return the number of vibration modes with zero frequency

Returns :nzero_modes : int


Zero modes can come from a number of different sources. You will have one zero mode for every symmetry in the Hamiltonian. e.g. 3 zero modes for translational invariance and 3 zero modes for rotational invariance. If you have extra degrees of freedom, from say frozen particles they will contribute zero modes.

Harmonic modes are necessary to calculate the free energy of a minimum in the harmonic approximation. The density of states is inversly proportional to the product of the frequencies. If the zero modes are not accounted for correctly then the product will be trivially zero and the free energy will be completely wrong.

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