fix ave/atom command
Syntax
fix ID group-ID ave/atom Nevery Nrepeat Nfreq value1 value2 ...
ID, group-ID are documented in fix command
ave/atom = style name of this fix command
Nevery = use input values every this many timesteps
Nrepeat = # of times to use input values for calculating averages
Nfreq = calculate averages every this many timesteps
one or more input values can be listed
value = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID[i], f_ID, f_ID[i], v_name
x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component) c_ID = per-atom vector calculated by a compute with ID c_ID[I] = Ith column of per-atom array calculated by a compute with ID, I can include wildcard (see below) f_ID = per-atom vector calculated by a fix with ID f_ID[I] = Ith column of per-atom array calculated by a fix with ID, I can include wildcard (see below) v_name = per-atom vector calculated by an atom-style variable with name
Examples
fix 1 all ave/atom 1 100 100 vx vy vz
fix 1 all ave/atom 10 20 1000 c_my_stress[1]
fix 1 all ave/atom 10 20 1000 c_my_stress[*]
Description
Use one or more per-atom vectors as inputs every few timesteps, and average them atom by atom over longer timescales. The resulting per-atom averages can be used by other output commands such as the fix ave/chunk or dump custom commands.
The group specified with the command means only atoms within the group have their averages computed. Results are set to 0.0 for atoms not in the group.
Each input value can be an atom attribute (position, velocity, force component) or can be the result of a compute or fix or the evaluation of an atom-style variable. In the latter cases, the compute, fix, or variable must produce a per-atom vector, not a global quantity or local quantity. If you wish to time-average global quantities from a compute, fix, or variable, then see the fix ave/time command.
Each per-atom value of each input vector is averaged independently.
Computes that produce per-atom vectors or arrays are those which have the word atom in their style name. See the doc pages for individual fixes to determine which ones produce per-atom vectors or arrays. Variables of style atom are the only ones that can be used with this fix since they produce per-atom vectors.
Note that for values from a compute or fix, the bracketed index I can
be specified using a wildcard asterisk with the index to effectively
specify multiple values. This takes the form “*” or “*n” or “m*” or
“m*n”. If
Using a wildcard is the same as if the individual columns of the array had been listed one by one. For example, these two fix ave/atom commands are equivalent, since the compute stress/atom command creates a per-atom array with six columns:
compute my_stress all stress/atom NULL
fix 1 all ave/atom 10 20 1000 c_my_stress[*]
fix 1 all ave/atom 10 20 1000 c_my_stress[1] c_my_stress[2] &
c_my_stress[3] c_my_stress[4] &
c_my_stress[5] c_my_stress[6]
The
For example, if
The atom attribute values (x, y, z, vx, vy, vz, fx, fy, and fz) are self-explanatory. Note that other atom attributes can be used as inputs to this fix by using the compute property/atom command and then specifying an input value from that compute.
Note
The x, y, and 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 of 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, and zu attributes.
If a value begins with “c_”, a compute ID must follow which has been
previously defined in the input script. If no bracketed term is
appended, the per-atom vector calculated by the compute is used. If a
bracketed term containing an index
If a value begins with “f_”, a fix ID must follow which has been previously
defined in the input script. If no bracketed term is appended, the per-atom
vector calculated by the fix is used. If a bracketed term containing an index
If a value begins with “v_”, a variable name must follow which has been previously defined in the input script as an atom-style variable. Variables of style atom can reference thermodynamic keywords or invoke other computes, fixes, or variables when they are evaluated, so this is a very general means of generating per-atom quantities to time average.
Restart, fix_modify, output, run start/stop, minimize info
No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global scalar or vector quantities are stored by this fix for access by various output commands.
This fix produces a per-atom vector or array which can be accessed by
various output commands. A vector is produced if
only a single quantity is averaged by this fix. If two or more
quantities are averaged, then an array of values is produced. The
per-atom values can only be accessed on timesteps that are multiples
of
No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.
Restrictions
none
Default
none