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manifold_gaussian_bump.cpp
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Thu, Jun 27, 07:34

manifold_gaussian_bump.cpp
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#include "manifold_gaussian_bump.h"
#include "comm.h"
#include "error.h"
using namespace LAMMPS_NS;
using namespace user_manifold;
// This manifold comes with some baggage;
// it needs a taper function that is sufficiently smooth
// so we use a cubic Hermite interpolation and then for
// efficiency we construct a lookup table for that....
class cubic_hermite
{
public:
// Represent x-polynomial as a * t^3 + b * t^2 + c*t + d.
double a, b, c, d;
// And y-polynomial as s * t^3 + u * t^2 + v * t + w.
double s, u, v, w;
double x0, x1, y0, y1, yp0, yp1;
LAMMPS_NS::Error *err;
cubic_hermite( double x0, double x1, double y0, double y1,
double yp0, double yp1, LAMMPS_NS::Error *err ) :
x0(x0), x1(x1), y0(y0), y1(y1), yp0(yp0), yp1(yp1),
a( 2*x0 + 2 - 2*x1 ),
b( -3*x0 - 3 + 3*x1 ),
c( 1.0 ),
d( x0 ),
s( 2*y0 - 2*y1 + yp0 + yp1 ),
u( -3*y0 + 3*y1 - 2*yp0 - yp1 ),
v( yp0 ),
w( y0 ),
err(err)
{
test();
}
void test()
{
if( fabs( x(0) - x0 ) > 1e-8 ) err->one(FLERR, "x0 wrong");
if( fabs( x(1) - x1 ) > 1e-8 ) err->one(FLERR, "x1 wrong");
if( fabs( y(0) - y0 ) > 1e-8 ) err->one(FLERR, "y0 wrong");
if( fabs( y(1) - y1 ) > 1e-8 ) err->one(FLERR, "y1 wrong");
}
double get_t_from_x( double xx ) const
{
if( xx < x0 || xx > x1 ){
char msg[2048];
sprintf(msg, "x ( %g ) out of bounds [%g, %g]", xx, x0, x1 );
err->one(FLERR, msg);
}
// Newton iterate to get right t.
double t = xx - x0;
double dx = x1 - x0;
// Reasonable initial guess I hope:
t /= dx;
double tol = 1e-8;
int maxit = 500;
double ff = x(t) - xx;
double ffp = xp(t);
// double l = 1.0 / ( 1 + res*res );
for( int i = 0; i < maxit; ++i ){
t -= ff / ffp;
ff = x(t) - xx;
ffp = xp(t);
double res = ff;
if( fabs( res ) < tol ){
return t;
}
}
err->warning(FLERR, "Convergence failed");
return t;
}
double x( double t ) const
{
double t2 = t*t;
double t3 = t2*t;
return a*t3 + b*t2 + c*t + d;
}
double y_from_x( double x ) const
{
double t = get_t_from_x( x );
return y(t);
}
double yp_from_x( double x ) const
{
double t = get_t_from_x( x );
return yp(t);
}
double y( double t ) const
{
double t2 = t*t;
double t3 = t2*t;
return s*t3 + u*t2 + v*t + w;
}
void xy( double t, double &xx, double &yy ) const
{
xx = x(t);
yy = y(t);
}
double xp( double t ) const
{
double t2 = t*t;
return 3*a*t2 + 2*b*t + c;
}
double yp( double t ) const
{
double t2 = t*t;
return 3*t2*s + 2*u*t + v;
}
double xpp( double t ) const
{
return 6*a*t + 2*b;
}
};
// Manifold itself:
manifold_gaussian_bump::manifold_gaussian_bump(class LAMMPS* lmp,
int narg, char **arg)
: manifold(lmp), lut_z(NULL), lut_zp(NULL) {}
manifold_gaussian_bump::~manifold_gaussian_bump()
{
if( lut_z ) delete lut_z;
if( lut_zp ) delete lut_zp;
}
// The constraint is z = f(r = sqrt( x^2 + y^2) )
// f(x) = A*exp( -x^2 / 2 l^2 ) if x < rc1
// = Some interpolation if rc1 <= rc2
// = 0 if x >= rc2
//
double manifold_gaussian_bump::g( const double *x )
{
double xf[3];
xf[0] = x[0];
xf[1] = x[1];
xf[2] = 0.0;
double x2 = dot(xf,xf);
if( x2 < rc12 ){
return x[2] - gaussian_bump_x2( x2 );
}else if( x2 < rc22 ){
double rr = sqrt( x2 );
double z_taper_func = lut_get_z( rr );
return x[2] - z_taper_func;
}else{
return x[2];
}
}
void manifold_gaussian_bump::n( const double *x, double *nn )
{
double xf[3];
xf[0] = x[0];
xf[1] = x[1];
xf[2] = 0.0;
nn[2] = 1.0;
double x2 = dot(xf,xf);
if( x2 < rc12 ){
double factor = gaussian_bump_x2(x2);
factor /= (ll*ll);
