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<div class="section" id="fix-ave-correlate-long-command">
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<span id="index-0"></span><h1>fix ave/correlate/long command<a class="headerlink" href="#fix-ave-correlate-long-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID ave/correlate/long Nevery Nfreq value1 value2 ... keyword args ...
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>ave/correlate/long = style name of this fix command</li>
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<li>Nevery = use input values every this many timesteps</li>
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<li>Nfreq = save state of the time correlation functions every this many timesteps</li>
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<li>one or more input values can be listed</li>
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<li>value = c_ID, c_ID[N], f_ID, f_ID[N], v_name</li>
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</ul>
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<div class="highlight-python"><div class="highlight"><pre>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
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</pre></div>
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</div>
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<ul class="simple">
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<li>zero or more keyword/arg pairs may be appended</li>
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<li>keyword = <em>type</em> or <em>start</em> or <em>file</em> or <em>overwrite</em> or <em>title1</em> or <em>title2</em> or <em>ncorr</em> or <em>p</em> or <em>m</em></li>
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</ul>
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<pre class="literal-block">
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<em>type</em> arg = <em>auto</em> or <em>upper</em> or <em>lower</em> or <em>auto/upper</em> or <em>auto/lower</em> or <em>full</em>
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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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<em>start</em> args = Nstart
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Nstart = start accumulating correlations on this timestep
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<em>file</em> arg = filename
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filename = name of file to output correlation data to
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<em>overwrite</em> arg = none = overwrite output file with only latest output
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<em>title1</em> arg = string
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string = text to print as 1st line of output file
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<em>title2</em> arg = string
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string = text to print as 2nd line of output file
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<em>ncorr</em> arg = Ncorrelators
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Ncorrelators = number of correlators to store
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<em>nlen</em> args = Nlen
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Nlen = length of each correlator
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<em>ncount</em> args = Ncount
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Ncount = number of values over which succesive correlators are averaged
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>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
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>This fix is similar in spirit and syntax to the <a class="reference internal" href="fix_ave_correlate.html"><em>fix ave/correlate</em></a>. 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 <a class="reference internal" href="#ramirez"><span>(Ramirez)</span></a> that decreases the resolution
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of the stored correlation function with time.</p>
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<p>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.</p>
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<p>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 <a class="reference internal" href="fix_ave_correlate.html"><em>fix ave/correlate</em></a> doc page for details.</p>
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<p>The <em>Nevery</em> and <em>Nfreq</em> 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 <em>Nrepeat</em> argument, unlike the <a class="reference internal" href="fix_ave_correlate.html"><em>fix ave/correlate</em></a> command.</p>
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<p>The optional keywords <em>ncorr</em>, <em>nlen</em>, and <em>ncount</em> 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. <em>Nlen</em> and
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<em>ncount</em> determine the amount of averaging done at longer correlation
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times. The default values <em>nlen=16</em>, <em>ncount=2</em> 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).</p>
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<p>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.</p>
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<p>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.</p>
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<p>For the meaning of the additional optional keywords, see the <a class="reference internal" href="fix_ave_correlate.html"><em>fix ave/correlate</em></a> doc page.</p>
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</div>
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<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
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<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
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<p>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.</p>
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<p>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.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>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 <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="fix_ave_correlate.html"><em>fix ave/correlate</em></a></p>
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<p><strong>Default:</strong> none</p>
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<p>The option defaults for keywords that are also keywords for the <a class="reference internal" href="fix_ave_correlate.html"><em>fix ave/correlate</em></a> 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 <a class="reference internal" href="fix_ave_correlate.html"><em>fix ave/correlate</em></a> doc page.</p>
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<p>The option defaults for keywords unique to this command are as
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follows: ncorr=20, nlen=16, ncount=2.</p>
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<hr class="docutils" />
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<p id="ramirez"><strong>(Ramirez)</strong> J. Ramirez, S.K. Sukumaran, B. Vorselaars and
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A.E. Likhtman, J. Chem. Phys. 133, 154103 (2010).</p>
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