The geometric, energetic, and vibrational features of the hydrogarnet defect in chabazite (CHA) and sodalite (SOD) have been studied at the Hartree-Fock and B3-LYP levels by using the periodic ab initio CRYSTAL code based on localized Gaussian basis functions. The geometry of the defective structures (CHA-HG and SOD-HG) has been fully optimized at both levels of theory by fixing the unit cell parameters to the values of the defect-free structures. The local structure of the defect is dictated by the hydrogen bond interaction among the OH groups, the strength of the shortest hydrogen bond in SOD-HG being 47 and 55 kJ/mol at the HF and B3-LYP levels, respectively. The reaction of CHA and SOD with gas phase water has been shown to be exothermic when either orthosilicic acid or alpha-quartz are formed as a product. On the contrary, the reactions are strongly endothermic with respect to liquid water. The vibrational spectra in the high-frequency OH stretching region for both SOD-HG and CHA-HG have been simulated using the ab initio harmonic OH stretching frequencies and compared to the experimental spectra of the mineral katoite and defective silicalite. Structure, geometry, hydrogen bond strength, defect formation energy, and OH stretching frequencies turn out to be significantly dependent on the adopted Hamiltonian for both CHA-HG and SOD-HG.