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units command
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bond_coeff command
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bond_style command
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bond_write command
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boundary command
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comm_style command
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create_atoms command
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dielectric command
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dihedral_coeff command
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dihedral_style command
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dimension command
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displace_atoms command
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dump command
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fix_modify command
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group2ndx command
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if command
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improper_coeff command
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improper_style command
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include command
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info command
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jump command
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kspace_modify command
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kspace_style command
</a></li>
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>
label command
</a></li>
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href=
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>
lattice command
</a></li>
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log command
</a></li>
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>
mass command
</a></li>
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min_modify command
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min_style command
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minimize command
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molecule command
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neb command
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neigh_modify command
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neighbor command
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"newton.html"
>
newton command
</a></li>
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href=
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>
next command
</a></li>
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package command
</a></li>
<li
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href=
"pair_coeff.html"
>
pair_coeff command
</a></li>
<li
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"toctree-l2"
><a
class=
"reference internal"
href=
"pair_modify.html"
>
pair_modify command
</a></li>
<li
class=
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class=
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href=
"pair_style.html"
>
pair_style command
</a></li>
<li
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"toctree-l2"
><a
class=
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href=
"pair_write.html"
>
pair_write command
</a></li>
<li
class=
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><a
class=
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href=
"partition.html"
>
partition command
</a></li>
<li
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>
prd command
</a></li>
<li
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>
print command
</a></li>
<li
class=
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class=
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href=
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>
processors command
</a></li>
<li
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python command
</a></li>
<li
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quit command
</a></li>
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>
read_data command
</a></li>
<li
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class=
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"read_dump.html"
>
read_dump command
</a></li>
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>
read_restart command
</a></li>
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>
region command
</a></li>
<li
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>
replicate command
</a></li>
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rerun command
</a></li>
<li
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reset_timestep command
</a></li>
<li
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restart command
</a></li>
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run command
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<li
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run_style command
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<li
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set command
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shell command
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<li
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special_bonds command
</a></li>
<li
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suffix command
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tad command
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temper command
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thermo command
</a></li>
<li
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thermo_modify command
</a></li>
<li
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thermo_style command
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undump command
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unfix command
</a></li>
<li
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units command
</a><ul>
<li
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"#syntax"
>
Syntax
</a></li>
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Examples
</a></li>
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Description
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Restrictions
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Default
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variable command
</a></li>
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velocity command
</a></li>
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write_coeff command
</a></li>
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write_data command
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write_dump command
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write_restart command
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Computes
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Dihedral Styles
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<div
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<span
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"index-0"
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units command
</h1>
<div
class=
"section"
id=
"syntax"
>
<h2>
Syntax
</h2>
<div
class=
"highlight-default"
><div
class=
"highlight"
><pre><span></span><span
class=
"n"
>
units
</span>
<span
class=
"n"
>
style
</span>
</pre></div>
</div>
<ul
class=
"simple"
>
<li>
style =
<em>
lj
</em>
or
<em>
real
</em>
or
<em>
metal
</em>
or
<em>
si
</em>
or
<em>
cgs
</em>
or
<em>
electron
</em>
or
<em>
micro
</em>
or
<em>
nano
</em></li>
</ul>
</div>
<div
class=
"section"
id=
"examples"
>
<h2>
Examples
</h2>
<div
class=
"highlight-default"
><div
class=
"highlight"
><pre><span></span><span
class=
"n"
>
units
</span>
<span
class=
"n"
>
metal
</span>
<span
class=
"n"
>
units
</span>
<span
class=
"n"
>
lj
</span>
</pre></div>
</div>
</div>
<div
class=
"section"
id=
"description"
>
<h2>
Description
</h2>
<p>
This command sets the style of units used for a simulation. It
determines the units of all quantities specified in the input script
and data file, as well as quantities output to the screen, log file,
and dump files. Typically, this command is used at the very beginning
of an input script.
</p>
<p>
For all units except
<em>
lj
</em>
, LAMMPS uses physical constants from
www.physics.nist.gov. For the definition of Kcal in real units,
LAMMPS uses the thermochemical calorie = 4.184 J.
</p>
<p>
The choice you make for units simply sets some internal conversion
factors within LAMMPS. This means that any simulation you perform for
one choice of units can be duplicated with any other unit setting
LAMMPS supports. In this context
“
duplicate
”
means the particles will
have identical trajectories and all output generated by the simulation
will be identical. This will be the case for some number of timesteps
until round-off effects accumulate, since the conversion factors for
two different unit systems are not identical to infinite precision.
</p>
<p>
To perform the same simulation in a different set of units you must
change all the unit-based input parameters in your input script and
other input files (data file, potential files, etc) correctly to the
new units. And you must correctly convert all output from the new
units to the old units when comparing to the original results. That
is often not simple to do.
