For anharmonic theoretical vibrational spectroscopy, it can be advantageous to expand the potential energy surface in localized-mode coordinates instead of normal-mode coordinates. To this end, all vibrational modes that contribute to a specific band or are of interest are collected into the subset Q sub . For 2D IR spectroscopy, this will usually be only a small number of vibrational modes, such as the carbonyl stretching vibrations in metal carbonyls or the amide I vibrations in peptides. For this subset of modes, a unitary transformation U is determined such that the resulting modes,