def get_phonon_frequencies(self):
        """calculate phonon frequencies"""
        # TODO: the following is most likely not correct or suboptimal
        # hence for demonstration purposes only
        frequencies = []
        for k, v0 in self.data.iteritems():
            for v1 in v0.itervalues():
                vec = map(abs, v1['dynmat'][k - 1])
                frequency = math.sqrt(sum(vec)) * 2. * \
                            math.pi * 15.633302  # THz
                frequencies.append(frequency)
        return frequencies