Fast and Accurate Electronic Structure Methods for Correlated Electronic Systems
Open shell transition metal complexes form a gigantic pool for the discovery of functional materials or catalysts. Excited electronic states are essential for photochemistry processes related to new energy and human health, including photosynthesis, biosensors, and human vision. Unfortunately, first-principle simulations for these systems are notoriously difficult because of the challenges in describing the correlated electrons both accurately and efficiently. I aim to build fast and accurate electronic structure methods for correlated electronic systems.
In the excited state, TDDFT is incapable of correctly describing the topography of potential energy surfaces at conical intersections, an essential component of studying non-adiabatic dynamics in photochemistry. The state-interaction state-averaged restricted ensemble-referenced Kohn-Sham (SI-SA-REKS) method is a computationally efficient alternative. It retains the attractive scaling features of its KS-DFT sibling, while explicitly accounting for non-dynamic correlation through the ensemble DFT formalism. I derived and implemented the analytical energy derivatives for SI-SA-REKS on GPUs to enable efficient non-adiabatic simulation of photochemistry. My implementation affords mean-field computational cost while allowing an accurate description of conical intersections. Collaborating with my colleagues, we have successfully applied this method to the non-adiabatic QM/MM dynamics study of the ultrafast photoisomerization of Channelrhodopsinand Bacteriorhodopsin. To tackle more strongly correlated electrons manifesting as a Nagaoka transition, I have recently made a vital contribution to the ab initio modeling of the quantum analog simulator as a precursor for quantum computing.
F. Liu, M. Filatov, and T. J. Martínez, Analytical Derivatives of the Individual State Energies in Ensemble Density Functional Theory Method II: Implementation on Graphical Processing Units (GPUs), Preprint chemrxiv.79856 (2019)
M. Filatov, F. Liu, K. S. Kim, and T. J. Martínez, Self-Consistent Implementation of Ensemble Density Functional Theory Method for Multiple Strongly Correlated Electron Pairs, J. Chem. Phys. 145, 244104 (2016)
M. Filatov, F. Liu, K. S. Kim, T. J. Martínez, Analytical Derivatives of the Individual State Energies in Ensemble Density Functional Theory Method. I. General formalism, J. Chem. Phys. 147, 034113 (2017)
Yu, Jimmy, Ruibinng Liang, Fang Liu, and Todd J. Martinez. “Characterization of the Elusive I Fluorescent State and the Ultrafast Photoisomerization of Retinal Protonated Schiff Base in Bacteriorhodopsin by Nonadiabatic Dynamics Simulation.” J. Am. Chem. Soc. 141 (2019): 18193.
Liang, Ruibin, Fang Liu, and Todd J. Martínez. “Nonadiabatic Photodynamics of Retinal Protonated Schiff Base in Channelrhodopsin 2.” J. Phys. Chem. Lett. 10 (2019): 2862
Electronic structure of transition metal containing molecules
In the ground state, transition metal complexes are notoriously difficult to study accurately with approximate DFT. Few systematic studies have been done for the quantification and elimination of density delocalization error (DDE), although it determines many molecular properties. I focused on understanding and controlling DDE in DFT simulations of transition metal chemistry. To quantify the DDE and its impact on chemical properties, I curate accurate reference densities with correlated wave function theory (WFT) for a broad range of transition metal compounds. With the references, I investigated the DDE reducing effects of various techniques, including DFT+U and global/range-separated hybrid tuning. Based on these method advances, I further investigated the spin-state energetics of transition metal single-atom catalysts (SACs). To determine the uncertainty of first-principle calculations in predicting the energetics of SACs, I developed accurate benchmarks from correlated WFT. These benchmarks enable the parameter tuning of DFT functional for predicting spin ordering in larger SACs. This work provides broad recommendations for modeling of open-shell transition metal SACs.
F. Liu, and H. J. Kulik “Impact of Approximate DFT Density Delocalization Error on Potential Energy Surfaces in Transition Metal Chemistry.” J. Chem. Theory Comput. (2019).
F. Liu, T. Yang, J. Yang, E. Xu, A. Bajaj, and H. J. Kulik. “Bridging the homogeneous-heterogeneous divide: modeling spin and reactivity in single atom catalysis.” Frontiers in Chemistry 7 (2019): 219.
A. Bajaj, F. Liu, and H. J. Kulik. “Non-empirical, low-cost recovery of exact conditions with model-Hamiltonian inspired expressions in jmDFT.” The Journal of chemical physics 150 (2019): 154115.