Averaging Unwrapped Coordinates

Dear all,

I would like to compute the average positions of all atoms over a certain time span. According to the documentation for ‘fix ave/atom command’;

“The x,y,z attributes are values that are re-wrapped inside the periodic box whenever an atom crosses a periodic boundary. Thus if you time average an atom that spends half its time on either side of the periodic box, you will get a value in the middle of the box. If this is not what you want, consider averaging unwrapped coordinates, which can be provided by the compute property/atom command via its xu,yu,zu attributes.”

Therefore, I tried the following:

compute unwrap all property/atom xu yu zu

fix meanxyz all ave/atom 1 100 100 c_unwrap[*]

But I get the error:

ERROR: Fix ave/atom compute does not calculate a per-atom vector (…/fix_ave_atom.cpp:138)

What am I doing wrong?

Kind Regards,

Dear all,

I would like to compute the average positions of all atoms over a certain
time span. According to the documentation for ‘fix ave/atom command’;

“The x,y,z attributes are values that are re-wrapped inside the periodic box
whenever an atom crosses a periodic boundary. Thus if you time average an
atom that spends half its time on either side of the periodic box, you will
get a value in the middle of the box. If this is not what you want, consider
averaging unwrapped coordinates, which can be provided by the compute
property/atom command via its xu,yu,zu attributes.”

Therefore, I tried the following:

compute unwrap all property/atom xu yu zu

fix meanxyz all ave/atom 1 100 100 c_unwrap[*]

But I get the error:

ERROR: Fix ave/atom compute does not calculate a per-atom vector
(../fix_ave_atom.cpp:138)

What am I doing wrong?

works for me with the current version of LAMMPS (20 Jan 2017).

axel.