics and machine learning algorithms.

Modeling excitons in light-harvesting systems

Photochemically active materials are critical for applications in renewable energy, and are also of fundamental interest at the interface between physics, chemistry, and the life sciences. However, light conversion systems are poorly understood either experimentally or theoretically since photochemical processes therein are particularly complex which involve multiple intrinsically quantum phenomena: chemical bond breaking and making; electronic excited states coupled with nuclei; and environmental quantum dissipation.

In this program, we study the electronic structure of excitonic states during the initial photochemical steps and several important mysteries, such as the mechanism for how bound electron-hole pairs overcome the Coulomb barrier leading to efficient charge separation. We utilize both semi-empirical models and ab-initio simulation techniques for revealing these mysteries on a many-body basis, and seek answers to key open questions, such as the role of correlated electrons, excitonic structure, charge and energy transfer, as well as the polarization of complex environment, for harnessing better light-harvesting materials.

Light-driven molecular transformation

Light-driven molecular transformation regulates light energy transduction in many ways. An important example is the electron-nucleus coupled motion that is the mechanism behind photochemical reactivity. Other examples of the significance for light energy transduction include non-radiative processes, ranging from photobiology such as the non-photochemical quenching protection of plants and algae from excessive sunlight intensity, the primary human vision step commencing with the photoinduced isomerization of retinal chromophore in rhodopsin, to materials such as molecular switches, light-powered engines, etc.

D. Polli, et al., Conical intersection dynamics of the primary photoisomerization event in vision, Nature 2010, 467, 440-443

As a group of theoretical chemists, we are particularly interested in researching effective computational description for non-adiabatic transitions and dynamics within a variety of correlated quantum chemistry approaches (Coupled-Cluster, DMRG-SCF, DMRG-NEVPT2, etc) together with other strategies in exploring new wavefunction basis and transformation, transition properties, reaction pathways and conical intersections of coupled electronic states.

Organic molecular materials: energy modeling, structure and polymorphism

One of the greatest challenges in the chemistry of organic molecular materials is to have a better control of materials structure and polymorphism. Given the tiny energy difference between two polymorph structures, it is highly likely that a molecule crystalizes into distinct competing crystal structures. For example, BEDT-TTF, a well-known organic charger transfer salt, can be packed as divergently as metal, insulator or superconductor, exhibiting drastically different collective electronic transport properties. Engineering a crystal in a wrong structure is also a serious problem found in other areas such as pharmaceutical industry (eg, drugs for treating HIV-I and Parkinson’s disease).

In our continued efforts, we develop accessible and accurate ab-initio energy models that allow distinguishing and even predicting the morphology structures of these materials. Our key methodology ingredients combine together low-scaling techniques, fragment expansion method, analytical nuclear gradient, free energy approach and accurate description of long-range interactions (electrostatic and dispersion). We are also interested in developing molecular design principles for energy storage, electronic devices and solar materials.