Princeton University

In the past decades, people have used many kinds of classical and empirical methods to study mechanical properties of material. However, each such method was based on some severe assumptions and approximations. In order to obtain mechanical properties in great detail and accurately, first-principles calculations are needed. Density functional theory is a good choice for this job, but its computation cost for metal scales cubically with the system size in most algorithms, and a dense k-point sampling is needed. In my project, I use orbital free density functional theory (OFDFT) which scales linearly with the system size and eliminates k-point sampling. By using OFDFT, accurate and large scale quantum mechanical simulations of materials are made possible. (Back)

Although quantum mechanics can reliably describe materials, the methods are computationally expensive and can only model materials up to the nanometer scale. To accurately study a macroscopic material, a multiscale model is needed. One such model is the quasicontinuum method, which links continuum mechanics with an atomic-resolution model, using a finite element framework. I am developing new implementations of orbital-free density functional theory in the quasicontinuum method. (Back)

Photocatalysts can be used for solar fuel production by inducing water splitting to produce H₂ and O₂ or by catalyzing CO₂ reduction by water to produce O₂ and small hydrocarbons. We will use quantum approaches to design new affordable materials with optimal photocatalytic properties, such as correct band gap width and position and long excited state lifetimes. New materials studied will be biologically motivated, by looking at complexes used in nature to catalyze related reactions and choosing similar metal oxides or sulfides. The challenge lies in choosing the correct theoretical technique for predicting different properties. Ground state properties will be determined using Density Functional Theory (DFT), or an ab initio DFT+U approach if appropriate for the material. Localized excited electronic states will be calculated using our embedded configuration interaction (ECI) theory, and delocalized excited states will be calculated using the Green’s functions techniques of the GW approximation and the Bethe-Salpeter equation. ECI will also be used to calculate the chemistry of the active site, by modeling the charged cluster and determining the barriers and rate constants for redox reactions. With this approach, we should be able to use quantum mechanics to predict a photocatalytic material’s excitonic states and describe subsequent chemical reactions.  (Back)

Theory is needed to deepen the insight into observed relationships between the function and structure of materials thereby allowing for rational design of desirable features. On the atomic level, quantum calculations offer the ability to explain phenomena that manifest in macroscopic properties. With this in mind, an accurate treatment of the optical properties of transition metal oxides and sulfides is sought for both the challenges it poses to computational methodology and for its application to the design of more efficient photovoltaic and photocatalytic materials.  (Back)

Iron oxides (FeO, Fe₂O₃, Fe₃O₄) are products of corrosion of steel. Finding ways to simulate their behavior to partner with experiments is critical for developing techniques to stop corrosion.However, as they belong to the group of 3d late transition metal oxides, they are not well treated by Density Functional Theory (DFT). This is because the iron ions have partially filled d-shells, with 3d electrons inherently tightly localized on the iron atoms. When applying DFT to this system, artificial self-interaction between localized electrons are not correctly cancelled out by the approximate exchange term, which leads to the wrong prediction of a metallic ground state, as in the case of FeO. Therefore, a modified scheme called DFT+U is adopted. In DFT+U, the Hamiltonian is parameterized by two parameters U and J that will favor integer occupation for the d states. We use an ab initio approach to calculate these U and J parameters. This method is promising in giving a reasonable and fully ab initio description of iron oxides. (Back)

Solid oxide fuel cells (SOFCs) are electrochemical devices which potentially may help to meet our energy demands in the future. A SOFC directly converts the energy stored in fuels such as hydrogen and light hydrocarbons into electrical energy. The factor that limits the performance of existing SOFC devices is the kinetics of the oxygen reduction reaction occuring at the cathode. The oxygen molecules in the gas phase must react to become oxygen ions in the bulk of the solid oxide electrolyte. This reaction involves a complex set of steps with generally undetermined kinetics including adsorption, diffusion and reduction rates. The goal of our research is to use ab initio quantum mechanical methods to understand how material composition determines the reaction rates of the oxygen reduction reaction (and in turn the cathode performance). Through understanding how the best existing materials operate, we will seek to find materials which improve SOFC cathode performance. The results of the research will serve as the starting point for higher level modeling including kinetic monte carlo methods. (Back)

In the past decades scientists have been trying to develop both fast and accurate methods to study materials. However, conventional first principles electronic structure methods are still time consuming even for a few hundred atoms and empirical potentials are not transferable enough to investigate systems outside the regime to which they were fit. In order to study properties of materials both quickly and accurately, OF-DFT (Orbital-Free Density Functional Theory) can be a satisfying choice combining the accuracy of DFT with the computational efficiency of the OF approach. I am studying mechanical and structural properties of aluminum and magnesium alloys for lightweight vehicles applications with OF-DFT as well as the theory itself.  (Back)