Most physical quantities carry a dimension, and their numeric values are meaningful only in conjuction with a suitable unit. A computer, on the other hand, processes just plain numbers. The interpretation of such a numeric value as physical quantity depends on the—completely arbitrary—specification of the associated unit. Within a given simulation, the only constraint is that all units are derived from the same set of base units, e.g., for length, time, mass, temperature, and current/charge.
For example, an interaction range “” of the Lennard-Jones potential may be interpreted as , , or even (for argon). Another more abstract interpretation of “” is that all lengths are measured relative to .
Typical choices for base units along with some derived units are given in the table:
|physical dimension||symbol||SI base units||cgs system||abstract units (Lennard-Jones potential)|
|current||I||ampère||franklin / second|
|dynamic viscosity||M×L⁻¹×T⁻¹||pascal × second||poise|
|charge||I×T||ampère × second||franklin|