Vibrational spectrum of brucite, Mg(OH)2: a periodic ab initio quantum mechanical calculation including OH anharmonicity

The harmonic frequency spectrum of bulk Mg(OH)2, brucite, has been computed with the CRYSTAL periodic code, using four different hamiltonians, namely Hartree–Fock (HF), local density (LDA), gradient corrected PW91 and hybrid B3LYP. The anharmonicity of the OH stretching frequency has also been evaluated, as well as the transverse/longitudinal optical (TO/LO) splitting. Comparison with the frequencies of the single layer shows that the bulk OH stretching splitting is due to interlayer interaction. Agreement with experiment is excellent for B3LYP (less than 10 cm−1 differences), less so for PW91 and much less satisfactory for both LDA and HF.