Calculate harmonic vibrational frequencies and normal modes. The hessian is calculated analytically or numerically by finite differences in 3N cartesian coordinates (Z-Matrix coordinates will be destroyed on entry). If analytic gradients are available these are differentiated once to build the hessian, otherwise the energy is differentiated twice. If for the wavefunction method dipole moments are available, the dipole derivatives and the IR intensities are also calculated. Note that numerical hessians cannot be computed when dummy atoms holding basis functions are present. To get reasonable results it is necessary to do a geometry optimization before using the frequency calculation.