where V LL→CC is the corrected PES, V LL is a PES fit to low-level DFT electronic data, and ∆V CC-LL is the correction PES based on high-level coupled cluster energies. This approach has been shown to achieve its objective with at 10-100 times fewer energies than the DFT dataset. The application to tunneling splittings in tropolone was perhaps the biggest success of the method. In this example the splittings for an MP2 PIP-based were brought to excellent agreement with experiment using a ∆-ML PIP PES. It also has been shown