[gmx-users] g_velacc again

[Anton Feenstra]

Nuno R. L. Ferreira wrote:
> David wrote:
> 
>>Have you looked at the graph in xmgrace? It is probably easiest to start
>>there, use the integration function to see how ill-behaved the ACF is.
>>It may just fluctuate wildly. Then use the non-linear fit to fit your
>>ACF to a function of your choosing and integrate again. You can probably
>>get the same result from gromacs if you use some more flags like
>>-beginfit and -endfit
> 
> 
> I've done the integration and the value is similar to the correlation time
> given by g_velacc. So basically the Correlation time is the VACF integral?
> But if it is, why does it come in ps? Should't be in nm2.s-1 or something
> similar ?

Correlation time is time, so it comes in time units, in this case ps.

The relation with 'nm2', I think, would come in due to your system size,
or dimensionality. I've gone through some of the basic formula's some long
time ago, and IIRC the units match up (between MSD and VACF) at some point.
IIRC (again), the 'nm2' actually comes in as something like 'nm3/nm'...


-- 
Groetjes,

Anton
  _____________ _______________________________________________________
|             |                                                       |
|  _   _  ___,| K. Anton Feenstra                                     |
| / \ / \'| | | Dept. of Pharmacochem. - Vrije Universiteit Amsterdam |
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