ISDF
ISDF stands for interpolative separable density fitting, a numerical technique used in quantum chemistry to approximate electron repulsion integrals (ERIs). The method seeks a low-rank, separable representation of the four-index ERI tensor by introducing a small set of interpolation points and associated interpolation vectors. In practice, products of basis functions are approximated using information evaluated at these points, allowing the ERIs to be expressed as a sum over a modest number of auxiliary terms. This reduces both memory requirements and computational cost compared with conventional density fitting schemes, enabling more efficient HF, DFT, and post-HF calculations for larger systems or higher-accuracy treatments.
ISDF yields near-linear scaling with system size under common implementations and complements other acceleration techniques such