Towards Accurate Quantum Mechanical Thermochemistry: (1) Extensible Implementation and Comparison of Bond Additivity Corrections and Isodesmic Reactions

To obtain accurate enthalpies of formation of a chemical species, Hf, one needs a procedure to quantitatively connect the results from quantum mechanical (QM) calculations with the experimental enthalpies of elements in their standard state, often along with additional empirical corrections. Although QM methods continue to improve, accuracy gains from new methods are usually realized if the calculations for Hf are properly corrected. One such approach is to use atomization energy corrections (AECs) followed by bond additivity corrections (BACs), such as those defined by Petersson et al. or Anantharaman and Melius. Another approach is to utilize isodesmic reactions (IDRs) which are error-canceling reactions that can be constructed systematically as shown by Buerger et al. In this work, we implement all of these approaches in Arkane, an open-source software that can calculate species thermochemistry from computations done by a wide variety of QM software packages. As part of this work, we curate a diverse database of over 400 reference species from the available literature for use in fitting BACs or to participate in IDRs, and also provide AECs and BACs for 15 commonly used levels of theory using the workflow implemented. When comparing the approaches, we find that both BAC types yield results of similar accuracy, though the Anantharaman and Melius-type BACs seem to generalize better. Further, although the IDRs approach is often as accurate as the BAC approaches and sometimes even more advantageous, we find that this method appears to be less robust than the BAC methods. In particular, the accuracy of the IDRs approach is sensitive to the chosen reference species and reactions. Therefore, in addition to providing the automated AECs and BACs fitting procedure, we also propose improved methods to select IDRs.