# Simulation units¶

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)
length L metre centimetre
time T second second
mass M kilogram gram
temperature Θ kelvin
current I ampère franklin / second

energy M×L²×T⁻² joule erg
force M×L×T⁻² newton dyne
pressure M×L⁻¹×T⁻² pascal barye
dynamic viscosity M×L⁻¹×T⁻¹ pascal × second poise
charge I×T ampère × second franklin