where two Gaussians are used to represent polar headgroups at ±𝑧 𝐻 and one Gaussian to model the methyl trough region at the bilayer midplane. The widths of these Gaussians are given by 𝜎 𝐻 and 𝜎 𝐶 , respectively. The ratio of the headgroup amplitude against the negative methyl group amplitude is the minimum to maximum electron density contrast, given as 𝜌 𝑅 = 𝜌 𝐶 /𝜌 𝐻 .