We have developed and validated a new force field for cations in zeolites, which explicitly distinguishes Si and Al atoms, as well as different types of oxygens in the framework. Our new force field gives excellent agreement with experimental data on cation positions, site occupancies and vibrational frequencies. Energy minimizations show that Na cations in site I are not at the centers of hexagonal prisms, but rather are in one of two symmetric S-I sites displaced by about 0.6 Angstrom along the [111] direction. Molecular dynamics (MD) simulations show that most cations are immobile in Na-X and Na-Y on the MD time scale, even at 1000 K. Only Na-X cations in site III' exhibit diffusive motion at 1000 K, with a self-diffusivity from MD of 3.6 x 10(-10) m(2) s(-1). The MD simulations also show that cation movement is highly correlated, composed of jumps involving at least 4 cations or more.