Quantum mechanics/molecular mechanics methodology for metals based on orbital-free density functional theory

We present a quantum mechanics QM/molecular mechanics MM formalism for coupling density functional theory DFT based quantum simulations to classical atomistic simulations for metals. The multiscale methodology is applicable to systems where important quantum phenomena are confined to a small region, but their impacts could be felt over much larger scales. The concurrent coupling between QM and MM regions is treated quantum mechanically via the orbital-free density functional theory OFDFT. We propose two energetic formulations for the QM region: one is based on OFDFT and the other based on the Kohn-Sham KS DFT. In the first case, the degree of freedom is the electron charge density in the QM region, and the total energy functional is directly minimized with respect to the charge density. In the second case, the degrees of freedom are KS orbitals in the QM region. An embedding potential representing the influence of the larger MM region onto the QM region is included in the KS Hamiltonian for the QM region, which is solved selfconsistently. Calculations for a perfect lattice and vacancy clusters of aluminum demonstrate that the present QM/MM approaches yield excellent results both in terms of energetics and electron density.