A highly accurate potential energy surface for carbonyl sulphide (OCS): how important are the ab initio calculations?

Ab initio quantum chemical methods can produce accurate molecular potential energy surfaces (PESs) capable of predicting the fundamental vibrational wavenumbers to within 1 cm^-1. However, for high-resolution applications this is simply not good enough and empirical refinement is necessary, i.e. adjusting the PES to better match laboratory spectroscopic data. Here, the impact of the underlying ab initio calculations is rigorously investigated within the context of empirical refinement. For carbonyl sulphide (OCS), state-of-the-art electronic structure calculations are employed to construct higher- and lower-level ab initio PESs, which are then empirically refined in near-identical procedures. The initial ab initio calculations are shown to considerably affect the accuracy of the final refined PES, with an order-of-magnitude improvement in computed rotation-vibration energy levels achieved for OCS. In demonstrating this, the most accurate PES of the electronic ground state of OCS is produced, reproducing the fundamentals with a root-mean-square error (RMSE) of 0.004 cm^-1, and 884 rovibrational energy levels below 14 000 cm^-1 with an RMSE of 0.060 cm^-1.