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meam_funcs.cpp
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Mon, Nov 4, 22:06

meam_funcs.cpp
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/* ----------------------------------------------------------------------
LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
http://lammps.sandia.gov, Sandia National Laboratories
Steve Plimpton, sjplimp@sandia.gov
Copyright (2003) Sandia Corporation. Under the terms of Contract
DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
certain rights in this software. This software is distributed under
the GNU General Public License.
See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */
/* ----------------------------------------------------------------------
Contributing author: Sebastian Hütter (OvGU)
------------------------------------------------------------------------- */
#include "meam.h"
#include "math_special.h"
#include <math.h>
using namespace LAMMPS_NS;
//-----------------------------------------------------------------------------
// Compute G(gamma) based on selection flag ibar:
// 0 => G = sqrt(1+gamma)
// 1 => G = exp(gamma/2)
// 2 => not implemented
// 3 => G = 2/(1+exp(-gamma))
// 4 => G = sqrt(1+gamma)
// -5 => G = +-sqrt(abs(1+gamma))
//
double
MEAM::G_gam(const double gamma, const int ibar, int& errorflag) const
{
double gsmooth_switchpoint;
switch (ibar) {
case 0:
case 4:
gsmooth_switchpoint = -gsmooth_factor / (gsmooth_factor + 1);
if (gamma < gsmooth_switchpoint) {
// e.g. gsmooth_factor is 99, {:
// gsmooth_switchpoint = -0.99
// G = 0.01*(-0.99/gamma)**99
double G = 1 / (gsmooth_factor + 1) * pow((gsmooth_switchpoint / gamma), gsmooth_factor);
return sqrt(G);
} else {
return sqrt(1.0 + gamma);
}
case 1:
return MathSpecial::fm_exp(gamma / 2.0);
case 3:
return 2.0 / (1.0 + exp(-gamma));
case -5:
if ((1.0 + gamma) >= 0) {
return sqrt(1.0 + gamma);
} else {
return -sqrt(-1.0 - gamma);
}
}
errorflag = 1;
return 0.0;
}
//-----------------------------------------------------------------------------
// Compute G(gamma and dG(gamma) based on selection flag ibar:
// 0 => G = sqrt(1+gamma)
// 1 => G = exp(gamma/2)
// 2 => not implemented
// 3 => G = 2/(1+exp(-gamma))
// 4 => G = sqrt(1+gamma)
// -5 => G = +-sqrt(abs(1+gamma))
//
double
MEAM::dG_gam(const double gamma, const int ibar, double& dG) const
{
double gsmooth_switchpoint;
double G;
switch (ibar) {
case 0:
case 4:
gsmooth_switchpoint = -gsmooth_factor / (gsmooth_factor + 1);
if (gamma < gsmooth_switchpoint) {
// e.g. gsmooth_factor is 99, {:
// gsmooth_switchpoint = -0.99
// G = 0.01*(-0.99/gamma)**99
double G = 1 / (gsmooth_factor + 1) * pow((gsmooth_switchpoint / gamma), gsmooth_factor);
G = sqrt(G);
dG = -gsmooth_factor * G / (2.0 * gamma);
return G;
} else {
G = sqrt(1.0 + gamma);
dG = 1.0 / (2.0 * G);
return G;
}
case 1:
G = MathSpecial::fm_exp(gamma / 2.0);
dG = G / 2.0;
return G;
case 3:
G = 2.0 / (1.0 + MathSpecial::fm_exp(-gamma));
dG = G * (2.0 - G) / 2;
return G;
case -5:
if ((1.0 + gamma) >= 0) {
G = sqrt(1.0 + gamma);
dG = 1.0 / (2.0 * G);
return G;
} else {
G = -sqrt(-1.0 - gamma);
dG = -1.0 / (2.0 * G);
return G;
}
}
dG = 1.0;
return 0.0;
}
//-----------------------------------------------------------------------------
// Compute ZBL potential
//
double
MEAM::zbl(const double r, const int z1, const int z2)
{
int i;
const double c[] = { 0.028171, 0.28022, 0.50986, 0.18175 };
const double d[] = { 0.20162, 0.40290, 0.94229, 3.1998 };
