This type specifies the Lennard-Jones potential with
periodic boundary conditions (minimum image convention.
The box lengths in the , and
directions and the cutoff (as a fraction of the smallest box length)
must be specified using the PARAMS keyword in the odata file.
Setting a negative value for the cutoff fraction means that all minimum
images are included.
Subroutines used: ljpdiff.
If the BINARY keyword is specified then a binary Lennard-Jones
potential is used along with subroutine ljpbin or ljpshift.