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rLAMMPS lammps
units.html
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<HTML>
<CENTER><A
HREF =
"http://lammps.sandia.gov"
>
LAMMPS WWW Site
</A>
-
<A
HREF =
"Manual.html"
>
LAMMPS Documentation
</A>
-
<A
HREF =
"Section_commands.html#comm"
>
LAMMPS Commands
</A>
</CENTER>
<HR>
<H3>
units command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
units style
</PRE>
<UL><LI>
style =
<I>
lj
</I>
or
<I>
real
</I>
or
<I>
metal
</I>
or
<I>
si
</I>
or
<I>
cgs
</I>
or
<I>
electron
</I>
or
<I>
micro
</I>
or
<I>
nano
</I>
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
units metal
units lj
</PRE>
<P><B>
Description:
</B>
</P>
<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
<I>
lj
</I>
, 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>
<P>
For style
<I>
lj
</I>
, 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><LI>
mass = mass or m
<LI>
distance = sigma, where x* = x / sigma
<LI>
time = tau, where t* = t (epsilon / m / sigma^2)^1/2
<LI>
energy = epsilon, where E* = E / epsilon
<LI>
velocity = sigma/tau, where v* = v tau / sigma
<LI>
force = epsilon/sigma, where f* = f sigma / epsilon
<LI>
torque = epsilon, where t* = t / epsilon
<LI>
temperature = reduced LJ temperature, where T* = T Kb / epsilon
<LI>
pressure = reduced LJ pressure, where P* = P sigma^3 / epsilon
<LI>
dynamic viscosity = reduced LJ viscosity, where eta* = eta sigma^3 / epsilon / tau
<LI>
charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2
<LI>
dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2
<LI>
electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon
<LI>
density = mass/volume, where rho* = rho sigma^dim
</UL>
<P>
Note that for LJ units, the default mode of thermodyamic output via
the
<A
HREF =
"thermo_style.html"
>
thermo_style
</A>
command is to normalize energies
by the number of atoms, i.e. energy/atom. This can be changed via the
<A
HREF =
"thermo_modify.html"
>
thermo_modify norm
</A>
command.
</P>
<P>
For style
<I>
real
</I>
, these are the units:
</P>
<UL><LI>
mass = grams/mole
<LI>
distance = Angstroms
<LI>
time = femtoseconds
<LI>
energy = Kcal/mole
<LI>
velocity = Angstroms/femtosecond
<LI>
force = Kcal/mole-Angstrom
<LI>
torque = Kcal/mole
<LI>
temperature = Kelvin
<LI>
pressure = atmospheres
<LI>
dynamic viscosity = Poise
<LI>
charge = multiple of electron charge (1.0 is a proton)
<LI>
dipole = charge*Angstroms
<LI>
electric field = volts/Angstrom
<LI>
density = gram/cm^dim
</UL>
<P>
For style
<I>
metal
</I>
, these are the units:
</P>
<UL><LI>
mass = grams/mole
<LI>
distance = Angstroms
<LI>
time = picoseconds
<LI>
energy = eV
<LI>
velocity = Angstroms/picosecond
<LI>
force = eV/Angstrom
<LI>
torque = eV
<LI>
temperature = Kelvin
<LI>
pressure = bars
<LI>
dynamic viscosity = Poise
<LI>
charge = multiple of electron charge (1.0 is a proton)
<LI>
dipole = charge*Angstroms
<LI>
electric field = volts/Angstrom
<LI>
density = gram/cm^dim
</UL>
<P>
For style
<I>
si
</I>
, these are the units:
</P>
<UL><LI>
mass = kilograms
<LI>
distance = meters
<LI>
time = seconds
<LI>
energy = Joules
<LI>
velocity = meters/second
<LI>
force = Newtons
<LI>
torque = Newton-meters
<LI>
temperature = Kelvin
<LI>
pressure = Pascals
<LI>
dynamic viscosity = Pascal*second
<LI>
charge = Coulombs (1.6021765e-19 is a proton)
<LI>
dipole = Coulombs*meters
<LI>
