We calculate the spectrum of internal vibrations of a single ferrocene Fe(C5H5)(2) molecule using ab initio density functional theory (without free parameters) and compare this with inelastic neutron scattering data on ferrocene in the solid state at 28 K. Due to the good agreement, we can assign each vibrational mode to each observed peak in the neutron spectrum and so remove ambiguities existing in the literature. There is also consistency between the calculated potential energy of a single ferrocene molecule for different orientations, Phi, of the two cyclopentadienyl C5H5 rings with respect to each other, which shows a potential barrier of 0.9 kcal/mol, and electron diffraction, and between the calculated shallow minimum at Phi=9 deg and x-ray diffraction.