This exercise studies a well-known phase transition in potassium chloride  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.
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.)
dl_poly/datadirectory and unpack:
cd dl_poly/data (i.e. go to dl_poly/data directory) gunzip TEST1.tar.gz tar -xf TEST1.tarOnce unpacked, you may delete the
Now go to the
dl_poly/execute directory and start up the
cd dl_poly/execute (i.e. go to dl_poly/execute directory) java -jar ../java/GUI.jar
Using the Store/Fetch option (i.e. 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.