To avoid the handling of small numbers, choose units appropriate to
the model.

*Lennard-Jones:*
Energy unit
; length .
The pair energy is then

where
and
.

As the third mechanical unit, choose the atomic mass
.

The time unit is now the combination
.

*Electrical charge:* best measured in multiples of the
electron charge,
.

*Number density:* is a large number; therefore we
reduce it by a suitable standard density,
:
.

*Temperature:*

*Hard spheres:*
no ``natural'' unit of energy; therefore choose
self-consistent time unit
.

For hard spheres of diameter
the customary
standard density is
; thus:
.

F. J. Vesely / University of Vienna