Plane-wave density functional theory (DFT-PW) calculations were performed on bulk SnO₂ (cassiterite) and the (100), (110), (001), and (101) surfaces with and without H₂O present. A classical interatomic force field has been developed to describe bulk SnO₂ and SnO ₂-H₂O surface interactions. Periodic density functional theory calculations using the program VASP (Kresse et al., 1996) and molecular cluster calculations using Gaussian 03 (Frisch et al., 2003) were used to derive the parametrization of the force field. The program GULP (Gale, 1997) was used to optimize parameters to reproduce experimental and ab initio results. The experimental crystal structure and elastic constants of SnO₂ are reproduced reasonably well with the force field. Furthermore, surface atom relaxations and structures of adsorbed H₂O molecules agree well between the ab initio and force field predictions. H₂O addition above that required to form a monolayer results in consistent structures between the DFT-PW and classical force field results as well.