To elucidate the molecular structure and water-clay interaction of interlayer water in sodium type smectite (hydrated layered alminosilicate), vibrational spectra of water were investigated using molecular dynamics (MD) simulations based on the free flexible force field model. Vibrational spectra of water were obtained by Fourier transformation of the velocity autocorrelation function of the hydrogen atom. Two distinct bands were found at 3365 and 3500 cm(-1) in the stretching vibrational spectrum of interlayer water. The former band was assigned to an O-H bond unbound to the clay surface, while the latter was attributed to O-H vibrations bound to the clay surface through hydrogen bonding. The hydrogen bond distance (the H...O distance) between water and the clay surface (H-water...O-clay = 0.22 nm) was larger than that between water molecules (H-water...O-water = 0.19 nm). Detailed comparison of simulation results with IR spectroscopic observations indicated good agreement. The hydrogen bond structure and the vibrational spectrum of interlayer water suggest no rigid network structure of water molecules (icelike water) near the smectite surface.