Density fitting of two-electron integrals in local multireference single and double excitation configuration interaction calculations

Density fitting of the two-electron four-centre integrals accelerates the speed of electronic structure calculations. In this work, we evaluate the performance of using auxiliary fitting functions generated on-the-fly from the underlying atomic orbital basis within the atomic compact Cholesky decomposition (acCD) method in configuration interaction (CI) calculations. The fitting functions, in addition to being free from inherent bias towards any quantum chemical setup, can be systematically tuned via the decomposition threshold within the acCD procedure. Results of using acCD within a local multireference single and double excitation CI (LMRSDCI) method are presented. The acCD-LMRSDCI method is able to retain the accuracy of LMRSDCI for energy differences (but not for absolute energies) at a substantially reduced cost.