The above simulation was designed for a system with piston so that the volume changed according to external apply modification: change pressure, temperature or number of particles.
I modified [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=632.0]another simulation[/url] to simulate what you want.
You can select [b]Volume fixed[/b] checkbox, then you can change number of particle,temperature and the pressure will change according to your input (you can also change volume,too!).
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop.