A new quantum transition state theory

An old challenge in rate theory is the formulation of a quantum thermodynamic theory of rates which gives accurate estimates but does not demand any real time propagation. In this paper we attempt to answer the challenge by extending an idea suggested by Voth, Chandler and Miller [J. Phys. Chem. 93, 7009 (1989)]. A new quantum expression for the rate is derived by replacing the exact time dependent dynamics with the analytically known dynamics of a parabolic barrier and utilizing the symmetrized thermal flux operator.The new rate expression is exact for a parabolic barrier, and leads by derivation rather than by ansatz to a:phase space integration of a Wigner thermal flux distribution function. The semiclassical limit is similar but not identical to Miller's semiclassical transition state theory. Numerical computations on the symmetric and asymmetric one dimensional Eckart barrier give results which are equal to or greater than the exact ones, as expected from a transition state theory. In contrast to other approaches, the present theory is a leading term in an expansion which may be used to systematically improve the results and assess their validity. (C) 1998 American Institute of Physics.