Herein, the vibrational density of states (phonon spectrum) for Li and Li variants of Li10SnP2S12 and Li6PS5Cl was calculated by the correlation function of Li-ion velocities from AIMD simulations (Figure ). AIMD simulation has the distinct benefit of being able to investigate isotope effects on the vibrational frequencies of lithium ions since neutron scattering studies on Li are not possible due to its high neutron absorption cross-section. Furthermore, AIMD simulation allows for the direct assessment of the lithium projected density of states, from which the average lithium frequency can be calculated (see Figure ). The ratios of the average 7 Li to Li frequencies are 92.7% (ℏ𝜔 7 / ℏ𝜔 6 = 32.9 meV / 35.5 meV) for Li10SnP2S12 and 94.6% (ℏ𝜔 7 / ℏ𝜔 6 = 35.2 meV / 37.2 meV) for Li6PS5Cl, in excellent agreement with the value expected from theory (Eq. 1). This indicates that a single-mode harmonic approximation can be used to describe the lowest-order phonon behavior in these materials. We also note that no discernable changes to the rest of the total density of states could be identified (for example, see the peak around 75 meV in Figure and Supplemental Note 3). This confirms that the force constants governing the lithium vibrations, as well as the other interatomic interactions in the solid, are independent of the isotope mass. Thus, we conclude that the isotope effect must pertain to the vibrational frequencies of the lithium ions only, due to their mass dependence.