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<div class="lammps_version">Version: <b>19 Nov 2024</b></div>
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<li class="toctree-l1"><a class="reference internal" href="Intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Install.html">2. Install LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Build.html">3. Build LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Run_head.html">4. Run LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Commands.html">5. Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Examples.html">9. Example scripts</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Tools.html">10. Auxiliary tools</a></li>
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<li class="toctree-l1 current"><a class="reference internal" href="Errors.html">11. Errors</a><ul class="current">
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<li class="toctree-l2 current"><a class="current reference internal" href="#">11.1. Common problems</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Errors_details.html">11.2. Error and warning details</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Errors_bugs.html">11.3. Reporting bugs</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Errors_debug.html">11.4. Debugging crashes</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Errors_messages.html">11.5. Error messages</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Errors_warnings.html">11.6. Warning messages</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Library.html">1. LAMMPS Library Interfaces</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Python_head.html">2. Use Python with LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Modify.html">3. Modifying & extending LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Developer.html">4. Information for Developers</a></li>
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<p class="caption" role="heading"><span class="caption-text">Command Reference</span></p>
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<li class="toctree-l1"><a class="reference internal" href="fixes.html">Fix Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="computes.html">Compute Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="pairs.html">Pair Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="bonds.html">Bond Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="angles.html">Angle Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="dihedrals.html">Dihedral Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="impropers.html">Improper Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="fix_modify_atc_commands.html">fix_modify AtC commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Bibliography.html">Bibliography</a></li>
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<li class="breadcrumb-item"><a href="Errors.html"><span class="section-number">11. </span>Errors</a></li>
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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
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<section id="common-problems">
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<h1><span class="section-number">11.1. </span>Common problems<a class="headerlink" href="#common-problems" title="Link to this heading"></a></h1>
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<p>If two LAMMPS runs do not produce the exact same answer on different
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machines or different numbers of processors, this is typically not a
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bug. In theory you should get identical answers on any number of
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processors and on any machine. In practice, numerical round-off can
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cause slight differences and eventual divergence of molecular dynamics
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phase space trajectories within a few 100s or few 1000s of timesteps.
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However, the statistical properties of the two runs (e.g. average
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energy or temperature) should still be the same.</p>
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<p>If the <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a> command is used to set initial atom
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velocities, a particular atom can be assigned a different velocity
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when the problem is run on a different number of processors or on
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different machines. If this happens, the phase space trajectories of
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the two simulations will rapidly diverge. See the discussion of the
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<em>loop</em> option in the <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a> command for details and
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options that avoid this issue.</p>
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<p>Similarly, the <a class="reference internal" href="create_atoms.html"><span class="doc">create_atoms</span></a> command generates a
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lattice of atoms. For the same physical system, the ordering and
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numbering of atoms by atom ID may be different depending on the number
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of processors.</p>
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<p>Some commands use random number generators which may be setup to
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produce different random number streams on each processor and hence
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will produce different effects when run on different numbers of
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processors. A commonly-used example is the <a class="reference internal" href="fix_langevin.html"><span class="doc">fix langevin</span></a> command for thermostatting.</p>
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<p>A LAMMPS simulation typically has two stages, setup and run. Most
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LAMMPS errors are detected at setup time; others like a bond
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stretching too far may not occur until the middle of a run.</p>
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<p>LAMMPS tries to flag errors and print informative error messages so
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you can fix the problem. For most errors it will also print the last
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input script command that it was processing. Of course, LAMMPS cannot
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figure out your physics or numerical mistakes, like choosing too big a
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timestep, specifying erroneous force field coefficients, or putting 2
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atoms on top of each other! If you run into errors that LAMMPS
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does not catch that you think it should flag, please send an email to
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the <a class="reference external" href="https://www.lammps.org/authors.html">developers</a> or create an new
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topic on the dedicated <a class="reference external" href="https://matsci.org/lammps/">MatSci forum section</a>.</p>
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<p>If you get an error message about an invalid command in your input
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script, you can determine what command is causing the problem by
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looking in the log.lammps file or using the <a class="reference internal" href="echo.html"><span class="doc">echo command</span></a>
