VPT2 Meets MLPs: IR and Raman Spectroscopy of Eight Molecules, Including 74-Atom Cholesterol, Using MACE-OFF and MACE-MDP

Vibrational second-order perturbation theory is a widely used method to perform postharmonic calculations of vibrational energies and infrared (IR) and Raman spectroscopy. We recently reported an application of this approach for vibrational energies using Fortran and Python software with machine-learned potentials (MLPs), with an application to 21-atom aspirin. Here, the software reported previously is extended to perform calculations of IR and Raman spectroscopy. Applications are made to six molecules, with up to 31 atoms, considered in a recent paper by Kapil and co-workers using MACE-based software for the MLP (MACE-OFF) and dipole moment/polarizability (MACE-MDP) to perform harmonic calculations of IR and Raman spectra with a linear approximation for the dipole and polarizability. Large differences are seen between those and the vibrational second-order perturbation theory (VPT2) spectra for XH-stretch fundamentals, and comparisons with experimental IR spectra for three of these molecules plus cholesterol show significant improvement of the VPT2 spectra over the harmonic ones. The extendability of the new software is demonstrated for 74-atom cholesterol using MACE software. The weakness of the VPT2 approach is shown for the IR spectrum of the formic acid dimer, using our PIP potential and dipole moment surface.