Computer simulations of the water adsorption isotherm of zeolite Na-MAP have been performed. Both grand canonical molecular dynamics and a combination of Monte Carlo calculations and lattice relaxations (pseudodynamic simulations) have been used to account for the lattice dynamics of the zeolite framework during water adsorption. A number of different forcefields have been evaluated to find the forcefield best suited for this kind of study. It is shown that the augmented CVFF is the only forcefield which is currently able to predict water adsorption in zeolites in a reasonable and affordable manner. The water adsorption isotherm of Na-MAP can be correctly predicted qualitatively. To reach quantitative agreement with experiment the way the excess chemical potential is determined has to be improved. An explanation of the reasons for the shape of the isotherm based on energetics is provided.