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99
doc/pair_meam.html
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@ -23,6 +23,10 @@ pair_coeff * * ../potentials/library.meam Ni Al NULL Ni Al Ni Ni
</PRE>
<P><B>Description:</B>
</P>
<P>NOTE: The behavior of the MEAM potential for alloy systems has changed
as of November 2010; see description below of the mixture_ref_t
parameter
</P>
<P>Style <I>meam</I> computes pairwise interactions for a variety of materials
using modified embedded-atom method (MEAM) potentials
<A HREF = "#Baskes">(Baskes)</A>. Conceptually, it is an extension to the original
@ -116,6 +120,32 @@ you wish to read in. There can be multiple entries in the library
file with the same "elt" value; LAMMPS reads the 1st matching entry it
finds and ignores the rest.
</P>
<P>Other parameters in the MEAM library file correspond to single-element
potential parameters:
</P>
<PRE>lat = lattice structure of reference configuration
z = number of nearest neighbors in the reference structure
ielement = atomic number
atwt = atomic weight
alat = lattice constant of reference structure
esub = energy per atom (eV) in the reference structure at equilibrium
asub = "A" parameter for MEAM (see e.g. <A HREF = "#Baskes">(Baskes)</A>)
</PRE>
<P>The alpha, b0, b1, b2, b3, t0, t1, t2, t3 parameters correspond to the
standard MEAM parameters in the literature <A HREF = "#Baskes">(Baskes)</A> (the b
parameters are the standard beta parameters). The rozero parameter is
an element-dependent density scaling that weights the reference
background density (see e.g. equation 4.5 in <A HREF = "#Gullet">(Gullet)</A>) and
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
</P>
<PRE> 0 => G = sqrt(1+Gamma)
1 => G = exp(Gamma/2)
2 => not implemented
3 => G = 2/(1+exp(-Gamma))
4 => G = sqrt(1+Gamma)
</PRE>
<P>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
@ -160,15 +190,19 @@ lattce(I,J) = lattice structure of I-J reference structure:
hcp = hexagonal close-packed
c11 = MoSi2 structure
l12 = Cu3Au structure (lower case L, followed by 12)
b2 = CsCl structure (interpenetrating simple cubic)
nn2(I,J) = turn on second-nearest neighbor MEAM formulation for
I-J pair (see for example <A HREF = "#Lee">(Lee)</A>). Only valid for I=J.
I-J pair (see for example <A HREF = "#Lee">(Lee)</A>).
0 = second-nearest neighbor formulation off
1 = second-nearest neighbor formulation on
default = 0
attrac(I,J) = additional cubic attraction term in Rose energy I-J pair potential
default = 0
repuls(I,J) = additional cubic repulsive term in Rose energy I-J pair potential
default = 0
default = 0
zbl(I,J) = blend the MEAM I-J pair potential with the ZBL potential for small
atom separations <A HREF = "#ZBL">(ZBL)</A>
default = 1
gsmooth_factor = factor determining the length of the G-function smoothing
region; only significant for ibar=0 or ibar=4.
99.0 = short smoothing region, sharp step
@ -179,6 +213,21 @@ augt1 = integer flag for whether to augment t1 parameter by
0 = don't augment t1
1 = augment t1
default = 1
ialloy = integer flag to use alternative averaging rule for t parameters,
for comparison with the DYNAMO MEAM code
0 = standard averaging (matches ialloy=0 in DYNAMO)
1 = alternative averaging (matches ialloy=1 in DYNAMO)
2 = no averaging of t (use single-element values)
default = 0
mixture_ref_t = integer flag to use mixture average of t to compute the background
reference density for alloys, instead of the single-element values
(see description and warning elsewhere in this doc page)
0 = do not use mixture averaging for t in the reference density
1 = use mixture averaging for t in the reference density
default = 0
erose_form = integer value to select the form of the Rose energy function
(see description below).
default = 0
</PRE>
<P>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
@ -211,13 +260,28 @@ automatically. When parameter values are fit using the modified
density function, as in more recent literature, augt1 should be set to
0.
