gcmc Ar 87K

An isotherm is a collection of points at different pressure.

​no, an isotherm is a collection of points at the _same temperature_.​
that is why it is called an *iso**therm*.

I said that that the temperature is 87K and it is obviously that it is
costant in a isotherm.

In my system is present a free volume and an atom of argon.

​your thermo output speaks differently.​ you have one atom _initially_! but
then it keeps changing with obviously fix gcmc removing and inserting atoms.

Volume 1000 + 1 ar

If I want know many atoms are present in volume at definite pressure or
fugacity, I must inset more atoms in a box, in indipendent mode, as in
monte carlo procedure, and not manually.

​so please explain, what is different from the point of thermodynamics, if
you keep the number of atoms fixed and determine the average pressure,
versus a simulation where you try to maintain a constant pressure through
inserting/removing atoms and then determine the average number of atoms.
this all at constant volume, of course.