  We present an accurate and efficient finite-difference formulation and
parallel implementation of Kohn-Sham Density (Operator) Functional Theory (DFT)
for non periodic systems embedded in a bulk environment. Specifically,
employing non-local pseudopotentials, local reformulation of electrostatics,
and truncation of the spatial Kohn-Sham Hamiltonian, and the Linear Scaling
Spectral Quadrature method to solve for the pointwise electronic fields in
real-space and the non-local component of the atomic force, we develop a
parallel finite difference framework suitable for distributed memory computing
architectures to simulate non-periodic systems embedded in a bulk environment.
Choosing examples from magnesium-aluminum alloys, we first demonstrate the
convergence of energies and forces with respect to spectral quadrature
polynomial order, and the width of the spatially truncated Hamiltonian. Next,
we demonstrate the parallel scaling of our framework, and show that the
computation time and memory scale linearly with respect to the number of atoms.
Next, we use the developed framework to simulate isolated point defects and
their interactions in magnesium-aluminum alloys. Our findings conclude that the
binding energies of divacancies, Al solute-vacancy and two Al solute atoms are
anisotropic and are dependent on cell size. Furthermore, the binding is
favorable in all three cases.
