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<div class="section" id="mass-command">
<span id="index-0"></span><h1>mass command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">mass</span> <span class="n">I</span> <span class="n">value</span>
</pre></div>
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<ul class="simple">
<li>I = atom type (see asterisk form below)</li>
<li>value = mass</li>
</ul>
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<div class="section" id="examples">
<h2>Examples</h2>
<pre class="literal-block">
mass 1 1.0
mass * 62.5
mass 2* 62.5
</pre>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Set the mass for all atoms of one or more atom types. Per-type mass
values can also be set in the <a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> data file
using the &#8220;Masses&#8221; keyword. See the <a class="reference internal" href="units.html"><span class="doc">units</span></a> command for
what mass units to use.</p>
<p>The I index can be specified in one of two ways. An explicit numeric
value can be used, as in the 1st example above. Or a wild-card
asterisk can be used to set the mass for multiple atom types. This
takes the form &#8220;*&#8221; or &#8220;*n&#8221; or &#8220;n*&#8221; or &#8220;m*n&#8221;. If N = the number of
atom types, then an asterisk with no numeric values means all types
from 1 to N. A leading asterisk means all types from 1 to n
(inclusive). A trailing asterisk means all types from n to N
(inclusive). A middle asterisk means all types from m to n
(inclusive).</p>
<p>A line in a <a class="reference internal" href="read_data.html"><span class="doc">data file</span></a> that follows the &#8220;Masses&#8221;
keyword specifies mass using the same format as the arguments of the
mass command in an input script, except that no wild-card asterisk can
be used. For example, under the &#8220;Masses&#8221; section of a data file, the
line that corresponds to the 1st example above would be listed as</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="mi">1</span> <span class="mf">1.0</span>
</pre></div>
</div>
<p>Note that the mass command can only be used if the <a class="reference internal" href="atom_style.html"><span class="doc">atom style</span></a> requires per-type atom mass to be set.
Currently, all but the <em>sphere</em> and <em>ellipsoid</em> and <em>peri</em> styles do.
They require mass to be set for individual particles, not types.
Per-atom masses are defined in the data file read by the
<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> command, or set to default values by the
<a class="reference internal" href="create_atoms.html"><span class="doc">create_atoms</span></a> command. Per-atom masses can also be
set to new values by the <a class="reference internal" href="set.html"><span class="doc">set mass</span></a> or <a class="reference internal" href="set.html"><span class="doc">set density</span></a>
commands.</p>
<p>Also note that <a class="reference internal" href="pair_eam.html"><span class="doc">pair_style eam</span></a> and <a class="reference internal" href="pair_bop.html"><span class="doc">pair_style bop</span></a> commands define the masses of atom types in their
respective potential files, in which case the mass command is normally
not used.</p>
<p>If you define a <a class="reference internal" href="atom_style.html"><span class="doc">hybrid atom style</span></a> which includes one
(or more) sub-styles which require per-type mass and one (or more)
sub-styles which require per-atom mass, then you must define both.
However, in this case the per-type mass will be ignored; only the
per-atom mass will be used by LAMMPS.</p>
</div>
<div class="section" id="restrictions">
<h2>Restrictions</h2>
<p>This command must come after the simulation box is defined by a
<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a>, <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a>, or
<a class="reference internal" href="create_box.html"><span class="doc">create_box</span></a> command.</p>
<p>All masses must be defined before a simulation is run. They must also
all be defined before a <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a> or <a class="reference internal" href="fix_shake.html"><span class="doc">fix shake</span></a> command is used.</p>
<p>The mass assigned to any type or atom must be &gt; 0.0.</p>
<p><strong>Related commands:</strong> none</p>
<p><strong>Default:</strong> none</p>
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