# Units¶

The unit system used internally by Lumol is the following:

- Angstrom (
`A`

) for distances; - Femtosecond (
`fs`

) for time; - Unified atomic mass unit (
`u`

or`Da`

) for mass; - Kelvin (
`K`

) for temperature; - Number of particles for quantity of matter;
- Radian (
`rad`

) for angles;

Any other internal unit is derived from this set:

- The internal unit of energy is
`u A^2 fs^-2`

; - The internal unit of force is
`u A fs^-2`

; - The internal unit of pressure is
`u A^-1 fs^-2`

; *etc.*

For convenience Lumol provides convertion facilities for any value in these internal unit to any others units. The following table lists available units that can be converted:

Quantity | Accepted units |
---|---|

Distance | A, Å, nm, pm, fm, m, bohr |

Time | fs, ps, ns |

Mass | u, Da, kDa, g, kg |

Matter | mol |

Angle | rad, deg |

Energy | J, kJ, kcal, eV, H, Ry |

Force | N |

Pressure | Pa, kPa, MPa, bar, atm |

In the input files, the units are specified as strings, and must be
spelled exactly as in the above table. They can be combined with other
units using `*`

for multiplication, `/`

for division, and `^`

for
exponentiation. Parentheses can be used to group sub-units together. Some
valid unit strings are `kcal/mol`

, `(J / mol) * A^-2`

, and
`m*fs^-1`

.