Regarding Autocorrelation calculation

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#1

Dear Sir,

I am trying to calculate the auto correlation of a quantity during my simulation but a bit confused about the output information of ‘fix ave/correlate’ command. It says in the manual that for keyword of ‘auto’ and ‘file’ (which is what I need for auto correlation), there will be ‘Npair+2’ columns in the output file. And since I just have one input quantity to compute, there should be 3 columns. But in my output file there was actually 4 columns like this: (Nevery = 1, Nrepeat = 50, Nfreq = 1000)

…

2000 50
1 0 1001 43.6729
2 1 1000 43.678
3 2 999 43.683
4 3 998 43.6879
5 4 997 43.6928
…

46 45 956 43.8488
47 46 955 43.8515
48 47 954 43.8541
49 48 953 43.8566
50 49 952 43.8591
…

I assume the last column is the auto correlation values (for different lags), but what are the means for other columns ?

Thank you very much for your time in advance!

Best Regards,
Zenan Qi

College of Engineering
Boston University
8 Saint Marry Street
Boston, MA, 02215
U.S.A

#2

Dear Sir,

I am trying to calculate the auto correlation of a quantity during my
simulation but a bit confused about the output information of 'fix
ave/correlate' command. It says in the manual that for keyword of 'auto' and
'file' (which is what I need for auto correlation), there will be 'Npair+2'
columns in the output file. And since I just have one input quantity to
compute, there should be 3 columns. But in my output file there was actually
4 columns like this: (Nevery = 1, Nrepeat = 50, Nfreq = 1000)

the first column is not a computed column; it is the index of the row
that is printed and thus doesn't count toward the number of columns.

axel.