## Description#

`gmx rotacf`

calculates the rotational correlation function

for molecules. Atom triplets (i,j,k) must be given in the index

file, defining two vectors ij and jk. The rotational ACF

is calculated as the autocorrelation function of the vector

n = ij x jk, i.e. the cross product of the two vectors.

Since three atoms span a plane, the order of the three atoms

does not matter. Optionally, by invoking the `-d`

switch, you can

calculate the rotational correlation function for linear molecules

by specifying atom pairs (i,j) in the index file.

EXAMPLES

`gmx rotacf -P 1 -nparm 2 -fft -n index -o rotacf-x-P1`

-fa expfit-x-P1 -beginfit 2.5 -endfit 20.0

This will calculate the rotational correlation function using a first

order Legendre polynomial of the angle of a vector defined by the index

file. The correlation function will be fitted from 2.5 ps until 20.0 ps

to a two-parameter exponential.

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