Editing P4V (section)

== Determine Velocities AutoCorrelations in Molecular Dynamics ==

One of the most powerful tool in P4Vasp is the possibility to calculate the autocorrelation of all the velocities in a system just with a few clicks. For molecular dynamics results, this autocorrelation can be usefull, especially for Infra-Red measurements. Here is how to use it:

* Firstly, the autocorrelation can be obtained only from the vasprun.xml file resulting for a finished calculation. If your molecular dynamics was aborted before reaching its last step, then you have to modify the end of your vasprun.xml file. [[nice_termination|Here is a little tool]] written in Python 2.7 to get a proper termination for files recovered from unfinished run.

* Once the file is loaded, you can look at the evolution of the system in time and also choose the "VAC" option in the "MD" menu.

* Then, the autocorrelation can be calculated for all the atoms simply with the "Read VAC" button.  

* For a molecular dynamics with N steps, the autocorrelation will be calculated with N-2 positions. Here is a molecular dynamics over 1000 steps.  

[[Image:VAC2.png]]

* The Fourier Transformation can be obtained by the "FFT" button and will display useful information about the frequencies existing in the system.  

* To get the data in a usable form, the photo camera button will three files, a postscript picture, a dat file with the data and a file with a .agr extension describing the graph.