Why does the result of velocity autocorrelation function at the beginning time equal 1? (velacc)

User discussions
1

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?

2

It seems that the option: -normalize didn’t work for some unknown reasons. After I tried the order referred in my question again, the -normalize option did work this time.
And the value of velocity autocorrelation function at the beginning is not 1, which is reasonable.
Then, this question is solved.
thank you for your attention!