hartree
The hartree is the atomic unit of energy, about 4.359744722 × 10⁻¹⁸ joules, or 27.211 electronvolts. It is twice the ionisation energy of hydrogen and the energy scale that falls out when the electron mass, elementary charge, reduced Planck constant and Coulomb constant are all set to one.
Watch out: The value is measured rather than exact, since it depends on the electron mass in kilograms. It is also easy to confuse with the rydberg, which is exactly half a hartree and is the natural unit in atomic spectroscopy. Chemical accuracy, about 1 kcal/mol, is 0.0016 hartree, which is why quantum chemistry output is printed to six decimal places.
| 1 Eh | 4.3597447e-18 J |
Named for Douglas Hartree, whose self-consistent field method made quantum chemistry computable. Every electronic structure code in existence reports its total energies in hartrees.