where N f is the number of degrees of freedom. For example, N f = 1 in the case of the diatomic molecule H 2 + . The results in Figure show that the minimum energy is reached at τ = 2 fs. Note, at this value the average distance ⟨r⟩ and the amplitude l are minimal, as Figure shows. The total vibrational energy E vib = ⟨E tot ⟩ -E e was compared to the quantum zero-point vibrational energy (ZPVE) of the system. Similar to the amplitude, ZPVE is dominated by the harmonic potential. Therefore, the harmonic and anharmonic ZPVE values are very close. There is also another consideration regarding the τ parameter. According to the equipartition theorem, which assumes the dominating role of the harmonic potential, the nuclear temperature can be used to guide the choice of the τ parameter in SWS. This is also confirmed by the numerical test demonstrated in Figure .