Correlation Functions

This exercise is to familiarize yourself with correlation functions.

• Change to the clamps/ex2 directory and run CLAMPS. The run id is ex2, and it should produce the output files ex2.out,ex2.crd, and ex2.sca. (The input file can also be downloaded: ex2.in)

  This input file is in 3 dimensions with 256 particles with a Lennard-Jones potential with the parameters for argon.

• DataSpork can be used to calculate g(r) and S(k) from the configurations saved in the .crd file. Open the .crd file using the "File/Open Datasets" menu, with the PairDistributionCalculator reader. Once the file is opened in its own tabbed pane, you can calculate g(r) or S(k) by right-clicking on the filename, and choosing an interpreter from the popup menu. (It may take a minute or two to calculate the pair distribution functions -- Java is slow.) For g(r) you can set the bin width dr, and the "start cutoff", which is the number of initial configurations to exclude. For S(k) you can set the number of k-shells, which determines the maximum range in k-space S(k) is calculated over.

• Is this system a liquid or solid? How can you tell by looking at g(r) and S(k)? Change the density while holding the temperature fixed until the system changes state. Roughly at what density does this happen?

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