Exercise 1: Anisotropic Constant Pressure MD

Difficulty: Medium

Summary

This exercise studies a well-known phase transition in potassium chloride [1] using constant pressure molecular dynamics. The objective is to develop the best practice in using such algorithms and to learn how phase transitions can be induced, detected and monitored in a simulation.

Background

Potassium chloride at ambient temperature and pressure adopts the cubic rocksalt structure, in which each ion is surrounded by six ions of opposite charge in an octahedral arrangement. Under high pressure this structure transforms to something more close packed - the so-called caesium chloride structure, where the nearest neighbour coordination rises to eight ions. (Using the model potential adopted here, this occurs at about 1.4 Gpa.)

In this exercise the student will have the opportunity to see this phase transition using the method of anisotropic constant pressure molecular dynamics. Commencing with the rocksalt crystal structure and applying a fixed external pressure it is possible to induce the phase transition in a simulation. Similarly it is possible to see the reverse transition back to rocksalt. However it is not necessarily trivial to make these transitions happen in any given simulation (though you may be lucky the first time!) Your task will be to find the conditions under which the phase transition occurs. This will not be entirely a matter of finding the right conditions of temperature and pressure, but will also involve setting up the control parameters for the simulation so as to encourage the phase transition to occur. (Even if the transformation is thermodynamically permitted, it does not follow that it will happen in the lifetime of a simulation.)

Instructions

Copy the TEST1.tar.gz file into the dl_class/data directory and unpack:

cd dl_class/data
gunzip TEST1.tar.gz
tar -xf TEST1.tar

Once unpacked, you may delete the TEST1.tar file.

Now go to the dl_class/execute directory and start up the GUI:

cd dl_class/execute
java -jar ../java/GUI.jar

Task

Using the Store/Fetch option of the data archiver on the GUI Execute menu, copy of the contents of the sub-subdirectory TEST1 of the data subdirectory into the execute subdirectory. You will obtain the files CONTROL, FIELD and CONFIG. The last of these is a crystal of potassium chloride at ambient temperature and pressure (i.e. in the rocksalt structure). You should proceed as follows.

  1. Load the CONTROL file into the GUI CONTROL panel under the FileMaker menu. Select one of the constant stress options indicated on the CONTROL panel and set appropriate relaxation times for the thermostat and barostat. Choose an appropriate starting temperature and pressure and perform a reference simulation of the system at ambient temperature and pressure (i.e. set DLPOLY.X running - 2000 time steps is quite sufficient). Examine the resulting OUTPUT file and display the final REVCON file and simulation RDFs to see what structure you have.
  2. Repeat the simulation at a different state point, where you might expect a phase transition to occur. Examine the result graphically once again (using the REVCON file) and try to deduce how the phase transition occurred. Look at the RDF plots and try to determine what phase the structure is now in.
  3. If you do not see a phase transition, experiment with the control parameters (e.g. change the relaxation times, temperature or pressure, as you think fit) until you see one. Be as systematic as you can, using whatever insight you gain to rationalise what's going on.
  4. If you believe have obtained the phase transition, a number of other options are open to you:

Bibliography

1
M. Parrinello and A. Rahman, J. Phys. C. (1981) 6 511.