Database-assisted local unitary transformation method for two-electron integrals in two-component relativistic calculations

This letter presents an efficient algorithm for local unitary transformation based on the spin-free infinite-order two-component relativistic method for the two-electron interaction, which is assisted by one-center relativistic two-electron integral (TEI) database. The database stores a set of TEIs, one for each element-basis set combination. The algorithm is numerically assessed for hydrogen halide chains, (HX)n (X = Cl and At), Aun, Ir(ppy)3, Pt3(C7H7)2(HCN)3, and PtCl2(NH3)2. The computational cost (time and memory size) at the Hartree-Fock level is lower than that of the conventional method, especially for small and medium-sized molecules.