Effective Approximations for Hartree-Fock Exchange Potential

The Hartree-Fock exchange potential is fundamental for capturing quantum mechanical exchange effects but faces critical challenges in large-scale applications due to its nonlocal and computationally intensive nature. This study introduces a generalized framework for constructing approximate Fock exchange operators in Hartree-Fock theory, addressing the computational bottlenecks caused by the nonlocal nature. By employing low-rank decomposition and incorporating adjustable variables, the proposed method ensures high accuracy for occupied orbitals while maintaining Hermiticity and structural consistency with the exact Fock exchange operator. This low-rank approximation constitutes the core contribution of the presented study. Meanwhile, a two-level nested self-consistent field iteration strategy is developed to decouple the exchange operator stabilization (outer loop) and electron density refinement (inner loop), significantly reducing overall computational costs. Numerical experiments on several molecules demonstrate that the approximate exchange operators achieve near-identical energies compared to that of the exact exchange operator and the NWChem references with substantial improvements in computational efficiency.