In this ab initio thermodynamics framework, ∆ A H of the target is anchored to the chemical potentials of the elements µ k , which we need to determine. Theoretically, it is possible to use the DFT energies of the elements as the chemical potentials. However, it is impractical to reliably determine the chemical potentials (or electronic energies) of the standard states of the elements with DFT. For example, the graphite formation energy is sensitive to vdW forces, which are not accurately accounted for in many functionals, and the O 2 energy requires an accurate treatment of the triplet spin state of oxygen, which is difficult at the DFT level of theory and depends on the spin state. Total atomization energies offer a convenient alternative in principle, but in practice, they suffer from similar issues. Treatment of bare atoms in DFT is generally inaccurate due to unpaired electrons. Also, it requires careful convergence of spin states that can cause ambiguity in the results due to potentially large numerical errors. Calculating accurate atomization energies demands the highest level of theory and presents the most demanding calculations. To avoid these challenges, we can derive the chemical potentials of the elements via two different approaches: (i) by hand-picking closed-shell reference species that are supposedly more accurate and use them to calculate the chemical potentials in a sequential approach, or (ii) by formulating a linear algebra problem that can be solved via linear regression, avoiding explicitly defined reference species. The first approach is described in this section, and the second is described in Section 4.2.3.