which clearly predicts a linear increase in Δ𝐸 A with increasing 𝐸 A whose slope is defined by the ratio 𝜔 ̅ 7 /𝜔 ̅ 6 . Compared with our experimental results, Eq. 5 is able to capture the general trend and magnitude of Δ𝐸 A pretty well (see Figure , the upper bound of the grey shading corresponds to 𝜔 ̅ 7 /𝜔 ̅ 6 = 0.927 and the lower bound corresponds to 𝜔 ̅ 7 /𝜔 ̅ 6 = 0.946, as determined from the average lithium frequencies found from AIMD simulations of the different materials in Figure ).