The differences in the vibrational spectra of a condensed matter between inelastic neutron scattering and photon scattering originate from the different corresponding interaction mechanisms. However, investigators used to ignore them due to difficulties to analyse theoretically. Herein, we devise an approach to link neutron scattering with photon scattering experiments by computing simulation methods and shed light on new physical insights for the assignments of crystalline materials. By using the first-principles method based on density functional theory (DFT), the vibrational spectrum of ice VIII is investigated. Comparisons between the computing vibration modes and the experimental data of ice VIII, i.e., Raman and neutron scattering data, mutually validate the reliability. The assignments of the dynamic process of 33 normal modes at the Brillouin zone (BZ) center are precisely illustrated from the vibrational analysis based on CASTEP code. The physical insights for special vibration modes are discussed individually as well as in terms of the correlated peak from neutron and Raman scattering.