forked from lijiext/lammps
145 lines
6.1 KiB
Plaintext
145 lines
6.1 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix ave/correlate/long command :h3
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[Syntax:]
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fix ID group-ID ave/correlate/long Nevery Nfreq value1 value2 ... keyword args ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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ave/correlate/long = style name of this fix command :l
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Nevery = use input values every this many timesteps :l
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Nfreq = save state of the time correlation functions every this many timesteps :l
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one or more input values can be listed :l
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value = c_ID, c_ID\[N\], f_ID, f_ID\[N\], v_name :l
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c_ID = global scalar calculated by a compute with ID
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c_ID\[I\] = Ith component of global vector calculated by a compute with ID
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f_ID = global scalar calculated by a fix with ID
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f_ID\[I\] = Ith component of global vector calculated by a fix with ID
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v_name = global value calculated by an equal-style variable with name :pre
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zero or more keyword/arg pairs may be appended :l
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keyword = {type} or {start} or {file} or {overwrite} or {title1} or {title2} or {ncorr} or {p} or {m} :l
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{type} arg = {auto} or {upper} or {lower} or {auto/upper} or {auto/lower} or {full}
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auto = correlate each value with itself
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upper = correlate each value with each succeeding value
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lower = correlate each value with each preceding value
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auto/upper = auto + upper
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auto/lower = auto + lower
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full = correlate each value with every other value, including itself = auto + upper + lower
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{start} args = Nstart
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Nstart = start accumulating correlations on this timestep
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{file} arg = filename
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filename = name of file to output correlation data to
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{overwrite} arg = none = overwrite output file with only latest output
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{title1} arg = string
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string = text to print as 1st line of output file
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{title2} arg = string
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string = text to print as 2nd line of output file
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{ncorr} arg = Ncorrelators
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Ncorrelators = number of correlators to store
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{nlen} args = Nlen
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Nlen = length of each correlator
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{ncount} args = Ncount
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Ncount = number of values over which succesive correlators are averaged :pre
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:ule
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[Examples:]
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fix 1 all ave/correlate/long 5 1000 c_myTemp file temp.correlate
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fix 1 all ave/correlate/long 1 10000 &
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c_thermo_press\[1\] c_thermo_press\[2\] c_thermo_press\[3\] &
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type upper title1 "My correlation data" nlen 15 ncount 3 :pre
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[Description:]
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This fix is similar in spirit and syntax to the "fix
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ave/correlate"_fix_ave_correlate.html. However, this fix allows the
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efficient calculation of time correlation functions on the fly over
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extremely long time windows without too much CPU overhead, using a
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multiple-tau method "(Ramirez)"_#Ramirez that decreases the resolution
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of the stored correlation function with time.
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The group specified with this command is ignored. However, note that
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specified values may represent calculations performed by computes and
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fixes which store their own "group" definitions.
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Each listed value can be the result of a compute or fix or the
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evaluation of an equal-style variable. See the "fix
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ave/correlate"_fix_ave_correlate.html doc page for details.
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The {Nevery} and {Nfreq} arguments specify on what timesteps the input
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values will be used to calculate correlation data, and the frequency
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with which the time correlation functions will be output to a file.
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Note that there is no {Nrepeat} argument, unlike the "fix
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ave/correlate"_fix_ave_correlate.html command.
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The optional keywords {ncorr}, {nlen}, and {ncount} are unique to this
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command and determine the number of correlation points calculated and
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the memory and CPU overhead used by this calculation. {Nlen} and
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{ncount} determine the amount of averaging done at longer correlation
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times. The default values {nlen=16}, {ncount=2} ensure that the
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systematic error of the multiple-tau correlator is always below the
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level of the statistical error of a typical simulation (which depends
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on the ensemble size and the simulation length).
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The maximum correlation time (in time steps) that can be reached is
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given by the formula (nlen-1) * ncount^(ncorr-1). Longer correlation
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times are discarded and not calculated. With the default values of
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the parameters (ncorr=20, nlen=16 and ncount=2), this corresponds to
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7864320 time steps. If longer correlation times are needed, the value
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of ncorr should be increased. Using nlen=16 and ncount=2, with
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ncorr=30, the maximum number of steps that can be correlated is
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80530636808. If ncorr=40, correlation times in excess of 8e12 time
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steps can be calculated.
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The total memory needed for each correlation pair is roughly
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4*ncorr*nlen*8 bytes. With the default values of the parameters, this
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corresponds to about 10 KB.
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For the meaning of the additional optional keywords, see the "fix
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ave/correlate"_fix_ave_correlate.html doc page.
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[Restart, fix_modify, output, run start/stop, minimize info:]
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Since this fix in intended for the calculation of time correlation
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functions over very long MD simulations, the information about this
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fix is written automatically to binary restart files, so that the time
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correlation calculation can continue in subsequent simulations. None
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of the fix_modify options are relevant to this fix.
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No parameter of this fix can be used with the start/stop keywords of
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the run command. This fix is not invoked during energy minimization.
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[Restrictions:]
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This compute is part of the USER-MISC package. It is only enabled if
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LAMMPS was built with that package. See the "Making
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LAMMPS"_Section_start.html#start_3 section for more info.
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[Related commands:]
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"fix ave/correlate"_fix_ave_correlate.html
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[Default:] none
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The option defaults for keywords that are also keywords for the "fix
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ave/correlate"_fix_ave_correlate.html command are as follows: type =
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auto, start = 0, no file output, title 1,2 = strings as described on
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the "fix ave/correlate"_fix_ave_correlate.html doc page.
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The option defaults for keywords unique to this command are as
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follows: ncorr=20, nlen=16, ncount=2.
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:line
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:link(Ramirez)
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[(Ramirez)] J. Ramirez, S.K. Sukumaran, B. Vorselaars and
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A.E. Likhtman, J. Chem. Phys. 133, 154103 (2010).
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