Anharmonicity and localization of atomic vibrations in vitreous silica

A numerical analysis has been made of anharmonicity effects for vibrational eigenmodes in a model of vitreous silica constructed by molecular dynamics. It is shown that in macroscopic systems all eigenmodes are generally harmonic. Strong anharmonicity is found for the bare localized modes in the low-frequency region around the boson peak. This anharmonicity is related to the rotational motion of coupled SiO4 tetrahedra and differs in character from the anharmonicity of the high-frequency localized eigenmodes which are mainly related to symmetric and asymmetric stretching of SiO4 tetrahedra.