In this section, we document the cost for XSAPT calculations and how it scales with system size. All calculations were performed using the Q-Chem program, which remains the only platform in which XSAPT is implemented. Timing benchmarks were performed using a pre-release copy of Q-Chem v. 6.3, in order to document improved multithreading for conventional SAPT0 calculations. The resolution-of-identity (RI) technique leads to considerable reduction in the cost of this O(N 5 ) method and the SAPT0 timings reported in Section 2.2.1 use this technique. Additional parallelization improvements were made in Q-Chem v. 6.0, as documented in Fig. , so that multithreaded performance should be significantly improved as compared to Q-Chem v. 5.4. Further improvements in parallel efficiency since that release are documented in Section 2.2.1. These timing benchmarks use an SCF convergence threshold of 10 -5 E h in the electronic energy gradient FP -PF , 182 and a screening threshold of 10 -12 a.u.. (The latter is appropriate for medium-size systems with diffuse basis functions. 183 ) Timing benchmarks are run on dedicated nodes.