Transfer learned potential energy surfaces: accurate anharmonic vibrational dynamics and dissociation energies for...

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.