A practical way to study electron excitation computationally is to first use the Kohn-Sham density functional theory (DFT) to perform a ground state calculation, and then solve a Dyson's equation satisfied by a one-particle or two particle Green's function derived from a many-body perturbation theory or linear response analysis. This type of calculation is often computationally demanding because it requires diagonalizing a Hamiltonian that is more costly to construct and potentially much larger in dimension. I will describe how a recently developed low rank approximation technique called interpolative separable density fitting (ISDF) can be used to significantly reduce the computational cost. I will explain why low rank approximation is effective in these post-DFT calculations and demonstrate the accuracy and computational efficiency of such an approach.
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