Atomic units seem to be the preferred system in the atomic and molecular physics community. Internally, the code works in atomic units. However, for output, some people like to use a system based on electron-Volts (eV) for energies and Angstroms (Å) for length.
Normally time units are derived from energy and length units, so it is measured in $\hbar$/Hartree or $\hbar$/eV.
Warning 1: All files read on input will also be treated using these units, including XYZ geometry files.
Warning 2: Some values are treated in their most common units, for example atomic masses (a.m.u.), electron effective masses (electron mass), vibrational frequencies (cm-1) or temperatures (Kelvin). The unit of charge is always the electronic charge e.
Electronvolts for energy, Angstroms for length, the rest of the
units are derived from these and $\hbar=1$.
The XYZ will be assumed to be in Bohr atomic units.
XYZ files will be assumed to be always in Angstrom,
independently of the units used by Octopus. This ensures
compatibility with most programs, that assume XYZ files have
coordinates in Angstrom.