Research

In short, I am committed to developing fast and accurate computational theories and methods, which can be used to accurately describe the properties of mesoscale systems. I am also interested in theoretical modeling of condensed matter phenomena. Therefore, I am mainly working on four directions:

1. Velocity-Gauge Real-Time DFTB

We first derived and implemented a new velocity-gauge real-time, time-dependent density functional tight-binding (VG-rtTDDFTB) method in the open-source DFTB+ software package for probing electronic excitations in large, condensed matter systems. Our VG-rtTDDFTB approach enables real-time electron dynamics simulations of large, periodic, condensed matter systems containing thousands of atoms with a favorable computational scaling as a function of system size. Taken together, our VG-rtTDDFTB approach enables new electron dynamics simulations of complex systems that require large periodic supercells, such as crystal defects, complex surfaces, nanowires, and amorphous materials.

• Theoretical Development. See Ref.Q. Xu et al., JCTC (2023)
• Ehrenfest Dynamics. See Ref.Q. Xu et al., JACS (2024)

2. Orbital-Free DFT (Read more »)

Orbital-free density functional theory (OFDFT) is an electronic structure method with a low computational cost that scales linearly with the number of simulated atoms, making it suitable for large-scale material simulations (1-100 million atoms, 1-100 nm). However, the accuracy of OFDFT simulations depends strongly on the quality of the non-interacting kinetic energy and the electron-ion (or electron-pseudocore) interaction energy employed in the simulations. In the past few years, we derived the advanced kinetic energy density functionals for OFDFT calculations. Furthermore, we first proposed the nonlocal pseudopotential method in this field, which defies the belief that nonlocal pseudopotentials are not applicable to OFDFT, leading to the creation for an alternate theoretical framework of OFDFT that works superior to the traditional approach.

• Kinetic Energy Density Funtional (KEDF)
  • Xu-Wang-Ma (XWM) functional for periodic systems. See Ref.Q. Xu et al., PRB (2019).
  • LDAK-X functionals for isolated systems. See Ref.Q. Xu et al., PRB (2020).
• Nonlocal Pesudopotential Energy Density Functional (NLPPF).
  • For metals: See Ref.Q. Xu et al., NC (2022).
  • For semiconductors: See Ref. C. Ma et al., PRB (2024).
• Free Energy Density Funtional (FEDF)
  • XWM-FEDF (XWMF) functional for warm dense matter. See Ref.C. Ma et al., PRB (2024).

3. Kohn-Sham DFT

We presented a Kohn-Sham DFT based calculation package (ARES), utilizing real-space finite-difference method, advanced Chebyshev subspace filtering iteration solver, versatile non-periodic/periodic boundary conditions, and highly parallelizable features making the simulation of large-scale systems possible.

• ARES Development. See Ref.Q. Xu et al., JPCM (2019)
  • Massively parallel is on going
• Real-Space Pesudopotential Core-Level Binding Energies Calculation Method (CEBE). See Ref.Q. Xu et al., JCTC (2022).

4. Theoretical Models

• Mechanism of superconductivity in molecular hydrides. See Ref.Liu et al., Sci. Adv. (2025)