Ab initio periodic calculations have been performed with the CRYSTAL code, which is based on Gaussian basis sets, to investigate the structure and OH vibrational features (both harmonic and anharmonic) of Mg(OH)(2) and beta-Be(OH)(2) crystals, which represent two extreme situations in which the OH group can be involved. In the latter, it participates in H-bonds of intermediate strength, whereas, in the former, it is essentially free. Hartree-Fock (HF), local density (LDA), gradient-corrected (PW91), and hybrid (B3LYP) density functionals have been used and the results compared with experiment. For Mg(OH)(2) the B3LYP OH frequencies are in very good agreement with experiment, whereas, for beta-Be(OM)(2), they are similar to100 cm(-1) too low. LDA grossly underestimates the OH frequency (330 and 800 cm(-1) for Mg(OH)(2) and beta-Be(OH)(2), respectively), and PW91 moderately does (170 and 400 cm(-1), respectively).