</P>
<P>The parameters attrac and repuls 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:
<P>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
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
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.
</P>
<P>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:
</P>
<PRE>astar = alpha * (r/re - 1.d0)
erose = -Ec*(1+astar+a3*(astar**3)/(r/re))*exp(-astar)
if erose_form = 0: erose = -Ec*(1+astar+a3*(astar**3)/(r/re))*exp(-astar)
if erose_form = 1: erose = -Ec*(1+astar+(-attrac+repuls/r)*(astar**3))*exp(-astar)
if erose_form = 2: erose = -Ec*(1 +astar + a3*(astar**3))*exp(-astar)
a3 = repuls, astar < 0
a3 = attrac, astar >= 0
</PRE>
@ -225,6 +289,14 @@ a3 = attrac, astar >= 0
Setting repuls=attrac=delta corresponds to the form used in several
recent published MEAM parameter sets, such as <A HREF = "#Vallone">(Vallone)</A>
</P>
<P>NOTE: 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
literature). An alternative form (see e.g. <A HREF = "#Lee2">(Lee2)</A>) is
available using erose_form = 2.
</P>
<HR>
<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
@ -278,12 +350,21 @@ This report may be accessed on-line via <A HREF = "http://infoserve.sandia.gov/s
<P><B>(Lee)</B> Lee, Baskes, Phys. Rev. B, 62, 8564-8567 (2000).
</P>
<A NAME = "Wang"></A>
<A NAME = "Lee2"></A>
<P><B>(Wang)</B> Wang, Van Hove, Ross, Baskes, J. Chem. Phys., 121, 5410 (2004).
<P><B>(Lee2)</B> Lee, Baskes, Kim, Cho. Phys. Rev. B, 64, 184102 (2001).
</P>
<A NAME = "Valone"></A>
<P><B>(Valone)</B> Valone, Baskes, Martin, Phys. Rev. B, 73, 214209 (2006).
</P>
<A NAME = "Wang"></A>
<P><B>(Wang)</B> Wang, Van Hove, Ross, Baskes, J. Chem. Phys., 121, 5410 (2004).
</P>
<A NAME = "ZBL"></A>
<P><B>(ZBL)</B> J.F. Ziegler, J.P. Biersack, U. Littmark, 'Stopping and Ranges
of Ions in Matter' Vol 1, 1985, Pergamon Press.
</P>
</HTML>

100
doc/pair_meam.txt
View File

@ -20,6 +20,10 @@ pair_coeff * * ../potentials/library.meam Ni Al NULL Ni Al Ni Ni :pre
[Description:]
NOTE: The behavior of the MEAM potential for alloy systems has changed
as of November 2010; see description below of the mixture_ref_t
parameter
Style {meam} computes pairwise interactions for a variety of materials
using modified embedded-atom method (MEAM) potentials
"(Baskes)"_#Baskes. Conceptually, it is an extension to the original
@ -113,6 +117,32 @@ you wish to read in. There can be multiple entries in the library
file with the same "elt" value; LAMMPS reads the 1st matching entry it
finds and ignores the rest.
Other parameters in the MEAM library file correspond to single-element
potential parameters:
lat = lattice structure of reference configuration
z = number of nearest neighbors in the reference structure
ielement = atomic number
atwt = atomic weight
alat = lattice constant of reference structure
esub = energy per atom (eV) in the reference structure at equilibrium
asub = "A" parameter for MEAM (see e.g. "(Baskes)"_#Baskes) :pre
The alpha, b0, b1, b2, b3, t0, t1, t2, t3 parameters correspond to the
standard MEAM parameters in the literature "(Baskes)"_#Baskes (the b
parameters are the standard beta parameters). The rozero parameter is
an element-dependent density scaling that weights the reference
background density (see e.g. equation 4.5 in "(Gullet)"_#Gullet) and
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
0 => G = sqrt(1+Gamma)
1 => G = exp(Gamma/2)
2 => not implemented
3 => G = 2/(1+exp(-Gamma))
4 => G = sqrt(1+Gamma) :pre
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
@ -157,15 +187,19 @@ lattce(I,J) = lattice structure of I-J reference structure:
hcp = hexagonal close-packed
c11 = MoSi2 structure
l12 = Cu3Au structure (lower case L, followed by 12)
b2 = CsCl structure (interpenetrating simple cubic)
nn2(I,J) = turn on second-nearest neighbor MEAM formulation for
I-J pair (see for example "(Lee)"_#Lee). Only valid for I=J.