const double azero = 0.4685;
const double cc = 14.3997;
double a, x;
// azero = (9pi^2/128)^1/3 (0.529) Angstroms
a = azero / (pow(z1, 0.23) + pow(z2, 0.23));
double result = 0.0;
x = r / a;
for (i = 0; i <= 3; i++) {
result = result + c[i] * MathSpecial::fm_exp(-d[i] * x);
}
if (r > 0.0)
result = result * z1 * z2 / r * cc;
return result;
}
//-----------------------------------------------------------------------------
// Compute Rose energy function, I.16
//
double
MEAM::erose(const double r, const double re, const double alpha, const double Ec, const double repuls,
const double attrac, const int form)
{
double astar, a3;
double result = 0.0;
if (r > 0.0) {
astar = alpha * (r / re - 1.0);
a3 = 0.0;
if (astar >= 0)
a3 = attrac;
else if (astar < 0)
a3 = repuls;
if (form == 1)
result = -Ec * (1 + astar + (-attrac + repuls / r) * pow(astar, 3)) * MathSpecial::fm_exp(-astar);
else if (form == 2)
result = -Ec * (1 + astar + a3 * pow(astar, 3)) * MathSpecial::fm_exp(-astar);
else
result = -Ec * (1 + astar + a3 * pow(astar, 3) / (r / re)) * MathSpecial::fm_exp(-astar);
}
return result;
}
//-----------------------------------------------------------------------------
// Shape factors for various configurations
//
void
MEAM::get_shpfcn(const lattice_t latt, double (&s)[3])
{
switch (latt) {
case FCC:
case BCC:
case B1:
case B2:
s[0] = 0.0;
s[1] = 0.0;
s[2] = 0.0;
break;
case HCP:
s[0] = 0.0;
s[1] = 0.0;
s[2] = 1.0 / 3.0;
break;
case DIA:
s[0] = 0.0;
s[1] = 0.0;
s[2] = 32.0 / 9.0;
break;
case DIM:
s[0] = 1.0;
s[1] = 2.0 / 3.0;
// s(3) = 1.d0
s[2] = 0.40;
break;
default:
s[0] = 0.0;
// call error('Lattice not defined in get_shpfcn.')
}
}
//-----------------------------------------------------------------------------
// Number of neighbors for the reference structure
//
int
MEAM::get_Zij(const lattice_t latt)
{
switch (latt) {
case FCC:
return 12;
case BCC:
return 8;
case HCP:
return 12;
case B1:
return 6;
case DIA:
return 4;
case DIM:
return 1;
case C11:
return 10;
case L12:
return 12;
case B2:
return 8;
// call error('Lattice not defined in get_Zij.')
}
return 0;
}
//-----------------------------------------------------------------------------
// Number of second neighbors for the reference structure
// a = distance ratio R1/R2
// S = second neighbor screening function
//
int
MEAM::get_Zij2(const lattice_t latt, const double cmin, const double cmax, double& a, double& S)
{
double C, x, sijk;
int Zij2 = 0, numscr = 0;
switch (latt) {
case FCC:
Zij2 = 6;
a = sqrt(2.0);
numscr = 4;
break;
case BCC:
Zij2 = 6;
a = 2.0 / sqrt(3.0);
numscr = 4;
break;
case HCP:
Zij2 = 6;
a = sqrt(2.0);
numscr = 4;
break;
case B1:
Zij2 = 12;
a = sqrt(2.0);
numscr = 2;
break;
case DIA:
Zij2 = 0;
a = sqrt(8.0 / 3.0);
numscr = 4;
if (cmin < 0.500001) {
// call error('can not do 2NN MEAM for dia')
}
break;
case DIM:
// this really shouldn't be allowed; make sure screening is zero
a = 1.0;
S = 0.0;
return 0;
case L12:
Zij2 = 6;
a = sqrt(2.0);
numscr = 4;
break;
case B2:
Zij2 = 6;
a = 2.0 / sqrt(3.0);
numscr = 4;
break;
case C11:
// unsupported lattice flag C11 in get_Zij
break;
default:
// unknown lattic flag in get Zij
// call error('Lattice not defined in get_Zij.')
break;
}
// Compute screening for each first neighbor
C = 4.0 / (a * a) - 1.0;
x = (C - cmin) / (cmax - cmin);
sijk = fcut(x);
// There are numscr first neighbors screening the second neighbors
S = MathSpecial::powint(sijk, numscr);
return Zij2;
}

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