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). 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.
+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)) ++
The alpha, b0, b1, b2, b3, t0, t1, t2, t3 parameters correspond to the +standard MEAM parameters in the literature (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)) 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) +
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 (Lee)). Only valid for I=J. + I-J pair (see for example (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) + 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
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.
-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@@ -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 (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)) is +available using erose_form = 2. +
Mixing, shift, table, tail correction, restart, rRESPA info: @@ -278,12 +350,21 @@ This report may be accessed on-line via + -
(Wang) Wang, Van Hove, Ross, Baskes, J. Chem. Phys., 121, 5410 (2004). +
(Lee2) Lee, Baskes, Kim, Cho. Phys. Rev. B, 64, 184102 (2001).
(Valone) Valone, Baskes, Martin, Phys. Rev. B, 73, 214209 (2006).
+ + +(Wang) Wang, Van Hove, Ross, Baskes, J. Chem. Phys., 121, 5410 (2004). +
+ + +(ZBL) J.F. Ziegler, J.P. Biersack, U. Littmark, 'Stopping and Ranges +of Ions in Matter' Vol 1, 1985, Pergamon Press. +