The H + (H2O)21 cluster, known for its exceptional stability, has been debated regarding whether its proton adopts an Eigen (H3O + ) or Zundel (H5O2 + ) structure. Torrent-Sucarrat and Anglada 57 applied vibrational second-order perturbation theory (VPT2) at a DFT level to model the infrared spectrum of H + (H2O)21, including anharmonic effects. Their calculations indicated that the characteristic H3O + stretches, typically observed near 2500 cm -1 , undergo a substantial redshift of over 500 cm -1 , shifting below 2000 cm -1 . This finding suggests that earlier studies, constrained to higher frequency ranges, may have overlooked crucial protonation signatures.