MuAPBEK: An Improved Analytical Kinetic Energy Density Functional for Quantum Chemistry

Abstract: Orbital-free density functional theory (OFDFT) offers a true realization of the Hohenberg-Kohn theorems, enabling full quantum-mechanical studies of electronic systems based solely on electron densities. However, OFDFT remains limited by the difficulty of formulating accurate kinetic-energy density functionals. In this paper, we substantially enhance the accuracy of OFDFT energies and densities by tuning, during density initialization, the parameter $\mu$ of the APBEK functional, which arises in the second-order gradient expansion of the kinetic energy for semiclassical neutral atoms. We augment this parameterized APBEK functional with two physically-motivated, non-empirical corrections derived from Kato's cusp condition and the virial theorem. The resulting functional, which we call MuAPBEK, is benchmarked against Kohn-Sham density functional theory (KSDFT) on atoms, organic molecules from the QM9 dataset, and the anti-malarial drug artemisinin. MuAPBEK achieves much lower energy errors than standard APBEK and Thomas-Fermi-von-Weizsacker functionals, even when the latter two are evaluated on converged KSDFT densities. Its mean absolute energy errors on atoms and molecules are 161 and 122 kcal/mol, respectively, indicating that MuAPBEK's errors do not scale with system size. MuAPBEK also yields accurate densities, with a mean integrated absolute density error of 1.8 electrons for molecules. Importantly, one step of our density optimization scheme is at least ten times faster than a single KSDFT self-consistent field cycle and exhibits a lower-order computational time complexity of $O(N^{1.96})$ with respect to system size, $N$. Our results indicate that highly-accurate OFDFT for large-scale quantum simulations beyond the practical limits of KSDFT is within reach.