Ultrafast full-accuracy QM/MM Monte Carlo simulation of molecules in polar solvents

by Janowski, Tomasz; Pulay, Peter

The std. MC approach to simulate mols. in polar solvents is to use Quantum Mechanics for the solute and Mol. Mechanics for the solvent. The long range electrostatic interaction requires the recalcn. of the solute QM wavefunction for each solvent configuration, making this method computationally extremely expensive. The first order interaction energy can be evaluated efficiently by a distributed multipole expansion (Yang and Thiel groups) or exactly (Tachikawa and Shiga). To compute the second-order (polarizability) contributions, we precalc. generalized polarizabilities using a Fourier expansion of the solvent elec. potential. This gives a very fast and accurate method. E.g., for guanine in TIP3P water the av. error compared to full RHF/6-31G** calcns. is only -0.08±0.05 kcal/mol while the first order approxn. alone gives an error of +5.6±1.26 kcal/mol. Speedups compared to quantum calcns. are large, 3000 in this example, and increase with the quality of the quantum calcn.