Physical units in simulations

In principle, it is possible to use arbitrary units of measurement in simulations. The only clear restriction is that the chosen set of units must be coherent - if the unit of energy is eV and the unit of distance is Å, the unit of force must be eV/Å. In practise, however, it is best to choose units so that the range of input values is close to 1. This improves readability of the values and prevents underflow and overflow in finite precision arithmetics. Obviously, it is also important to stick with the chosen set of units and not mix them.

The set of units used by these simulation tools are:

• charge: e (elementary charge)
• energy: eV (electron volt)
• distance: Å (Ångström)
• mass: u (atomic mass unit)
• temperature: K (Kelvin)
• time: fs (femtosecond)
• force: eV/Å
• velocity: Å/fs

These units represent the typical scale of quantities in the atomic scale. Another popular set of measurement units in atomic scale simulations are the atomic units. You are likely to encounter them later on if you end up in computational physics.