Hartree energy

Douglas Hartree built a unit system in which ħ, m_e, e and 4πε₀ all equal one, so that the Schrödinger equation for atoms loses its constants entirely. One hartree is then the natural energy: twice hydrogen's ionisation energy, and equal to the potential energy of two elementary charges one Bohr radius apart. Every serious electronic-structure code (Gaussian, VASP, Quantum ESPRESSO) reports total energies in hartrees.

Conversions worth keeping close: 1 E_h = 27.211 386 eV = 2625.5 kJ/mol = 627.51 kcal/mol = 219 474.6 cm⁻¹. The last one matters because spectroscopists work in wavenumbers while chemists work in kJ/mol. The classic trap remains the factor of two against the Rydberg: a plane-wave code quoting cutoff energies in Ry and a molecular code quoting them in hartree are not describing the same number, and mixing them silently doubles or halves your energy scale.