We propose a statistical-mechanical methodology to estimate 'slow' diffusivity from conventional molecular dynamics (MD) calculations. In this method, the temperature dependence of the Helmholtz free energy profiles is predicted from one MD simulation at a particular temperature. These profiles are used to estimate the diffusivity, combined with transition state theory. We apply this methodology to molecular diffusion in a zeolite nanopore. Our methodology promises the efficient estimation of molecular 'slow' diffusivity in nanopores, e.g., values less than 10 (14) m(2)/s. (c) 2008 Elsevier B. V. All rights reserved.