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Axel Kohlmeyer
2022-03-26 10:49:25 -04:00
parent 36444a1db1
commit 980b6cada7
1 changed files with 43 additions and 43 deletions
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doc/src/pair_meam.rst
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@ -75,16 +75,16 @@ N additional arguments after the second filename in the pair_coeff
command, where N is the number of LAMMPS atom types:
* MEAM library file
* Elem1, Elem2, ...
* Element1, Element2, ...
* MEAM parameter file
* N element names = mapping of MEAM elements to atom types
See the :doc:`pair_coeff <pair_coeff>` page for alternate ways
to specify the path for the potential files.
As an example, the potentials/library.meam file has generic MEAM
settings for a variety of elements. The potentials/SiC.meam file has
specific parameter settings for a Si and C alloy system. If your
As an example, the ``potentials/library.meam`` file has generic MEAM
settings for a variety of elements. The ``potentials/SiC.meam`` file
has specific parameter settings for a Si and C alloy system. If your
LAMMPS simulation has 4 atoms types and you want the first 3 to be Si,
and the fourth to be C, you would use the following pair_coeff command:
@ -118,30 +118,30 @@ that will be used with other potentials.
If the second filename is NULL, the element names between the two
filenames can appear in any order, e.g. "Si C" or "C Si" in the
example above. However, if the second filename is not NULL (as in the
example above), it contains settings that are Fortran-indexed for the
elements that precede it. Thus you need to insure you list the
elements between the filenames in an order consistent with how the
example above. However, if the second filename is **not** NULL (as in the
example above), it contains settings that are indexed **by numbers**
for the elements that precede it. Thus you need to insure that you list
the elements between the filenames in an order consistent with how the
values in the second filename are indexed. See details below on the
syntax for settings in the second file.
The MEAM library file provided with LAMMPS has the name
potentials/library.meam. It is the "meamf" file used by other MD
codes. Aside from blank and comment lines (start with #) which can
appear anywhere, it is formatted as a series of entries, each of which
``potentials/library.meam``. It is the "meamf" file used by other MD
codes. Aside from blank and comment lines (starting with # which can
appear anywhere), it is formatted as a series of entries, each of which
has 19 parameters and can span multiple lines:
elt, lat, z, ielement, atwt, alpha, b0, b1, b2, b3, alat, esub, asub,
t0, t1, t2, t3, rozero, ibar
The "elt" and "lat" parameters are text strings, such as elt = Si or
Cu and lat = dia or fcc. Because the library file is used by Fortran
The *elt* and *lat* parameters are text strings, such as *elt* = Si or
Cu and *lat* = dia or fcc. Because the library file is used by Fortran
MD codes, these strings may be enclosed in single quotes, but this is
not required. The other numeric parameters match values in the
formulas above. The value of the "elt" string is what is used in the
formulas above. The value of the *elt* string is what is used in the
pair_coeff command to identify which settings from the library file
you wish to read in. There can be multiple entries in the library
file with the same "elt" value; LAMMPS reads the first matching entry it
file with the same *elt* value; LAMMPS reads the first matching entry it
finds and ignores the rest.
Other parameters in the MEAM library file correspond to single-element
@ -157,13 +157,13 @@ potential parameters:
esub = energy per atom (eV) in the reference structure at equilibrium
asub = "A" parameter for MEAM (see e.g. :ref:`(Baskes) <Baskes>`)
The alpha, b0, b1, b2, b3, t0, t1, t2, t3 parameters correspond to the
The *alpha*, *b0*, *b1*, *b2*, *b3*, *t0*, *t1*, *t2*, *t3* parameters correspond to the
standard MEAM parameters in the literature :ref:`(Baskes) <Baskes>` (the b
parameters are the standard beta parameters). Note that only parameters
normalized to t0 = 1.0 are supported. The rozero parameter is
normalized to *t0 = 1.0* are supported. The *rozero* parameter is
an element-dependent density scaling that weights the reference
background density (see e.g. equation 4.5 in :ref:`(Gullet) <Gullet>`) and
is typically 1.0 for single-element systems. The ibar parameter
is typically 1.0 for single-element systems. The *ibar* parameter
selects the form of the function G(Gamma) used to compute the electron
density; options are
@ -180,7 +180,7 @@ If used, the MEAM parameter file contains settings that override or
complement the library file settings. Examples of such parameter
files are in the potentials directory with a ".meam" suffix. Their
format is the same as is read by other Fortran MD codes. Aside from
blank and comment lines (start with #) which can appear anywhere, each
blank and comment lines (start with # which can appear anywhere), each
line has one of the following forms. Each line can also have a
trailing comment (starting with #) which is ignored.
