Transfer Learned Potential Energy Surfaces: Accurate Anharmonic Vibrational Dynamics and Dissociation Energies for the Formic Acid Monomer and Dimer

The vibrational dynamics of the formic acid monomer (FAM) and dimer (FAD) is investigated from machine-learned potential energy surfaces at the MP2 (PESMP2) and transfer-learned (PESTL) 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 PESTL 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 PESTL 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 (similar to 100 cm(-1)) absorption band with center frequency estimated at similar to 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 PESTL. 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 D-0 = -14.23 +/- 0.08 kcal mol(-1) compared with an experimentally determined value of -14.22 +/- 0.12 kcal mol(-1).