fix ave/moments command — LAMMPS documentation

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# fix ave/moments command

## Syntax

    fix ID group-ID ave/moments Nevery Nrepeat Nfreq value1 value2 ... moment1 moment2 ... keyword args ...

- ID, group-ID are documented in fix command

- ave/moments = style name of this fix command

- Nevery = use input values every this many time steps

- Nrepeat = # of times to use input values for calculating averages

- Nfreq = calculate averages every this many time steps

- one or more input variables can be listed

- value = v_name

      c_ID = global scalar calculated by a compute with ID
      c_ID[I] = Ith component of global vector calculated by a compute with ID, I can include wildcard (see below)
      f_ID = global scalar calculated by a fix with ID
      f_ID[I] = Ith component of global vector calculated by a fix with ID, I can include wildcard (see below)
      v_name = value calculated by an equal-style variable with name
      v_name[I] = value calculated by a vector-style variable with name, I can include wildcard (see below)

- one or more moments to compute can be listed

- moment = mean or stddev or variance or skew or kurtosis, see exact definitions below.

- zero or more keyword/arg pairs may be appended

- keyword = start or history

      start args = Nstart
        Nstart = invoke first after this time step
      history args = Nrecent
        Nrecent = keep a history of up to Nrecent outputs

## Examples

    fix 1 all ave/moments 1 1000 100 v_volume mean stddev
    fix 1 all ave/moments 1 200 1000 v_volume variance kurtosis history 10

## Description

Added in version 12Jun2025.

Using one or more values as input, calculate the moments of the underlying (population) distributions based on samples collected every few time steps over a time step window. The definitions of the moments calculated are given below.

The group specified with this command is ignored. However, note that specified values may represent calculations performed by computes and fixes which store their own “group” definitions.

Each listed value can be the result of a compute or fix or the evaluation of an equal-style or vector-style variable. In each case, the compute, fix, or variable must produce a global quantity, not a per-atom or local quantity. If you wish to spatial- or time-average or histogram per-atom quantities from a compute, fix, or variable, then see the fix ave/chunk, fix ave/atom, or fix ave/histo commands. If you wish to sum a per-atom quantity into a single global quantity, see the compute reduce command.

Many computes and fixes produce global quantities. See their doc pages for details. Variables of style equal and vector are the only ones that can be used with this fix. Variables of style atom cannot be used, since they produce per-atom values.

The input values must all be scalars or vectors with a bracketed term appended, indicating the \(I^\text{th}\) value of the vector is used.

The result of this fix can be accessed as a vector, containing the interleaved moments of each input in order. If M moments are requested, then the moments of input 1 will be the first M values in the vector output by this fix. The moments of input 2 will the next M values, etc. If there are N values, the vector length will be N*M.

For input values from a compute or fix or variable, 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 \(N\) is the size of the vector, then an asterisk with no numeric values means all indices from 1 to \(N\). A leading asterisk means all indices from 1 to n (inclusive). A trailing asterisk means all indices from n to \(N\) (inclusive). A middle asterisk means all indices from m to n (inclusive).

Using a wildcard is the same as if the individual elements of the vector or cells of the array had been listed one by one. For examples, see the description of this capability in fix ave/time.

The \(N_\text{every}\), \(N_\text{repeat}\), and \(N_\text{freq}\) arguments specify on what time steps the input values will be used in order to contribute to the average. The final statistics are generated on time steps that are a multiple of \(N_\text{freq}\). The average is over a window of up to \(N_\text{repeat}\) quantities, computed in the preceding portion of the simulation once every \(N_\text{every}\) time steps.

Note

Contrary to most fix ave/* commands, it is not required that Nevery * Nrepeat <= Nfreq. This is to allow the user to choose the time window and number of samples contributing to the output at each Nfreq interval.

For example, if \(N_\text{freq}=100\) and \(N_\text{repeat}=5\) (and \(N_\text{every}=1\)), then on step 100 values from time steps 96, 97, 98, 99, and 100 will be used. The fix does not compute its inputs on steps that are not required. If \(N_\text{freq}=5\), \(N_\text{repeat}=8\) and \(N_\text{every}=1\), then values will first be calculated on step 5 from steps 1-5, on step 10 from 3-10, on step 15 from 8-15 and so on, forming a rolling average over timesteps that span a time window larger than Nfreq.

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 global scalar calculated by the compute is used. If a bracketed term is appended, the Ith element of the global vector calculated by the compute is used. See the discussion above for how I can be specified with a wildcard asterisk to effectively specify multiple values.

