  Proper consideration of anharmonicity is important for the calculation of the
thermal conductivity. However, how the anharmonicity influences the thermal
conduction in amorphous materials is still an open question. In this work, we
uncover the role of anharmonicity on the thermal conductivity of amorphous
silica (a-SiO2) by comparing the thermal conductivity predicted from the
harmonic theory and the anharmonic theory. Moreover, we explore the effect of
anharmonicity-induced frequency shift on the prediction of the thermal
conductivity. It is found that the thermal conductivity calculated by the
recently developed anharmonic theory (quasi-harmonic Green-Kubo approximation,
QHGK) is higher than that by the harmonic theory developed by Allen and
Feldman. The use of anharmonic vibrational frequencies also leads to a higher
thermal conductivity compared with that calculated using harmonic vibrational
frequencies. The anharmonicity induced frequency shifts is a mechanism for the
positive temperature dependence of the thermal conductivity of a-SiO2 at higher
temperatures. Further investigation on mode diffusivity suggests that although
anharmonicity has larger influence on locons than diffusons, the increase of
the thermal conductivity due to the anharmonicity is mainly contributed by the
anharmonicity induced increase of the diffusivity of diffusons. Finally, it is
found that the cross-correlations between diffusons and diffusons contribute
most to the thermal conductivity of a-SiO2, and the locons contribute to the
thermal conductivity mainly through collaboration with diffusons. These results
offer new insights into the nature of the thermal conduction in a-SiO2.
