Orbital-free density functional theory study of amorphous Li-Si alloys and introduction of a simple density decomposition formalism

We propose a simple density decomposition formalism within orbital-free (OF) density functional theory (DFT) based on the Wang-Govind-Carter-decomposition (WGCD) kinetic energy density functional (KEDF). The resulting simple-WGCD (sWGCD) KEDF provides efficient density optimization, full cell relaxation, reasonable bulk properties for various materials compared to both the original OFDFT-WGCD and the Kohn-Sham (KS) DFT values, and has various numerical benefits including more stable convergence and lower computational cost (twice as fast as the WGCD KEDF). We also study amorphous (a-) Li-Si alloys with KSDFT and OFDFT using the Huang-Carter (HC), WGCD, and sWGCD KEDFs. The a-Li-Si alloy samples are prepared with the anneal-and-quench method using NVT molecular dynamics simulations. We report structural properties, equilibrium volumes, bulk moduli, and alloy formation energies for each a-alloy. The HC, WGCD, and sWGCD KEDFs within OFDFT all predict accurate equilibrium volumes compared against KSDFT benchmarks. The HC KEDF bulk moduli agree with KSDFT benchmarks whereas the WGCD/sWGCD KEDFs generally overestimate the bulk moduli, especially for alloys with low Li concentrations. All three KEDFs show limited ability to predict alloy formation energies, which indicates the lack of transferability of these KEDFs among such systems and motivates future developments in OFDFT and KEDF formalisms.