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to see it on the screen. If you get an error like “Invalid …
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style”, with … being fix, compute, pair, etc, it means that you
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mistyped the style name or that the command is part of an optional
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package which was not compiled into your executable. The list of
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available styles in your executable can be listed by using
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<a class="reference internal" href="Run_options.html"><span class="doc">the -h command-line switch</span></a>. The installation and
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compilation of optional packages is explained on the
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<a class="reference internal" href="Build_package.html"><span class="doc">Build packages</span></a> doc page.</p>
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<p>For a given command, LAMMPS expects certain arguments in a specified
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order. If you mess this up, LAMMPS will often flag the error, but it
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may also simply read a bogus argument and assign a value that is
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valid, but not what you wanted. E.g. trying to read the string “abc”
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as an integer value of 0. Careful reading of the associated doc page
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for the command should allow you to fix these problems. In most cases,
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where LAMMPS expects to read a number, either integer or floating point,
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it performs a stringent test on whether the provided input actually
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is an integer or floating-point number, respectively, and reject the
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input with an error message (for instance, when an integer is required,
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but a floating-point number 1.0 is provided):</p>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>ERROR: Expected integer parameter instead of '1.0' in input script or data file
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</pre></div>
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</div>
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<p>Some commands allow for using variable references in place of numeric
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constants so that the value can be evaluated and may change over the
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course of a run. This is typically done with the syntax <em>v_name</em> for a
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parameter, where name is the name of the variable. On the other hand,
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immediate variable expansion with the syntax ${name} is performed while
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reading the input and before parsing commands,</p>
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<div class="admonition note">
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<p class="admonition-title">Note</p>
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<p>Using a variable reference (i.e. <em>v_name</em>) is only allowed if
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the documentation of the corresponding command explicitly says it is.
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Otherwise, you will receive an error message of this kind:</p>
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</div>
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<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>ERROR: Expected floating point parameter instead of 'v_name' in input script or data file
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</pre></div>
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</div>
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<p>Generally, LAMMPS will print a message to the screen and logfile and
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exit gracefully when it encounters a fatal error. Sometimes it will
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print a WARNING to the screen and logfile and continue on; you can
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decide if the WARNING is important or not. A WARNING message that is
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generated in the middle of a run is only printed to the screen, not to
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the logfile, to avoid cluttering up thermodynamic output. If LAMMPS
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crashes or hangs without spitting out an error message first then it
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could be a bug (see <a class="reference internal" href="Errors_bugs.html"><span class="doc">this section</span></a>) or one of the following
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cases:</p>
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<p>LAMMPS runs in the available memory a processor allows to be
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allocated. Most reasonable MD runs are compute limited, not memory
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limited, so this should not be a bottleneck on most platforms. Almost
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all large memory allocations in the code are done via C-style malloc’s
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which will generate an error message if you run out of memory.
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Smaller chunks of memory are allocated via C++ “new” statements. If
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you are unlucky you could run out of memory just when one of these
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small requests is made, in which case the code will crash or hang (in
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parallel), since LAMMPS does not trap on those errors.</p>
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<p>Illegal arithmetic can cause LAMMPS to run slow or crash. This is
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typically due to invalid physics and numerics that your simulation is
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computing. If you see wild thermodynamic values or NaN values in your
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LAMMPS output, something is wrong with your simulation. If you
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suspect this is happening, it is a good idea to print out
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thermodynamic info frequently (e.g. every timestep) via the
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<a class="reference internal" href="thermo.html"><span class="doc">thermo</span></a> so you can monitor what is happening.
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Visualizing the atom movement is also a good idea to ensure your model
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is behaving as you expect.</p>
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<p>In parallel, one way LAMMPS can hang is due to how different MPI
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implementations handle buffering of messages. If the code hangs
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without an error message, it may be that you need to specify an MPI
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setting or two (usually via an environment variable) to enable
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buffering or boost the sizes of messages that can be buffered.</p>
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</section>
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