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### fix ave/correlate/long command

Syntax:

    fix ID group-ID ave/correlate/long Nevery Nfreq value1 value2 ... keyword args ...

- ID, group-ID are documented in fix command
- ave/correlate/long = style name of this fix command
- Nevery = use input values every this many timesteps
- Nfreq = save state of the time correlation functions every this many timesteps
- one or more input values can be listed
- value = c_ID, c_ID[N], f_ID, f_ID[N], 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
        f_ID = global scalar calculated by a fix with ID
        f_ID[I] = Ith component of global vector calculated by a fix with ID
        v_name = global value calculated by an equal-style variable with name

- zero or more keyword/arg pairs may be appended
- keyword = type or start or file or overwrite or title1 or title2 or ncorr or p or m

        type arg = auto or upper or lower or auto/upper or auto/lower or full
          auto = correlate each value with itself
          upper = correlate each value with each succeeding value
          lower = correlate each value with each preceding value
          auto/upper = auto + upper
          auto/lower = auto + lower
          full = correlate each value with every other value, including itself = auto + upper + lower
        start args = Nstart
          Nstart = start accumulating correlations on this timestep
        file arg = filename
          filename = name of file to output correlation data to
        overwrite arg = none = overwrite output file with only latest output
        title1 arg = string
          string = text to print as 1st line of output file
        title2 arg = string
          string = text to print as 2nd line of output file
        ncorr arg = Ncorrelators
          Ncorrelators = number of correlators to store
        nlen args = Nlen
          Nlen = length of each correlator
        ncount args = Ncount
          Ncount = number of values over which succesive correlators are averaged

Examples:

    fix 1 all ave/correlate/long 5 1000 c_myTemp file temp.correlate
    fix 1 all ave/correlate/long 1 10000 &
              c_thermo_press[1] c_thermo_press[2] c_thermo_press[3] &
              type upper title1 "My correlation data" nlen 15 ncount 3

Description:

This fix is similar in spirit and syntax to the fix ave/correlate. However, this fix allows the efficient calculation of time correlation functions on the fly over extremely long time windows without too much CPU overhead, using a multiple-tau method (Ramirez) that decreases the resolution of the stored correlation function with time.

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 variable. See the fix ave/correlate doc page for details.

The Nevery and Nfreq arguments specify on what timesteps the input values will be used to calculate correlation data, and the frequency with which the time correlation functions will be output to a file. Note that there is no Nrepeat argument, unlike the fix ave/correlate command.

The optional keywords ncorr, nlen, and ncount are unique to this command and determine the number of correlation points calculated and the memory and CPU overhead used by this calculation. Nlen and ncount determine the amount of averaging done at longer correlation times. The default values nlen=16, ncount=2 ensure that the systematic error of the multiple-tau correlator is always below the level of the statistical error of a typical simulation (which depends on the ensemble size and the simulation length).

The maximum correlation time (in time steps) that can be reached is given by the formula (nlen-1) * ncount^(ncorr-1). Longer correlation times are discarded and not calculated. With the default values of the parameters (ncorr=20, nlen=16 and ncount=2), this corresponds to 7864320 time steps. If longer correlation times are needed, the value of ncorr should be increased. Using nlen=16 and ncount=2, with ncorr=30, the maximum number of steps that can be correlated is 80530636808. If ncorr=40, correlation times in excess of 8e12 time steps can be calculated.

The total memory needed for each correlation pair is roughly 4*ncorr*nlen*8 bytes. With the default values of the parameters, this corresponds to about 10 KB.

For the meaning of the additional optional keywords, see the fix ave/correlate doc page.

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

Since this fix in intended for the calculation of time correlation functions over very long MD simulations, the information about this fix is written automatically to binary restart files, so that the time correlation calculation can continue in subsequent simulations. None of the fix_modify options are relevant to this fix.

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 USER-MISC package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

Related commands:

fix ave/correlate

Default: none

The option defaults for keywords that are also keywords for the fix ave/correlate command are as follows: type = auto, start = 0, no file output, title 1,2 = strings as described on the fix ave/correlate doc page.

The option defaults for keywords unique to this command are as follows: ncorr=20, nlen=16, ncount=2.

(Ramirez) J. Ramirez, S.K. Sukumaran, B. Vorselaars and A.E. Likhtman, J. Chem. Phys. 133, 154103 (2010).