A vibrational analysis of the inelastic neutron scattering spectrum (WS) of pyrimidine has been carried out combined with quantum mechanical calculations at the RHF, MP2, and B3LYP levels using the 6-31G* basis set in all the cases. The INS spectrum was first calculated from the atomic displacement matrix in order to determine which level of theory gives the best agreement between observed and calculated INS intensities. In a second approach, force constants from a calculation at the MP2/6-31G* level were transformed to a symmetrized set (C-2v) and all, the diagonal and some off-diagonal force constants were fitted. Good agreement between observed and calculated wavenumbers and spectral INS intensities was achieved which demonstrates the validity of our force field.