We present an efficient Monte Carlo algorithm for simulating diffusion in tight-fitting host-guest systems, based on using zeolite normal modes. Computational efficiency is gained by sampling framework distortions using normal-mode coordinates, and by exploiting the fact that zeolite distortion energies are well approximated by harmonic estimates. Additional savings are obtained by performing local normal-mode analysis, i.e., only including the motions of zeolite atoms close to the jumping molecule, hence focusing the calculation on zeolite distortions relevant to guest diffusion. We performed normal-mode analysis on various silicalite structures to demonstrate the accuracy of the harmonic approximation. We computed free energy surfaces for benzene in silicalite, finding excellent agreement with previous theoretical studies. Our method is found to be orders-of-magnitude faster than comparable Monte Carlo calculations that use conventional forcefields to quantify zeolite distortion energies. For tight-fitting guests, the efficiency of our new method allows flexible-lattice simulations to converge in less CPU time than that required for fixed-lattice simulations, because of the increased likelihood of jumping through a flexible lattice. (C) 2003 American Institute of Physics.