Finally, all points with the high-level energy are used to fit the target high-level PES. In the new strategy, ∆VHL-LL is used to efficiently determine the energy difference for points that are not computed directly at the high-level method. The final PES is still represented by a single NN function, which is faster than Eq. (1) in evaluation. All NN based PES methods, PIP-NN, FI-NN, or the high-dimensional-NN (HD-NN) proposed by Behler et al., can be employed.