[gmx-users] Dihedral Autocorrelation Function with negative values

[Davide Mercadante]

Dear gmx users,

I am writing to ask why I am getting a dihedral autocorrelation function
(ACF) with negative values. I am trying to calculate the ACF using
g_angle (gromacs 4.5.5) with the following set of flags:

g_angle -f trajectory.xtc -type dihedral -oc acf.xvg -od angdist.xvg -n
angle.ndx

The ACF starts from 1 as expected and slowly drops (sometimes) to values
below 0 (approx. -0.2). Even when I use the flag -normalize (which I
would expect normalizes the ACF between 0 and 1) the output is identical.

Can you please help me to figure out why this happens?
Any help will be very much appreciated.

Thank you in advance.

Cheers,
David