Spectral approximation scheme for a hybrid, spin-density Kohn-Sham Density-Functional Theory in an external (nonuniform) magnetic field and a collinear exchange-correlation energy

We provide a mathematical justification of a spectral approximation scheme known as spectral binning for the Kohn-Sham spin density-functional theory in the presence of an external (nonuniform) magnetic field and a collinear exchange-correlation energy term. We use an extended density-only formulation for modeling the magnetic system. No current densities enter the description in this formulation, but the particle density is split into different spin components. By restricting the exchange-correlation energy functional to be of a collinear LSDA form, we prove a series of results which enable us to mathematically justify the spectral binning scheme using the method of Gamma-convergence, in conjunction with auxiliary steps involving recasting the electrostatic potentials, justifying the spectral approximation by making a spectral decomposition of the Hamiltonian and ``linearizing" the latter Hamiltonian.