See: Description
Class  Description 

CGS 
CentimeterGramSecond system of units.

LJ 
LennardJones system of units, such that all quantities are made
dimensionless with respect to a characteristic size (sigma), energy (epsilon)
and mass.

LJ.LJUnit  
MKS 
MeterKilogramSecond system of units

UnitSystem 
Specifies a system of units.

UnitSystem.Sim 
System of units based on simulation units of Daltons, picoseconds, and Angstroms

The package etomica.units
defines the Unit and Dimension classes.