Accuracy of Low-level Surface in Hierarchical Construction of Potential Energy Surface

The effects of low-level PES on the overall accuracy of the final surface constructed by using hierarchical construction were investigated with the constructions of a number of global surfaces for the H₃ system at UHF (UMP2, DFT-B3LYP, UCCSD(T))/vtz, and UCCSD(T)/avqz levels of theory. The total reaction probabilities for the H+H₂ reaction calculated on these surfaces revealed that the accuracy of UCCSD(T)/avqz surface is very close to the well-known BKMP2 surface, while the UCCSD(T)/vtz PES has a slightly higher barrier. In contrast, the low-level theories (UHF, UMP2, DFT-B3LYP) with vtz basis set can only provide a qualitative description of this simplest reaction despite the fact that they are widely used to study reactions in complex systems. On the other hand, although these theories are not accurate on describing the reaction, they can be used to provide the low-level PESs for hierarchical construction of the UCCSD(T)/avqz PES with the number of UCCSD(T)/avqz energies substantially reduced.