[gmx-users] Effects of -P and -fitfn in g_rotacf

[Yun Shi]

Hi all,

I am trying to extract rotational correlation time of a small molecule
by g_rotacf.

With little knowledge of the theory on auto-correlation function, I
tried different values of -P (Order of Legendre polynomial for ACF)
and -fitfn (Fit function: none, exp, aexp, exp_exp, vac, exp5, exp7,
exp9 or erffit).

I found that -beginfit and -endfit seemed to have no effect at all as
long as the input trajectory is the same, and that those exponential
fit functions ( -fitfn ), such as exp, aexp, exp_exp, exp5, gave me
the same correlation time.

The only thing matters seem to be different orders of Legendre
polynomial for ACF ( -P ), which resulted in dramatically different
correlation times.

Does anyone have any idea how to choose the order of Legendre
polynomial for ACF? How do I know which order suits my system?

Thanks,

Yun