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I want to calculate the veolcity correlation function ,but where I can get the velcity form the vasp output.
thank you in advance.

the velocity of the ions is written in CONTCAR (second block of data after the positions) if you do an MD.

Thank you for you quickly answer ,but you know the CONTCAR is overwrite at every time step .so how I can calculate the veolcity correlation function ,which ask for a lot of velocities collectives to calculate

Here is a perl script that might be helpful. It uses finite difference to generate velocities from the XDATCAR at each time step:

http://theory.cm.utexas.edu/vtsttools/d ... at2vdat.pl

A limitation is that it only works with orthogonal cells

You need to make a file of masses for the atoms called massfile. This is a simple file of the following form (for the example of CH4):

1 1 12.0
2 1 1.0
3 1 1.0
4 1 1.0
5 1 1.0

It doesn't matter what you put in the first two columns; only the third is read, and that is just to calculate kinetic energies. It is a short script, so feel free to modify it as needed.



<span class='smallblacktext'>[ Edited Thu Oct 11 2007, 04:25AM ]</span>

thank you very much ,it is very kind of you to share your script.