+
+ +
+

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.

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+ +
+ + +
+