quantum chemistry - How to calculate wavenumbers of normal modes from the eigenvalues of the Cartesian force constant matrix?

I would like to get the wavenumbers (Frequencies -- in Gaussian) from the eigenvalues of the force constant matrix (Force constants in Cartesian coordinates: in Gaussian).

Could someone please explain how to perform this calculation? The Cartesian force constants are given in Hartrees per Bohr squared, while the wavenumbers are in $\pu{cm^-1}$.