[gmx-users] g_energy averages

David spoel at xray.bmc.uu.se
Wed Jul 17 20:34:48 CEST 2002

On Wed, 2002-07-17 at 20:15, David L. Bostick wrote:
> .. So, if I want to find the standard error in an average value, I just
> take the fluctuation and divide it by sqrt(nsteps)?
> i.e.
> If I get from the energy file
> Energy                      Average       RMSD     Fluct.
> #Surf*SurfTen               147.805    2270.69    2263.84
> and the run was over 250000 steps, the standard error is
> 2263.84/sqrt(250000)
No you should divide by the sqrt of the number of *independent* data
points. For energy, there is a correlation on the order of a few
picoseconds (depends on system). For e.g. the dipole moment of the box
it is usually 10 ps (can be related to Debye relaxation). g_analyze may
help you estimating errors, but it is not trivial...

Groeten, David.
Dr. David van der Spoel, 	Biomedical center, Dept. of Biochemistry
Husargatan 3, Box 576,  	75123 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://zorn.bmc.uu.se/~spoel

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