Frequencies and polarization vectors of phonons: Results from force constants which are fitted to experimental data or calculated ab initio

The properties of phonons may be calculated from the dynamical matrix which is determined by force constants. Often the force constants are obtained by fitting them to experimental phonon frequencies, e. g., for wave vectors q on high-symmetry directions of the Brillouin zone. It is well known that these force constants do not necessarily lead to correct frequencies for wave vectors for nonsymmetrical q and to correct polarization vectors. In the present paper this is demonstrated by comparing for fcc Ni, fcc Al, and bcc Fe the frequencies and polarization vectors calculated from fitted force constants with the results from ab initio calculated force constants. However, for most regions of the Brillouin zone the differences between the results obtained from the two sets of force constants are not large.