Computational efficiency is very important for quantum chemistry in order treat molecules that are as large as possible for given computational resources (computer speed, memory size, etc). Our AB2 implementation uses exact 4-center integrals in a basis of Gaussian-type atomic orbitals (other alternatives such as using auxiliary basis expansions can also be readily implemented). Each step with its computational complexity is shown in Fig. . Note that for the figure and the discussion here we use O, V , and N for the number of bonding orbitals, virtual orbitals, and AO basis functions, respectively. We start by making a pseudo-density