From eae0731cae5b1e2d5f2326d777ccc110e072b02e Mon Sep 17 00:00:00 2001 From: sjplimp Date: Tue, 22 Feb 2011 20:30:51 +0000 Subject: [PATCH] git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@5700 f3b2605a-c512-4ea7-a41b-209d697bcdaa --- doc/pair_meam.html | 99 ++++++++++++++++++++++++++++++++++++++++---- doc/pair_meam.txt | 100 ++++++++++++++++++++++++++++++++++++++++----- 2 files changed, 180 insertions(+), 19 deletions(-) diff --git a/doc/pair_meam.html b/doc/pair_meam.html index 795ea71c16..bad3abf6c0 100644 --- a/doc/pair_meam.html +++ b/doc/pair_meam.html @@ -23,6 +23,10 @@ pair_coeff * * ../potentials/library.meam Ni Al NULL Ni Al Ni Ni

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. +

diff --git a/doc/pair_meam.txt b/doc/pair_meam.txt index c43e7e953b..db3c4ca8de 100644 --- a/doc/pair_meam.txt +++ b/doc/pair_meam.txt @@ -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. +