Transfer learned potential energy surfaces: accurate anharmonic vibrational dynamics and dissociation energies for the formic acid monomer and dimer

Abstract

The vibrational dynamics of the formic acid monomer (FAM) and dimer (FAD) is investigated from machine-learned potential energy surfaces at the MP2 (PES_MP2) and transfer-learned (PES_TL) to the CCSD(T) levels of theory. The normal mode (MAEs of 17.6 and 25.1 cm^-1) and second order vibrational perturbation theory (VPT2, MAEs of 6.7 and 17.1 cm^-1) frequencies from PES_TL for all modes below 2000 cm^-1 for FAM and FAD agree favourably with experiment. For the OH stretch mode the experimental frequencies are overestimated by more than 150 cm^-1 for both FAM and FAD from normal mode calculations. Conversely, VPT2 calculations on PES_TL for FAM reproduce the experimental OH frequency to within 22 cm^-1. For FAD the VPT2 calculations find the high-frequency OH stretch at 3011 cm^-1, compared with an experimentally reported, broad (∼100 cm^-1) absorption band with center frequency estimated at ∼3050 cm^-1. In agreement with earlier reports, MD simulations at higher temperature shift the position of the OH-stretch in FAM to the red, consistent with improved sampling of the anharmonic regions of the PES. However, for FAD the OH-stretch shifts to the blue and for temperatures higher than 1000 K the dimer partly or fully dissociates using PES_TL. Including zero-point energy corrections from diffusion Monte Carlo simulations for FAM and FAD and corrections due to basis set superposition and completeness errors yields a dissociation energy of D₀ = -14.23 ± 0.08 kcal mol^-1 compared with an experimentally determined value of -14.22 ± 0.12 kcal mol^-1.