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< title > mass command — LAMMPS documentation< / title >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_intro.html" > 1. Introduction< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_start.html" > 2. Getting Started< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_commands.html" > 3. Commands< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_packages.html" > 4. Packages< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_accelerate.html" > 5. Accelerating LAMMPS performance< / a > < / li >
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< li class = "toctree-l1" > < a class = "reference internal" href = "Section_perf.html" > 8. Performance & scalability< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_tools.html" > 9. Additional tools< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_modify.html" > 10. Modifying & extending LAMMPS< / a > < / li >
< li class = "toctree-l1" > < a class = "reference internal" href = "Section_python.html" > 11. Python interface to LAMMPS< / a > < / li >
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< li > mass command< / li >
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< a href = "Section_commands.html#comm" > Commands< / a >
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< span id = "index-0" > < / span > < h1 > mass command< a class = "headerlink" href = "#mass-command" title = "Permalink to this headline" > ¶< / a > < / h1 >
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< h2 > Syntax< a class = "headerlink" href = "#syntax" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > mass I value
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< li > I = atom type (see asterisk form below)< / li >
< li > value = mass< / li >
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< h2 > Examples< a class = "headerlink" href = "#examples" title = "Permalink to this headline" > ¶< / a > < / h2 >
< div class = "highlight-python" > < div class = "highlight" > < pre > mass 1 1.0
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mass * 62.5
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mass 2* 62.5
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< h2 > Description< a class = "headerlink" href = "#description" title = "Permalink to this headline" > ¶< / a > < / 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" > < em > read_data< / em > < / a > data file
using the “ Masses” keyword. See the < a class = "reference internal" href = "units.html" > < em > units< / em > < / a > command for
what mass units to use.< / p >
< p > The I index can be specified in one of two ways. An explicit numeric
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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
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takes the form “ *” or “ < em > n” or “ n< / em > ” or “ m*n” . If N = the number of
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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
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(inclusive).< / p >
< p > A line in a < a class = "reference internal" href = "read_data.html" > < em > data file< / em > < / a > that follows the “ Masses”
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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
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be used. For example, under the “ Masses” section of a data file, the
line that corresponds to the 1st example above would be listed as< / p >
< div class = "highlight-python" > < div class = "highlight" > < pre > 1 1.0
< / pre > < / div >
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< p > Note that the mass command can only be used if the < a class = "reference internal" href = "atom_style.html" > < em > atom style< / em > < / 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.
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They require mass to be set for individual particles, not types.
Per-atom masses are defined in the data file read by the
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< a class = "reference internal" href = "read_data.html" > < em > read_data< / em > < / a > command, or set to default values by the
< a class = "reference internal" href = "create_atoms.html" > < em > create_atoms< / em > < / a > command. Per-atom masses can also be
set to new values by the < a class = "reference internal" href = "set.html" > < em > set mass< / em > < / a > or < a class = "reference internal" href = "set.html" > < em > set density< / em > < / a >
commands.< / p >
< p > Also note that < a class = "reference internal" href = "pair_eam.html" > < em > pair_style eam< / em > < / a > and < a class = "reference internal" href = "pair_bop.html" > < em > pair_style bop< / em > < / a > commands define the masses of atom types in their
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respective potential files, in which case the mass command is normally
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not used.< / p >
< p > If you define a < a class = "reference internal" href = "atom_style.html" > < em > hybrid atom style< / em > < / a > which includes one
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(or more) sub-styles which require per-type mass and one (or more)
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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
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per-atom mass will be used by LAMMPS.< / p >
< / div >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< a class = "headerlink" href = "#restrictions" title = "Permalink to this headline" > ¶< / a > < / h2 >
< p > This command must come after the simulation box is defined by a
< a class = "reference internal" href = "read_data.html" > < em > read_data< / em > < / a > , < a class = "reference internal" href = "read_restart.html" > < em > read_restart< / em > < / a > , or
< a class = "reference internal" href = "create_box.html" > < em > create_box< / em > < / 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" > < em > velocity< / em > < / a > or < a class = "reference internal" href = "fix_shake.html" > < em > fix shake< / em > < / a > command is used.< / p >
< p > The mass assigned to any type or atom must be > 0.0.< / p >
< p > < strong > Related commands:< / strong > none< / p >
< p > < strong > Default:< / strong > none< / p >
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