@ -213,7 +213,7 @@ The recognized keywords for the parameter file are as follows:
delta(I,J) = heat of formation for I-J alloy; if Ec_IJ is input as
zero, then LAMMPS sets Ec_IJ = (Ec_II + Ec_JJ)/2 - delta_IJ
alpha(I,J) = alpha parameter for pair potential between I and J (can
be computed from bulk modulus of reference structure
be computed from bulk modulus of reference structure)
re(I,J) = equilibrium distance between I and J in the reference
structure
Cmax(I,J,K) = Cmax screening parameter when I-J pair is screened
@ -283,8 +283,8 @@ The recognized keywords for the parameter file are as follows:
1 = rho_bkgd = rho0_meam(a)\*Z_meam(a) (matches DYNAMO)
default = 0
Rc, delr, re are in distance units (Angstroms in the case of metal
units). Ec and delta are in energy units (eV in the case of metal
*Rc*, *delr*, *re* are in distance units (Angstroms in the case of metal
units). *Ec* and *delta* are in energy units (eV in the case of metal
units).
Each keyword represents a quantity which is either a scalar, vector,
@ -299,37 +299,37 @@ Thus these lines
rho0(2) = 2.25
alpha(1,2) = 4.37
set rho0 for the second element to the value 2.25 and set alpha for the
set *rho0* for the second element to the value 2.25 and set *alpha* for the
alloy interaction between elements 1 and 2 to 4.37.
The augt1 parameter is related to modifications in the MEAM
The *augt1* parameter is related to modifications in the MEAM
formulation of the partial electron density function. In recent
literature, an extra term is included in the expression for the
third-order density in order to make the densities orthogonal (see for
example :ref:`(Wang) <Wang2>`, equation 3d); this term is included in the
MEAM implementation in lammps. However, in earlier published work
this term was not included when deriving parameters, including most of
those provided in the library.meam file included with lammps, and to
account for this difference the parameter t1 must be augmented by
3/5\*t3. If augt1=1, the default, this augmentation is done
those provided in the ``library.meam`` file included with lammps, and to
account for this difference the parameter *t1* must be augmented by
3/5\**t3*. If *augt1* = 1, the default, this augmentation is done
automatically. When parameter values are fit using the modified
density function, as in more recent literature, augt1 should be set to
0.
The mixture_ref_t parameter is available to match results with those
The *mixture_ref_t* parameter is available to match results with those
of previous versions of lammps (before January 2011). Newer versions
of lammps, by default, use the single-element values of the t
of lammps, by default, use the single-element values of the *t*
parameters to compute the background reference density. This is the
proper way to compute these parameters. Earlier versions of lammps
used an alloy mixture averaged value of t to compute the background
reference density. Setting mixture_ref_t=1 gives the old behavior.
WARNING: using mixture_ref_t=1 will give results that are demonstrably
used an alloy mixture averaged value of *t* to compute the background
reference density. Setting *mixture_ref_t* = 1 gives the old behavior.
WARNING: using *mixture_ref_t* = 1 will give results that are demonstrably
incorrect for second-neighbor MEAM, and non-standard for
first-neighbor MEAM; this option is included only for matching with
previous versions of lammps and should be avoided if possible.
The parameters attrac and repuls, along with the integer selection
parameter erose_form, can be used to modify the Rose energy function
The parameters *attrac* and *repuls*, along with the integer selection
parameter *erose_form*, can be used to modify the Rose energy function
used to compute the pair potential. This function gives the energy of
the reference state as a function of interatomic spacing. The form of
this function is:
@ -343,19 +343,19 @@ this function is:
a3 = repuls, astar < 0
a3 = attrac, astar >= 0
Most published MEAM parameter sets use the default values attrac=repulse=0.
Setting repuls=attrac=delta corresponds to the form used in several
Most published MEAM parameter sets use the default values *attrac* = *repulse* = 0.
Setting *repuls* = *attrac* = *delta* corresponds to the form used in several
recent published MEAM parameter sets, such as :ref:`(Valone) <Valone>`
.. note::
The default form of the erose expression in LAMMPS was corrected
The default form of the *erose* expression in LAMMPS was corrected
in March 2009. The current version is correct, but may show different
behavior compared with earlier versions of lammps with the attrac
and/or repuls parameters are non-zero. To obtain the previous default
form, use erose_form = 1 (this form does not seem to appear in the
form, use *erose_form* = 1 (this form does not seem to appear in the
literature). An alternative form (see e.g. :ref:`(Lee2) <Lee2>`) is
available using erose_form = 2.
available using *erose_form* = 2.
----------
@ -364,13 +364,13 @@ Mixing, shift, table, tail correction, restart, rRESPA info
For atom type pairs I,J and I != J, where types I and J correspond to
two different element types, mixing is performed by LAMMPS with
user-specifiable parameters as described above. You never need to
specify a pair_coeff command with I != J arguments for this style.
user-specifiable parameters as described above.
This pair style does not support the :doc:`pair_modify <pair_modify>`
shift, table, and tail options.
*shift*, *table*, and *tail* options.
This pair style does not write its information to :doc:`binary restart files <restart>`, since it is stored in potential files. Thus, you
This pair style does not write its information to :doc:`binary restart files <restart>`,
since it is stored in potential files. Thus, you
need to re-specify the pair_style and pair_coeff commands in an input
script that reads a restart file.