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

The vibrational dynamics of formic acid monomer (FAM) and dimer (FAD) is investigated from machine-learned potential energy surfaces at the MP2 (PES$_{\rm MP2}$) and transfer-learned (PES$_{\rm TL}$) to the CCSD(T) levels of theory. The normal modes and anharmonic frequencies of all modes below 2000 cm$^{-1}$ agree favourably with experiment whereas the OH-stretch mode is challenging for FAM and FAD from normal mode analyses and finite-temperature MD simulations. VPT2 calculations on PES$_{\rm TL}$ for FAM reproduce the experimental OH frequency to within 22 cm$^{-1}$. For FAD the VPT2 calculations find the high-frequency OH stretch at 3011cm$^{-1}$, compared with an experimentally reported, broad ($\sim 100$ cm$^{-1}$) absorption band with center frequency estimated at $\sim 3050$ 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$_{\rm TL}$. Including zero-point energy corrections from diffusion Monte Carlo simulations for FAM and FAD and corrections due to basis set superposition and completeness errors yield a dissociation energy of $D_0 = -14.23 \pm 0.08$ kcal/mol compared with an experimentally determined value of $-14.22 \pm 0.12$ kcal/mol.
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