FORTRAN code for periodic boundary conditions

Here is the FORTRAN code that implements the periodic boundary conditions. This is only part of the larger programs for molecular dynamics and Monte Carlo simulations of simple monoatomic fluids with N particles. These larger programs are presented in the next two chapters. 

This next command is used to iterate and repeat executions, do-loops in FORTRAN end with an enddo command. 

The implementation of the periodic boundary conditions will be clear with an example, presented in the next section. 

Although this piece of code will result in exactly the same results as the original one, it will result in a considerably faster simulation, requiring a shorter wall-clock time to complete the computations. 

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