Local density functional theory within the framework of the Car-Parrinello method has been used to study the structural parameters and the energetics of offretite, when a Si4+ ion is substituted by (Al3+, H+). The calculations have been performed on a periodically repeated unit cell with 54 atoms and a proton. In agreement with previous cluster calculations, we conclude that the sites with the lowest (Al, H)/Si substitution energies are also those with the largest proton affinity. In addition a correlation previously reported between acidity and Al---O---Si bond angles is confirmed.