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 global scalar calculated by the fix is used. If a bracketed term is appended, the Ith element of the global vector calculated by the fix is used. See the discussion above for how I can be specified with a wildcard asterisk to effectively specify multiple values.

Note that some fixes only produce their values on certain time steps, which must be compatible with Nevery, else an error will result. Users can also write code for their own fix styles and add them to LAMMPS.

If a value begins with “v_”, a variable name must follow which has been previously defined in the input script. Only equal-style or vector-style variables can be used, which both produce global values. Vector-style variables require a bracketed term to specify the Ith element of the vector calculated by the variable.

Note that variables of style equal and vector define a formula which can reference individual atom properties or thermodynamic keywords, or they can invoke other computes, fixes, or variables when they are evaluated, so this is a very general means of specifying quantities to time average.

The moments are output in the order requested in the arguments following the last input. Any number and order of moments can be specified, although it does not make much sense to specify the same moment multiple times. All moments are computed using a correction of the sample estimators used to obtain unbiased cumulants \(k_{1..4}\) (see (Cramer)). The correction for variance is the standard Bessel correction. For other moments, see (Joanes).

For mean, the arithmetic mean \(\bar{x} = \frac{1}{n} \sum_{i=1}^{n} x_i\) is calculated.

For variance, the Bessel-corrected sample variance \(var = k_2 = \frac{1}{n - 1} \sum_{i=1}^{n} (x_i - \bar{x})^2\) is calculated.

For stddev, the Bessel-corrected sample standard deviation \(stddev = \sqrt{k_2}\) is calculated.

For skew, the adjusted Fisher–Pearson standardized moment \(G_1 = \frac{k_3}{k_2^{3/2}} = \frac{k_3}{stddev^3}\) is calculated.

For kurtosis, the adjusted Fisher–Pearson standardized moment \(G_2 = \frac{k_4}{k_2^2}\) is calculated.

Fix invocation and output can be modified by optional keywords.

The start keyword specifies that the first computation should be no earlier than the step number given (but will still occur on a multiple of Nfreq). The default is step 0. Often input values can be 0.0 at time 0, so setting start to a larger value can avoid including a 0.0 in a longer series.

The history keyword stores the Nrecent most recent outputs on Nfreq timesteps, so they can be accessed as global outputs of the fix. Nrecent must be >= 1. The default is 1, meaning only the most recent output is accessible. For example, if history 10 is specified and Nfreq = 1000, then on timestep 20000, the Nfreq outputs from steps 20000, 19000, … 11000 are available for access. See below for details on how to access the history values.

For example, this will store the outputs of the previous 10 Nfreq time steps, i.e. a window of 10000 time steps:

    fix 1 all ave/moments 1 200 1000 v_volume mean history 10

The previous results can be accessed as values in a global array output by this fix. Each column of the array is the vector output of the N-th preceding Nfreq timestep. For example, assuming a single moment is calculated, the most recent result corresponding to the third input value would be accessed as “f_name[3][1]”, “f_name[3][4]” is the 4th most recent and so on. The current vector output is always the first column of the array, corresponding to the most recent result.

To illustrate the utility of keeping output history, consider using this fix in conjunction with fix halt to stop a run automatically if a quantity is converged to within some desired tolerance:

    variable target equal etot
    fix aveg all ave/moments 1 200 1000 v_target mean stddev history 10
    variable stopcond equal "abs(f_aveg[1]-f_aveg[1][10])<f_aveg[2]"
    fix fhalt all halt 1000 v_stopcond == 1

In this example, every 1000 time steps, the average and standard deviation of the total energy over the previous 200 time steps are calculated. If the difference between the most recent and 10-th most recent average is lower than the most recent standard deviation, the run is stopped.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to binary restart files.

This fix produces a global vector and global array which can be accessed by various output commands. The values can be accessed on any time step, but may not be current.

A global vector is produced with the # of elements = number of moments * number of inputs. The moments are output in the order given in the fix definition. An array is produced having # of rows = length of vector output (with an ordering which matches the vector) and # of columns = value of history. There is always at least one column.

Each element of the global vector or array can be either “intensive” or “extensive”, depending on whether the values contributing to the element are “intensive” or “extensive”. If a compute or fix provides the value being time averaged, then the compute or fix determines whether the value is intensive or extensive; see the page for that compute or fix for further info. Values produced by a variable are treated as intensive.

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

This compute is part of the EXTRA-FIX package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

## Related commands

fix ave/time,

## Default

The option defaults are history = 1, start = 0.

(Cramer) Cramer, Mathematical Methods of Statistics, Princeton University Press (1946).

(Joanes) Joanes, Gill, The Statistician, 47, 183–189 (1998).