Dynamic Interaction Response Spectra Reveal the Modal Organization of Intermolecular Interactions

Static cross-coordinate response reports how one local coordinate relaxes when another is perturbed, but it does not show how individual normal modes contribute. We introduce dynamic interaction response spectra (DIRS), a signed mode- and frequency-resolved representation of cross-coordinate mechanical response. Across 37 coordinate-pair end points in nine intermolecular complexes, DIRS distinguishes coherent reinforcement, small-magnitude response, and cancellation-dominated response. A methane-dimer C–H/C–H cross-response is nearly zero because near-degenerate high-frequency C–H stretching contributions of opposite sign cancel, whereas a water-dimer H···O/O···O response is coherently built from low-frequency modes. Finite-resolution broadening preserves the methane cancellation while showing that its near-unity canonical modal activity is resolution sensitive. Independent ωB97X-D4 reoptimization of five representative systems gives max|Δη| = 0.0269 and signed-η Spearman rs = 0.948. DIRS thus exposes the modal construction of intermolecular mechanical response without defining a new interaction-strength scale.