(sec:orcaarchitecture.units)=
# Units and Conversion Factors

Internally the program uses atomic units. This means that the unit of
energy is the Hartree (Eh) and the unit of length is the Bohr radius
($a_0$). The following conversion factors to other units are used:

:::{list-table}
* - 1 Eh
  - =
  - 27.2113834 eV
  -
* - 1 eV
  - =
  - 8065.54477 cm$^{-1}$
  - = 23.0605 $\frac{\text{kcal} }{\text{mol} }$
* - 1 cm$^{-1}$
  - =
  - 29979.2458 MHz
  -
* - 1 $a_0$
  - =
  - 0.5291772083 Å
  -
* - 1 a.t.u.
  - =
  - 2.4188843 10$^{-17}$ s
  -
:::