Integrating Local and Normal Modes for Multilevel Vibrational Spectroscopy

Abstract We present a multilevel strategy for harmonic vibrational spectroscopy in which a lower cost calculation supplies the complete coupled force field and higher level energy calculations refine selected curvatures. Symmetry-oriented non-redundant internal coordinates retain chemical identity and coupling topology, while concordant normal-mode directions provide an efficient frequency end point. Their sequential use improves mode shapes without requiring a higher level Hessian. Across four validation fixtures, the local-coordinate models recover unsupplied coupling correlations of 0.987–0.999; the complete sequence uses at most 2N independent higher level curvatures for N normal modes.