</p>
<hr
class=
"docutils"
/>
<p>
For style
<em>
lj
</em>
, all quantities are unitless. Without loss of
generality, LAMMPS sets the fundamental quantities mass, sigma,
epsilon, and the Boltzmann constant = 1. The masses, distances,
energies you specify are multiples of these fundamental values. The
formulas relating the reduced or unitless quantity (with an asterisk)
to the same quantity with units is also given. Thus you can use the
mass
&
sigma
&
epsilon values for a specific material and convert the
results from a unitless LJ simulation into physical quantities.
</p>
<ul
class=
"simple"
>
<li>
mass = mass or m
</li>
<li>
distance = sigma, where x* = x / sigma
</li>
<li>
time = tau, where t* = t (epsilon / m / sigma^2)^1/2
</li>
<li>
energy = epsilon, where E* = E / epsilon
</li>
<li>
velocity = sigma/tau, where v* = v tau / sigma
</li>
<li>
force = epsilon/sigma, where f* = f sigma / epsilon
</li>
<li>
torque = epsilon, where t* = t / epsilon
</li>
<li>
temperature = reduced LJ temperature, where T* = T Kb / epsilon
</li>
<li>
pressure = reduced LJ pressure, where P* = P sigma^3 / epsilon
</li>
<li>
dynamic viscosity = reduced LJ viscosity, where eta* = eta sigma^3 / epsilon / tau
</li>
<li>
charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2
</li>
<li>
dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2
</li>
<li>
electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon
</li>
<li>
density = mass/volume, where rho* = rho sigma^dim
</li>
</ul>
<p>
Note that for LJ units, the default mode of thermodyamic output via
the
<a
class=
"reference internal"
href=
"thermo_style.html"
><span
class=
"doc"
>
thermo_style
</span></a>
command is to normalize all
extensive quantities by the number of atoms. E.g. potential energy is
extensive because it is summed over atoms, so it is output as
energy/atom. Temperature is intensive since it is already normalized
by the number of atoms, so it is output as-is. This behavior can be
changed via the
<a
class=
"reference internal"
href=
"thermo_modify.html"
><span
class=
"doc"
>
thermo_modify norm
</span></a>
command.
</p>
<p>
For style
<em>
real
</em>
, these are the units:
</p>
<ul
class=
"simple"
>
<li>
mass = grams/mole
</li>
<li>
distance = Angstroms
</li>
<li>
time = femtoseconds
</li>
<li>
energy = Kcal/mole
</li>
<li>
velocity = Angstroms/femtosecond
</li>
<li>
force = Kcal/mole-Angstrom
</li>
<li>
torque = Kcal/mole
</li>
<li>
temperature = Kelvin
</li>
<li>
pressure = atmospheres
</li>
<li>
dynamic viscosity = Poise
</li>
<li>
charge = multiple of electron charge (1.0 is a proton)
</li>
<li>
dipole = charge*Angstroms
</li>
<li>
electric field = volts/Angstrom
</li>
<li>
density = gram/cm^dim
</li>
</ul>
<p>
For style
<em>
metal
</em>
, these are the units:
</p>
<ul
class=
"simple"
>
<li>
mass = grams/mole
</li>
<li>
distance = Angstroms
</li>
<li>
time = picoseconds
</li>
<li>
energy = eV
</li>
<li>
velocity = Angstroms/picosecond
</li>
<li>
force = eV/Angstrom
</li>
<li>
torque = eV
</li>
<li>
temperature = Kelvin
</li>
<li>
pressure = bars
</li>
<li>
dynamic viscosity = Poise
</li>
<li>
charge = multiple of electron charge (1.0 is a proton)
</li>
<li>
dipole = charge*Angstroms
</li>
<li>
electric field = volts/Angstrom
</li>
<li>
density = gram/cm^dim
</li>
</ul>
<p>
For style
<em>
si
</em>
, these are the units:
</p>
<ul
class=
"simple"
>
<li>
mass = kilograms
</li>
<li>
distance = meters
</li>
<li>
time = seconds
</li>
<li>
energy = Joules
</li>
<li>
velocity = meters/second
</li>
<li>
force = Newtons
</li>
<li>
torque = Newton-meters
</li>
<li>
temperature = Kelvin
</li>
<li>
pressure = Pascals
</li>
<li>
dynamic viscosity = Pascal*second
</li>
<li>
charge = Coulombs (1.6021765e-19 is a proton)
</li>
<li>
dipole = Coulombs*meters
</li>
<li>
electric field = volts/meter
</li>
<li>
density = kilograms/meter^dim
</li>
</ul>
<p>
For style
<em>
cgs
</em>
, these are the units:
</p>
<ul
class=
"simple"
>
<li>
mass = grams
</li>
<li>
distance = centimeters
</li>
