The uncoupled anharmonic OH-stretching vibrational frequency for the layered mineral Mg(OH)2 (brucite) has been calculated in the pressure range 0−22 GPa. Quantum-mechanical electronic structure (DFT) calculations were performed, followed by quantum-mechanical vibrational energy calculations. The following findings emerged: (1) The calculated dν(OH)/dP slope is −4 cm−1/GPa, in agreement with the experimental literature value [taken as the average between the Raman and IR-measured slopes for Mg(OH)2]. (2) The calculated ν(OH) vs. R(O···O) correlation is linear and the slope is much smaller than that of traditional H-bond correlation curves in the literature. (3) The main origin of the small dν/dP and dν/dR(O···O) slopes is the small electric field variation as the mineral layers are pressed toward each other. (4) At high pressure, the OH− ions show some tendency to be tilted with respect to the c axis, and a larger tilt angle leads to a larger ν(OH) downshift. (5) The pressure variation of the D quadrupole coupling constant is approximately −1 kHz/GPa.