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

TL;DR

This study develops transfer-learned potential energy surfaces for formic acid monomer and dimer, enabling accurate vibrational frequency predictions and dissociation energy calculations that closely match experimental data.

Contribution

The paper introduces transfer learning techniques to improve potential energy surfaces for formic acid, achieving high accuracy in vibrational and dissociation energy predictions.

Findings

Normal mode frequencies agree with experiment below 2000 cm$^{-1}$.

VPT2 calculations reproduce OH frequencies within 22 cm$^{-1}$.

Dissociation energies closely match experimental values.

Abstract

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.