The exact tunnelling splitting of malonaldehyde from symmetrized path-integral molecular dynamics

A recently proposed path-integral molecular dynamics method is applied to compute the tunnelling splitting of malonaldehyde. The approach, which is based on symmetrised partition functions, rigorously projects out the ground rotational state and is formally exact for a given potential energy surface (PES). We obtain the result of 21.1±0.1cm−1, which can be compared with previous calculations on the same PES, including diffusion Monte Carlo (21.0±0.4cm−1) and a grid-based wavefunction method (21.7±0.3cm−1), as well as with experiment (21.6cm−1). This resolves the discrepancies of previous PIMD studies (19.3±0.2cm−1), which we find to be contaminated by contributions from higher rotational states. Our work demonstrates the accuracy that can be achieved from path-integral methods in polyatomic molecules and sets the stage for exact tunnelling-splitting calculations in even larger systems out of the range of traditional methods.