I-J pair (see for example "(Lee)"_#Lee).
0 = second-nearest neighbor formulation off
1 = second-nearest neighbor formulation on
default = 0
attrac(I,J) = additional cubic attraction term in Rose energy I-J pair potential
default = 0
repuls(I,J) = additional cubic repulsive term in Rose energy I-J pair potential
default = 0
default = 0
zbl(I,J) = blend the MEAM I-J pair potential with the ZBL potential for small
atom separations "(ZBL)"_#ZBL
default = 1
gsmooth_factor = factor determining the length of the G-function smoothing
region; only significant for ibar=0 or ibar=4.
99.0 = short smoothing region, sharp step
@ -175,7 +209,22 @@ augt1 = integer flag for whether to augment t1 parameter by
3/5*t3 to account for old vs. new meam formulations;
0 = don't augment t1
1 = augment t1
default = 1 :pre
default = 1
ialloy = integer flag to use alternative averaging rule for t parameters,
for comparison with the DYNAMO MEAM code
0 = standard averaging (matches ialloy=0 in DYNAMO)
1 = alternative averaging (matches ialloy=1 in DYNAMO)
2 = no averaging of t (use single-element values)
default = 0
mixture_ref_t = integer flag to use mixture average of t to compute the background
reference density for alloys, instead of the single-element values
(see description and warning elsewhere in this doc page)
0 = do not use mixture averaging for t in the reference density
1 = use mixture averaging for t in the reference density
default = 0
erose_form = integer value to select the form of the Rose energy function
(see description below).
default = 0 :pre
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
@ -208,13 +257,28 @@ automatically. When parameter values are fit using the modified
density function, as in more recent literature, augt1 should be set to
0.
The parameters attrac and repuls 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:
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
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
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
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:
astar = alpha * (r/re - 1.d0)
erose = -Ec*(1+astar+a3*(astar**3)/(r/re))*exp(-astar)
if erose_form = 0: erose = -Ec*(1+astar+a3*(astar**3)/(r/re))*exp(-astar)
if erose_form = 1: erose = -Ec*(1+astar+(-attrac+repuls/r)*(astar**3))*exp(-astar)
if erose_form = 2: erose = -Ec*(1 +astar + a3*(astar**3))*exp(-astar)
a3 = repuls, astar < 0
a3 = attrac, astar >= 0 :pre
@ -222,6 +286,14 @@ 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 "(Vallone)"_#Vallone
NOTE: 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
literature). An alternative form (see e.g. "(Lee2)"_#Lee2) is
available using erose_form = 2.
:line
[Mixing, shift, table, tail correction, restart, rRESPA info]:
@ -272,8 +344,16 @@ This report may be accessed on-line via "this link"_sandreport.
:link(Lee)
[(Lee)] Lee, Baskes, Phys. Rev. B, 62, 8564-8567 (2000).
:link(Wang)
[(Wang)] Wang, Van Hove, Ross, Baskes, J. Chem. Phys., 121, 5410 (2004).
:link(Lee2)
[(Lee2)] Lee, Baskes, Kim, Cho. Phys. Rev. B, 64, 184102 (2001).
:link(Valone)
[(Valone)] Valone, Baskes, Martin, Phys. Rev. B, 73, 214209 (2006).
:link(Wang)
[(Wang)] Wang, Van Hove, Ross, Baskes, J. Chem. Phys., 121, 5410 (2004).
:link(ZBL)
[(ZBL)] J.F. Ziegler, J.P. Biersack, U. Littmark, 'Stopping and Ranges
of Ions in Matter' Vol 1, 1985, Pergamon Press.