The changes in the spectra of the acidic group in chabazite are studied by quantum chem. calcns. The zeolite is modeled by two clusters consisting of eight tetrahedral atoms arranged in a ring and seven tetrahedral atoms coordinated around the zeolite OH group. The potential energy and dipole surfaces were constructed from the zeolite OH stretch, in-plane and out-of-plane bending coordinates, and the intermol. stretch coordinate that corresponds to a movement of the water mol. as a whole. Both the anharmonicities of the potential energy and dipole were taken into account by calcn. of the frequencies and intensities. The matrix elements of the vibrational Hamiltonian were calcd. within the discrete variable representation basis set. We have assigned the exptl. obsd. frequencies at .apprx.2900, .apprx.2400, and .apprx.1700 cm-1 to the strongly perturbed zeolite OH vibrations caused by the hydrogen bonding with the water mol. The ABC triplet is a Fermi resonance of the zeolite OH stretch mode with the overtone of the in-plane bending (the A band) and the overtone of the out-of-plane bending (the C band). In the B band the stretch is also coupled with the second overtone of the out-of-plane bending. The frequencies at .apprx.3700 and .apprx.3550 cm-1 we have assigned to the OH stretch frequencies of a slightly perturbed water mol.