Calculating the frequencies and intensities of strongly anharmonic modes of adsorbates on surfaces: a low-cost but accurate computational approach

We present a new method for calculating the frequencies and intensities of the vibrational modes of adsorbates on surfaces. Our method is based on density functional perturbation theory (DFPT) and provides accurate estimates of the vibrational intensities even when the vibrations are strongly anharmonic. Furthermore, it does so at a negligible additional computation cost compared to conventional DFPT calculation. We illustrate our method by calculating the vibrational spectra of three example systems — ethylidyne on Rh(111), benzene on Rh(111) coadsorbed with CO, and terephthalic acid (TPA) on Cu(100) — and comparing them to experimental measurements performed using High-Resolution Electron Energy Loss Spectroscopy (HREELS). We find excellent agreement between our predictions and the experimentally measured frequencies and intensities in all three cases.