<li>
time = seconds
</li>
<li>
energy = ergs
</li>
<li>
velocity = centimeters/second
</li>
<li>
force = dynes
</li>
<li>
torque = dyne-centimeters
</li>
<li>
temperature = Kelvin
</li>
<li>
pressure = dyne/cm^2 or barye = 1.0e-6 bars
</li>
<li>
dynamic viscosity = Poise
</li>
<li>
charge = statcoulombs or esu (4.8032044e-10 is a proton)
</li>
<li>
dipole = statcoul-cm = 10^18 debye
</li>
<li>
electric field = statvolt/cm or dyne/esu
</li>
<li>
density = grams/cm^dim
</li>
</ul>
<p>
For style
<em>
electron
</em>
, these are the units:
</p>
<ul
class=
"simple"
>
<li>
mass = atomic mass units
</li>
<li>
distance = Bohr
</li>
<li>
time = femtoseconds
</li>
<li>
energy = Hartrees
</li>
<li>
velocity = Bohr/atomic time units [1.03275e-15 seconds]
</li>
<li>
force = Hartrees/Bohr
</li>
<li>
temperature = Kelvin
</li>
<li>
pressure = Pascals
</li>
<li>
charge = multiple of electron charge (1.0 is a proton)
</li>
<li>
dipole moment = Debye
</li>
<li>
electric field = volts/cm
</li>
</ul>
<p>
For style
<em>
micro
</em>
, these are the units:
</p>
<ul
class=
"simple"
>
<li>
mass = picograms
</li>
<li>
distance = micrometers
</li>
<li>
time = microseconds
</li>
<li>
energy = picogram-micrometer^2/microsecond^2
</li>
<li>
velocity = micrometers/microsecond
</li>
<li>
force = picogram-micrometer/microsecond^2
</li>
<li>
torque = picogram-micrometer^2/microsecond^2
</li>
<li>
temperature = Kelvin
</li>
<li>
pressure = picogram/(micrometer-microsecond^2)
</li>
<li>
dynamic viscosity = picogram/(micrometer-microsecond)
</li>
<li>
charge = picocoulombs (1.6021765e-7 is a proton)
</li>
<li>
dipole = picocoulomb-micrometer
</li>
<li>
electric field = volt/micrometer
</li>
<li>
density = picograms/micrometer^dim
</li>
</ul>
<p>
For style
<em>
nano
</em>
, these are the units:
</p>
<ul
class=
"simple"
>
<li>
mass = attograms
</li>
<li>
distance = nanometers
</li>
<li>
time = nanoseconds
</li>
<li>
energy = attogram-nanometer^2/nanosecond^2
</li>
<li>
velocity = nanometers/nanosecond
</li>
<li>
force = attogram-nanometer/nanosecond^2
</li>
<li>
torque = attogram-nanometer^2/nanosecond^2
</li>
<li>
temperature = Kelvin
</li>
<li>
pressure = attogram/(nanometer-nanosecond^2)
</li>
<li>
dynamic viscosity = attogram/(nanometer-nanosecond)
</li>
<li>
charge = multiple of electron charge (1.0 is a proton)
</li>
<li>
dipole = charge-nanometer
</li>
<li>
electric field = volt/nanometer
</li>
<li>
density = attograms/nanometer^dim
</li>
</ul>
<p>
The units command also sets the timestep size and neighbor skin
distance to default values for each style:
</p>
<ul
class=
"simple"
>
<li>
For style
<em>
lj
</em>
these are dt = 0.005 tau and skin = 0.3 sigma.
</li>
<li>
For style
<em>
real
</em>
these are dt = 1.0 fmsec and skin = 2.0 Angstroms.
</li>
<li>
For style
<em>
metal
</em>
these are dt = 0.001 psec and skin = 2.0 Angstroms.
</li>
<li>
For style
<em>
si
</em>
these are dt = 1.0e-8 sec and skin = 0.001 meters.
</li>
<li>
For style
<em>
cgs
</em>
these are dt = 1.0e-8 sec and skin = 0.1 cm.
</li>
<li>
For style
<em>
electron
</em>
these are dt = 0.001 fmsec and skin = 2.0 Bohr.
</li>
<li>
For style
<em>
micro
</em>
these are dt = 2.0 microsec and skin = 0.1 micrometers.
</li>
<li>
For style
<em>
nano
</em>
these are dt = 0.00045 nanosec and skin = 0.1 nanometers.
</li>
</ul>
</div>
<div
class=
"section"
id=
"restrictions"
>
<h2>
Restrictions
</h2>
<p>
This command cannot be used after the simulation box is defined by a
<a
class=
"reference internal"
href=
"read_data.html"
><span
class=
"doc"
>
read_data
</span></a>
or
<a
class=
"reference internal"
href=
"create_box.html"
><span
class=
"doc"
>
create_box
</span></a>
command.
</p>
<p><strong>
Related commands:
</strong>
none
</p>
</div>
<div
class=
"section"
id=
"default"
>
<h2>
Default
</h2>
<div
class=
"highlight-default"
><div
class=
"highlight"
><pre><span></span><span
class=
"n"
>
units
</span>
<span
class=
"n"
>
lj
</span>
</pre></div>
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