COMPARISON OF 3 BROWNIAN-DYNAMICS ALGORITHMS FOR CALCULATING RATE CONSTANTS OF DIFFUSION-INFLUENCED REACTIONS

A new algorithm for calculating the rate constants of diffusion-influe nced reactions from Brownian-dynamics simulations is introduced and co mpared with two previous algorithms. It is based on the mean residence time of the pair of reactant molecules in the reactive region after t he molecules are started from that region. Of the previous algorithms, one is based on the capture probability of one reactant molecule star ted on a spherical surface enclosing the other reactant molecule [Nort hrup et al., J. Chem. Phys. 80, 1517 (1984)]; the other is based on th e survival probability of the pair of reactant molecules started in th e reactive region [Zhou, J. Phys. Chem. 94, 8794 (1990)]. In the imple mentation of the residence-time based algorithm, analogy can be drawn between diffusion-influenced bimolecular reactions and diffusive energ y-barrier crossing processes. When the reactive region is small, the p air of reactant molecules will be near the reactive region even after many multiples of the mean residence time have elapsed. Hence the resi dence time in the reactive region will not be significantly affected b y the presence of an interaction potential U if the potential is smoot h around the reactive region. This rationalizes an earlier analytic re sult k = k(0)[exp(-U/k(B)T)], where k and k(0) are the rate constants in the presence and absence of the potential. The three simulation alg orithms are applied to the binding of a pointlike ligand to an immobil e sphere with a reactive patch in the presence and absence of a Coulom b potential. The survival-probability based algorithm is always the mo st accurate and efficient one. (C) 1998 American Institute of Physics.