where the gas-phase frequency of NMA ω ref is used as reference point. The shift due to the nearest neighbors ∆ω NN is expressed as a function of the backbone dihedral angles ϕ i and ψ i as derived from quantum-chemical calculations on N -acetyl-glycine-N ′ -methylamide (glycine dipeptide, GLDP) . All three vibrational frequency maps considered here use the GLDP parametrization for the nearest-neighbor shifts, and express these as