Returns the nac_frequencies for the given direction (not necessarily a versor).
        None if the direction is not present or nac_frequencies has not been calculated.

        Args:
            direction: the direction as a list of 3 elements
        Returns:
            the frequencies as a numpy array o(3*len(structure), len(qpoints)).
            None if not found.

def get_nac_frequencies_along_dir(self, direction):
        """
        Returns the nac_frequencies for the given direction (not necessarily a versor).
        None if the direction is not present or nac_frequencies has not been calculated.

        Args:
            direction: the direction as a list of 3 elements
        Returns:
            the frequencies as a numpy array o(3*len(structure), len(qpoints)).
            None if not found.
        """
        versor = [i / np.linalg.norm(direction) for i in direction]
        for d, f in self.nac_frequencies:
            if np.allclose(versor, d):
                return f

        return None