The phonon density of states in the monoclinic phase of tetracyanoethylene is investigated at 20 K using inelastic neutron scattering, and found to be in fair agreement with theoretical calculations based on a semi-rigid molecule model and 6-exponential pair potentials. The results provide support for the atom-atom potential model which was used to explain the relative stability of different phases in this solid. The monoclinic crystal structure at 295 K is also refined using powder neutron diffraction data with a higher resolution than in earlier reports. Comparison is made with the molecular geometry in the cubic phase.