VPT2 Calculations of Vibrational Energies of CH ₃ COOC ₆ H ₄ COOH Done in Seconds on a Laptop Using a Machine Learned Potential

High Resolution Image Download MS PowerPoint Slide The determination of quartic force fields for use in vibrational second-order perturbation (VPT2) calculations, currently available in numerous electronic structure packages, becomes very expensive as the size of the molecule increases, especially if high-level coupled-cluster theory is used. Machine-learned potentials (MLPs) for large molecules and clusters offer a viable alternative to obtaining the quartic force field (QFF). Here, we report Fortran and Python software to determine the QFF and perform VPT2 calculations of energies from the MLPs. We describe this software and then apply it to H 2 O and protonated oxalate as the test cases. The Fortran software is applied to 21-atom aspirin using a fast MLP reported by us. Despite the fact that there are 32,509 unique cubic force constants for aspirin, the computer time to calculate them using this MLP is trivial, i.e., around 1 min. The new software provides an efficient way to calculate quantum anharmonic energies, using the established VPT2 methodology, for machine learned potentials of large molecules.