# A Note on Units

RuNNer uses atomic units internally.

• Length: $$a_0$$ (Bohr radius)
• Charge: $$e$$ (charge of an electron $$\rightarrow 1$$ in RuNNer units)
• Spin: $$\hbar$$ (spin of an electron $$\rightarrow \tfrac{1}{2}$$ in RuNNer units)
• Energy: $$E_\mathrm{h}$$ (Hartree)
$1\,E_\mathrm{h} = \frac{\hbar^2}{m_e a_0^2} = \frac{e^2}{4\pi \epsilon_0 a_0}$
• Force: $$E_\mathrm{h}/a_0$$
• Stress: $$E_\mathrm{h}/a_0^3$$

While in the standard output file of runner_mode 2 the unit can be switched to eV for energies and eV/$$E_\mathrm{h}$$ for forces by an input keyword (fitting_unit), please note that all other input and output files strictly use these atomic units, e.g. for the atomic positions, forces, and lattice vectors. According to experience, unit errors belong to the most frequent errors made by beginners using RuNNer.

Please also note that internally RuNNer uses energies normalized per atom and depending on the setting of the remove_atom_energies keyword these energies may not be total energies but atomic "binding energies".