We chose the direct approximation of polarization primarily for the sake of computational speed relative to a full SCF treatment of inducible dipoles. We compared the average speed of 15 ns NPT production simulations using a standard fixed-charge AM1-BCC model, our direct approximation AM1-BCC-dPol, and a full self-consistent mutual polarization model using the same parameters as AM1-BCC-dPol and also executed with the MPID plugin in OpenMM, using keyword "direct" and "mutual" for direct and mutual polarization, respectively. All calculations were run on the same hardware. Simulations with direct polarization average 3.6-fold slower than fixed charge simulations, while simulations with the full mutual polarization calculations, with convergence set to 10 -5 , average 4.5-fold slower. We also compared the speed of the direct approximation with the available OpenMM implementation of the empirical extrapolation scheme for efficient treatment of induced dipoles, namely the OPTn methods , for two sample systems, ethane and ethanol. As detailed in Table S5, the direct method is about twice as fast as OPT3 and 50% faster than OPT1. (Runs with OPT0, which is theoretically equivalent to the direct polarization, failed for currently unknown reasons, and hence are not included here.)