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.

also, please humor me and follow my advice to compute the estimated number
of ideal gas atoms for your given volume and pressure.

There is no different, but seems to be different for lammps.
I will try an empty box inserting atoms at costant volume...

In licterature to predict adsorbent property is used gran canonical monte
carlo tecnique.
Lammps can perform this type of simulation??

​yes, if it is performed correctly and with a meaningful system and