GROMACS version: 2018.4

GROMACS modification: No

I used the velacc module of Gromacs to compute the velocity autocorrelation function. But I’m confused about the output file by velacc.

Here is the order I used: gmx velacc -f water500.trr -s water500.tpr -n index.ndx -b 1 -e 9 -mol yes -normalize no -o vac.xvg

The equation of velocity autocorrelation function is: Cv(τ) = <vi(τ) · vi(0)>

The result of velocity autocorrelation function at the beginning time is 1 shown in vac.xvg file. But Cv(0) = <vi(0) · vi(0)>. and It seems that this value is not 1.

Then, Could anyone please help me to understand this?