The geometries of model antiparallel and parallel β-sheets (seven strands of capped Ala6; Figure and) were optimized in Gaussian 16 using the BP86 functional and the 6-31g* basis set, followed by normal mode analysis as performed in our previous work. After extracting the vibrational wavenumbers and dipole derivatives, in-plane polarized infrared spectra were calculated from the squared dipole derivative magnitudes in various orientations in 5° steps. 0° (or 180°) was defined as a polarization along the common main axis of the strands and 90° (or 270°) as the orthogonal inplane direction along the amide C=O bonds (see inset in Figure and). Out-of-plane polarized spectra are not shown as the transition dipole moments of amide I and amide II bands are within the β-sheet plane, such that their out-ofplane projections are negligible.