A simple and Efficient Way to Incorporate Anharmonicity within Density Functional Theory Calculations

Accurately predicting molecular spectroscopic properties requires including anharmonic corrections in theoretical methods such as density functional theory (DFT). In this work, we present an efficient way to calculate anharmonic vibrational frequencies for a given normal mode. We apply this method for a set of diatomic molecules and show that evaluating potential energy surface (PES) at only eight displacements per mode is sufficient to predict properties such as anharmonic frequencies and average displacements from the equilibrium geometry. We also show that in some molecules, PES calculated at those displacements along the normal modes provides useful insights about its stability than harmonic frequencies alone. Prospects for using the method to account for off-diagonal anharmonicity will be discussed.