about velocity in molecular dynamics

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tfb · #1 Post by **tfb** » Tue Oct 09, 2007 2:15 am

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

#2 Post by **admin** » Wed Oct 10, 2007 7:33 am

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

tfb · #3 Post by **tfb** » Thu Oct 11, 2007 12:56 am

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

graeme · #4 Post by **graeme** » Thu Oct 11, 2007 2:17 am

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>

tfb · #5 Post by **tfb** » Thu Oct 11, 2007 10:12 am

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