nn[0] = factor * x[0];
nn[1] = factor * x[1];
}else if( x2 < rc22 ){
double rr = sqrt( x2 );
double zp_taper_func = lut_get_zp( rr );
double inv_r = 1.0 / rr;
double der_part = zp_taper_func * inv_r;
nn[0] = der_part * x[0];
nn[1] = der_part * x[1];
}else{
nn[0] = nn[1] = 0.0;
}
}
double manifold_gaussian_bump::g_and_n( const double *x, double *nn )
{
double xf[3];
xf[0] = x[0];
xf[1] = x[1];
xf[2] = 0.0;
nn[2] = 1.0;
double x2 = dot(xf,xf);
if( x2 < rc12 ){
double gb = gaussian_bump_x2(x2);
double factor = gb / (ll*ll);
nn[0] = factor * x[0];
nn[1] = factor * x[1];
return x[2] - gb;
}else if( x2 < rc22 ){
double z_taper_func, zp_taper_func;
double rr = sqrt( x2 );
lut_get_z_and_zp( rr, z_taper_func, zp_taper_func );
double inv_r = 1.0 / rr;
double der_part = zp_taper_func * inv_r;
nn[0] = der_part * x[0];
nn[1] = der_part * x[1];
return x[2] - z_taper_func;
}else{
nn[0] = nn[1] = 0.0;
return x[2];
}
}
void manifold_gaussian_bump::post_param_init()
{
// Read in the params:
AA = params[0];
ll = params[1];
rc1 = params[2];
rc2 = params[3];
ll2 = 2.0*ll*ll;
rc12 = rc1*rc1;
rc22 = rc2*rc2;
dr = rc2 - rc1;
inv_dr = 1.0 / dr;
f_at_rc = gaussian_bump_x2 ( rc12 );
fp_at_rc = gaussian_bump_der( rc1 );
make_lut();
// test_lut();
}
double manifold_gaussian_bump::gaussian_bump( double x ) const
{
double x2 = x*x;
return gaussian_bump_x2( x2 );
}
double manifold_gaussian_bump::gaussian_bump_x2( double x2 ) const
{
return AA*exp( -x2 / ll2 );
}
double manifold_gaussian_bump::gaussian_bump_der( double x ) const
{
double x2 = x*x;
return gaussian_bump_x2( x2 )*( -x/(ll*ll) );
}
void manifold_gaussian_bump::make_lut()
{
lut_x0 = rc1;
lut_x1 = rc2;
lut_Nbins = 1024; // Don't make it too big, it should fit in cache.
lut_z = new double[lut_Nbins+1];
lut_zp = new double[lut_Nbins+1];
lut_dx = (lut_x1 - lut_x0) / lut_Nbins;
cubic_hermite pchip( lut_x0, lut_x1, f_at_rc, 0.0, fp_at_rc, 0.0, error );
double xx = lut_x0;
for( int i = 0; i <= lut_Nbins; ++i ){
lut_z[i] = pchip.y_from_x( xx );
lut_zp[i] = pchip.yp_from_x( xx );
xx += lut_dx;
}
}
double manifold_gaussian_bump::lut_get_z ( double rr ) const
{
double xs = rr - lut_x0;
double xss = xs / lut_dx;
int bin = static_cast<int>(xss);
double frac = xss - bin;
double zleft = lut_z[bin];
double zright = lut_z[bin+1];
return zleft * ( 1 - frac ) + frac * zright;
}
double manifold_gaussian_bump::lut_get_zp( double rr ) const
{
double xs = rr - lut_x0;
double xss = xs / lut_dx;
int bin = static_cast<int>(xss);
double frac = xss - bin;
double zleft = lut_zp[bin];
double zright = lut_zp[bin+1];
return zleft * ( 1 - frac) + frac * zright;
}
void manifold_gaussian_bump::lut_get_z_and_zp( double rr, double &zz,
double &zzp ) const
{
double xs = rr - lut_x0;
double xss = xs / lut_dx;
int bin = static_cast<int>(xss);
double frac = xss - bin;
double fmin = 1 - frac;
double zleft = lut_z[bin];
double zright = lut_z[bin+1];
double zpleft = lut_zp[bin];
double zpright = lut_zp[bin+1];
zz = zleft * fmin + zright * frac;
zzp = zpleft * fmin + zpright * frac;
}
void manifold_gaussian_bump::test_lut()
{
double x[3], nn[3];
if( comm->me != 0 ) return;
FILE *fp = fopen( "test_lut_gaussian.dat", "w" );
double dx = 0.1;
for( double xx = 0; xx < 20; xx += dx ){
x[0] = xx;
x[1] = 0.0;
x[2] = 0.0;
double gg = g( x );
n( x, nn );
double taper_z;
if( xx <= rc1 ){
taper_z = gaussian_bump(xx);
}else if( xx < rc2 ){
taper_z = lut_get_z( xx );
}else{
taper_z = 0.0;
}
fprintf( fp, "%g %g %g %g %g %g %g\n", xx, gaussian_bump(xx), taper_z,
gg, nn[0], nn[1], nn[2] );
}
fclose(fp);
}

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