electric field = volts/meter
<LI>
density = kilograms/meter^dim
</UL>
<P>
For style
<I>
cgs
</I>
, these are the units:
</P>
<UL><LI>
mass = grams
<LI>
distance = centimeters
<LI>
time = seconds
<LI>
energy = ergs
<LI>
velocity = centimeters/second
<LI>
force = dynes
<LI>
torque = dyne-centimeters
<LI>
temperature = Kelvin
<LI>
pressure = dyne/cm^2 or barye = 1.0e-6 bars
<LI>
dynamic viscosity = Poise
<LI>
charge = statcoulombs or esu (4.8032044e-10 is a proton)
<LI>
dipole = statcoul-cm = 10^18 debye
<LI>
electric field = statvolt/cm or dyne/esu
<LI>
density = grams/cm^dim
</UL>
<P>
For style
<I>
electron
</I>
, these are the units:
</P>
<UL><LI>
mass = atomic mass units
<LI>
distance = Bohr
<LI>
time = femtoseconds
<LI>
energy = Hartrees
<LI>
velocity = Bohr/atomic time units [1.03275e-15 seconds]
<LI>
force = Hartrees/Bohr
<LI>
temperature = Kelvin
<LI>
pressure = Pascals
<LI>
charge = multiple of electron charge (1.0 is a proton)
<LI>
dipole moment = Debye
<LI>
electric field = volts/cm
</UL>
<P>
For style
<I>
micro
</I>
, these are the units:
</P>
<UL><LI>
mass = picograms
<LI>
distance = micrometers
<LI>
time = microseconds
<LI>
energy = picogram-micrometer^2/microsecond^2
<LI>
velocity = micrometers/microsecond
<LI>
force = picogram-micrometer/microsecond^2
<LI>
torque = picogram-micrometer^2/microsecond^2
<LI>
temperature = Kelvin
<LI>
pressure = picogram/(micrometer-microsecond^2)
<LI>
dynamic viscosity = picogram/(micrometer-microsecond)
<LI>
charge = picocoulombs (1.6021765e-7 is a proton)
<LI>
dipole = picocoulomb-micrometer
<LI>
electric field = volt/micrometer
<LI>
density = picograms/micrometer^dim
</UL>
<P>
For style
<I>
nano
</I>
, these are the units:
</P>
<UL><LI>
mass = attograms
<LI>
distance = nanometers
<LI>
time = nanoseconds
<LI>
energy = attogram-nanometer^2/nanosecond^2
<LI>
velocity = nanometers/nanosecond
<LI>
force = attogram-nanometer/nanosecond^2
<LI>
torque = attogram-nanometer^2/nanosecond^2
<LI>
temperature = Kelvin
<LI>
pressure = attogram/(nanometer-nanosecond^2)
<LI>
dynamic viscosity = attogram/(nanometer-nanosecond)
<LI>
charge = multiple of electron charge (1.0 is a proton)
<LI>
dipole = charge-nanometer
<LI>
electric field = volt/nanometer
<LI>
density = attograms/nanometer^dim
</UL>
<P>
The units command also sets the timestep size and neighbor skin
distance to default values for each style:
</P>
<UL><LI>
For style
<I>
lj
</I>
these are dt = 0.005 tau and skin = 0.3 sigma.
<LI>
For style
<I>
real
</I>
these are dt = 1.0 fmsec and skin = 2.0 Angstroms.
<LI>
For style
<I>
metal
</I>
these are dt = 0.001 psec and skin = 2.0 Angstroms.
<LI>
For style
<I>
si
</I>
these are dt = 1.0e-8 sec and skin = 0.001 meters.
<LI>
For style
<I>
cgs
</I>
these are dt = 1.0e-8 sec and skin = 0.1 cm.
<LI>
For style
<I>
electron
</I>
these are dt = 0.001 fmsec and skin = 2.0 Bohr.
<LI>
For style
<I>
micro
</I>
these are dt = 2.0 microsec and skin = 0.1 micrometers.
<LI>
For style
<I>
nano
</I>
these are dt = 0.00045 nanosec and skin = 0.1 nanometers.
</UL>
<P><B>
Restrictions:
</B>
</P>
<P>
This command cannot be used after the simulation box is defined by a
<A
HREF =
"read_data.html"
>
read_data
</A>
or
<A
HREF =
"create_box.html"
>
create_box
</A>
command.
</P>
<P><B>
Related commands:
</B>
none
</P>
<P><B>
Default:
</B>
</P>
<PRE>
units lj
</PRE>
</HTML>
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