public class LJ
Lennard-Jones system of units, such that all quantities are made
dimensionless with respect to a characteristic size (sigma), energy (epsilon)
and mass. Values of sigma, epsilon and mass reside with the instance of
the LJ unit system, to which each unit refers when making its conversion.
Thus any changes in sigma, epsilon, or mass are immediately recognized
by the unit conversions.
Values of sigma, epsilon, and mass are specified in simulation units.
Default values are each 1.0.