Figure : The walltime on 32 cores for constructing the CIS state overlap matrix Φ J Φ K ′ using Method #1 and #2 and SHARC is plotted in a logarithmic scale as a function of the number of electronic states. In general, we find that Method #2 (red) is at least 2 orders of magnitude faster than Method #1 (blue). For the SHARC routine, the overlap matrix is constructed by predetermining a certain percentage of the determinants to be evaluated with Eq. ( ). Here we denote SHARC 0.9 (0.99) for the results where 90% (99%) of the determinants are evaluated. For N = 256 states, Method #2 can be more than 3 orders of magnitude faster than SHARC 0.99. Recall that Method #2 is exact.