Permutationally Invariant Polynomial Potential Energy Surfaces for Tropolone and H and D atom Tunneling Dynamics

We report permutationally invariant polynomial (PIP) fits to energies and gradients for 15-atom tropolone. These include standard, augmented, and fragmented PIP bases. Approximately 6600 energies and associated gradients are obtained from direct-dynamics calculations using DFT/B3LYP/6-31+G(d) supplemented by grid calculations spanning an energy range up to roughly 35 000 cm-1. Three fragmentation schemes are investigated with respect to efficiency and fit precision. In addition several fits are done with reduced weight for gradient data relative to energies. These do result in more precision for the H-transfer barrier height. Properties of the fits such as stationary points, harmonic frequencies and the barrier to H-atom transfer are reported and compared to direct calculations. A 1-d model to obtain the tunneling splitting for the ground vibrational state and qualitative predictions for excited vibrational states is employed. Several 1-d double well fits to the PES are developed and used to extrapolate H and D atom tunneling splittings to values at the CCSD(T)-F12 barrier. The level of agreement is within expectations for the method adapted and the level of the electronic structure theory employed.