The OH- ion in water is studied using a CPMD/BLYP + QM(electronic) + QM(vibrational) approach. The ion resides in a cage of water molecules, which are H-bonded among each other, and pinned by H-bonding to the ion's O atom. The water network keeps the 'on-top' water in place, despite the fact that this particular ion-water pair interaction is non-binding. The calculated OH- vibrational peak maximum is at similar to 3645 cm(-1) (experiment similar to 3625 cm(-1)) and the shift with respect to the gas-phase is similar to +90 cm(-1) (experiment +70 cm(-1)). The waters molecules on each side of the ion (O and H) induce a substantial OH- vibrational blueshift, but the net effect is much smaller than the sum. A parabolic 'frequency-field' relation qualitatively explains this non-additivity. The calculated 'in-liquid' nu(OH-) anharmonicity is 85 cm(-1). (C) 2011 Elsevier B.V. All rights reserved.