where the Hamiltonian operator (H) encompasses the range of electron/nuclear interaction terms and Ψ is the electron wavefunction, itself a functional of the electron density. In practice, absolute values of internal energy are rarely useful in isolation, and we must focus on energy differences (∆U ) between the different species for balanced chemical reactions such as those considered above. The total energies of different materials cannot be directly compared between codes and a set of common parameters must be chosen for each study. To overcome this limitation, systematic calculations are performed to compile a dataset, e.g. as curated within the Materials Project or the Open Quantum Materials Database. The internal energy is easily extended to include pressure effects through the enthalpy. The associated P V term is usually negligible for condensed phases at ambient conditions due to the low compressibility of solids, but becomes important at elevated pressures such as those in the Earth's mantle.