Unlike most molecular dynamics software, ATOMDEMO includes the force of gravity. This makes it easier to compare the simulations to everyday experience. Also unlike most molecular dynamics software, ATOMDEMO operates in two dimensions only. This simplifies display and comprehension.
ATOMDEMO runs well on any Windows PC. The faster the PC, the faster the simulations will run.
Upon startup, you will see 200 atoms (screenshot). These atoms are formed into a solid that is taller than wide. The atoms in the top half are colored red, the lower half are blue, and there is a single green atom. The colors are there only to help the eye follow individual atoms - all atoms behave alike. These atoms are confined within a hollow vessel (the dark area on the screen).
The program begins with the atoms in motion and the thermostat set to 200K. The program thermostat is like a house thermostat, it heats or cools the atoms to keep them at temperature, but the temperature can differ a little from what the thermostat reads.
Onscreen there are labeled buttons. These can be worked with the mouse or keyboard. The underlined letter on each button corresponds to the keyboard key which controls that button. The "Pause" button will stop the action. Pressing it again (when it is re-labeled "Continue") will resume. The "Raise Thermostat" and "Lower Thermostat" are self-explanatory. There are two equivalent button labeled with up and down arrows. The thermostat can be set between 0 and 2000 Kelvin, in intervals of 50 Kelvin. The "Quit" button ends the program.
The program starts with 200 atoms and the thermostat at 200 degrees Kelvin. At this temperature the atoms form a weak plastic solid which is not rigid enough to support its own weight. The solid, which is originally taller than wide, tips over and lands on its side. One can see while the atoms vibrate as result of heat energy, the solid maintains an orderly crystalline atomic arrangement.
If you observe the single green atom, you will see that although it vibrates, it remains close to its lattice site (on rare occasions you might see it move over one site).
When you're ready to melt the solid, raise the thermostat to 400 degrees Kelvin (400K). You will see the solid melt and the liquid flow to cover the bottom of the vessel. There will be a well defined (but unsteady) surface on the liquid. If you observe the green atom now, you will see that it does not stay close to one site, but slowly meanders around the liquid. This is the phenomenon of diffusion.
If you set the thermostat to 500K, you will see some of the liquid vaporize, but most of the atoms remain in the liquid state. Atoms which vaporize will slowly return to the liquid, being pulled down by the force of gravity.
Now set the thermostat to 700K. At this temperature all the liquid vaporizes into a gas. You can see that the gas is denser at the bottom of the container, thinner at the top. This is analogous to the thinning of the Earth's atmosphere at high elevations. In both cases, gravity pulls the atoms downward.
If you follow the green atom again, you can see it diffuse over greater
distances than when it was in the liquid state.
(2) Quenching, Sound, Crystalline Defects, and Annealing
Quenching is the rapid cooling of a material. You can observe this easily with ATOMDEMO. After the atoms have been at 700K long enough to become a gas, set the thermostat to 0K. At this setting, the atoms are cooled very rapidly until the reach 0K, at which time they will stop moving entirely. But they do not stop right away, because they must first fall to the bottom of the vessel.
The atoms will slowly collect at the bottom of the vessel and form themselves into a solid crystalline arrangement. The atoms in this arrangement will be very cold, 0K or close to it. The atoms which fall carry energy with them. When the falling atoms strike the solid they will start their new neighbors vibrating. This motion will continue until the solid cools down. If you see an atom hit the solid while the solid is perfectly still, you will see the atoms in the solid vibrate. The vibration starts at the first solid atom that was hit, and spreads outward to the surrounding atoms. This is a sound wave. It is created in much the same way as you would strike a drum. The sound wave quickly dies out due to the cooling effect of the thermostat.
Once the atoms stop moving, examine the atomic arrangement. It is not a perfect crystal, but rather includes several defects. The most common defects are the grain boundary and the vacancy. The grain boundary is where two regions of different crystal meet. In ATOMDEMO, the crystalline arrangement is always the same, but sometimes the crystal is turned differently in different regions. The place where two different regions meet is a grain boundary. Sometimes there will be no grain boundary. If not, try re-heating the atoms back up to 700K and repeat the process.
Another common defect is the vacancy. Do you see a "hole" in the lattice? A place where there should be an atom? This is a vacancy. Again, they do not always form. You may want to try again and repeat the demonstration.
If you see either (or both) defects, you can try annealing them. Annealing is a slow gentle heating of a solid which helps remove defects, but does not melt the solid. Raise the thermostat to 50K and watch the defects. Do they stay? If so, raise the thermostat another 50K. How high do you need to go to remove the defects?
