Local density functional theory within the framework of the Car-Parrinello molecular dynamics method has been used to study the structural parameters and the energetics of the zeolite offretite, when a Si⁴⁺ ion is substituted by (Al^3+,H⁺). The calculations have been performed for a bulk system made of a periodically repeated unit cell with 54 atoms and a proton. In agreement with previous cluster calculations, it is concluded that the sites with the lowest (Al,H)/Si substitution energies are also those with the largest proton affinities. In addition, a previously reported correlation between site acidity and Al-O-Si bond angles is confirmed.