smash::ClebschGordan Class Reference

#include <clebschgordan_lookup.h>

Class to store and retrieve/calculate Clebsch-Gordan coefficients.

Definition at line 22 of file clebschgordan_lookup.h.

Classes
struct	ThreeSpinHash
	This is one of the possible ways to prepare a hashing mechanism to use a custom object in a std::unordered_map container. More...
struct	ThreeSpins
	Auxiliary struct to be used as key in the look up table of Clebsch-Gordan coefficients. More...

Static Public Member Functions
static double	coefficient (const int j_a, const int j_b, const int j_c, const int m_a, const int m_b, const int m_c)
	Check in the Clebsch-Gordan lookup table if the requested coefficient is available. More...

Static Private Member Functions
static double	calculate_coefficient (const int j_a, const int j_b, const int j_c, const int m_a, const int m_b, const int m_c)
	Calculate Clebsch-Gordan coefficient \((-1)^{j_a - j_b + m_c} \sqrt{(2 j_c + 1)} \cdot [Wigner 3J symbol] \). More...

Static Private Attributes
static std::unordered_map< ThreeSpins, double, ThreeSpinHash >	lookup_table
	Tabulation of Clebsch-Gordan coefficients. More...

Member Function Documentation

coefficient()

double smash::ClebschGordan::coefficient ( const int  j_a, const int  j_b, const int  j_c, const int  m_a, const int  m_b, const int  m_c  )

static

Check in the Clebsch-Gordan lookup table if the requested coefficient is available.

If so, return it, otherwise calculate the requested one, store it in the lookup table and return it.

See also

calculate_coefficient for a description of function arguments and return value.

Definition at line 41 of file clebschgordan_lookup.cc.

                                                                               {
  const ThreeSpins spin_information = {j_a, j_b, j_c, m_a, m_b, m_c};
  if (auto search = lookup_table.find(spin_information);
      search != lookup_table.end()) {
    return search->second;
  } else {
    double result = calculate_coefficient(j_a, j_b, j_c, m_a, m_b, m_c);
    lookup_table.insert({spin_information, result});
    return result;
  }
}

calculate_coefficient()

double smash::ClebschGordan::calculate_coefficient ( const int  j_a, const int  j_b, const int  j_c, const int  m_a, const int  m_b, const int  m_c  )

staticprivate

Calculate Clebsch-Gordan coefficient \((-1)^{j_a - j_b + m_c} \sqrt{(2 j_c + 1)} \cdot [Wigner 3J symbol] \).

Parameters

[in]	j_a	isospin of first particle
[in]	j_b	isospin of second particle
[in]	j_c	isospin of resonance
[in]	m_a	z-component of isospin of first particle
[in]	m_b	z-component of isospin of second particle
[in]	m_c	z-component of isospin of resonance

Returns

Clebsch-Gordan coefficient for coupling of particles a, b and c

Note that the calculation assumes that the isospin values (j/m) have been multiplied by two (in order to be integer).

Definition at line 22 of file clebschgordan_lookup.cc.

                                                                          {
  const double wigner_3j = gsl_sf_coupling_3j(j_a, j_b, j_c, m_a, m_b, -m_c);
  if (std::abs(wigner_3j) < really_small) {
    return 0.;
  }
  assert((j_a - j_b + m_c) % 2 == 0);
  const int j = (j_a - j_b + m_c) / 2;
  double result = std::sqrt(j_c + 1) * wigner_3j;
  result *= (j % 2 == 0) * 2 - 1;  // == (-1)**j
 
  logg[LResonances].debug("CG: ", result, " I1: ", j_a, " I2: ", j_b,
                          " IR: ", j_c, " iz1: ", m_a, " iz2: ", m_b,
                          " izR: ", m_c);
 
  return result;
}

Member Data Documentation

lookup_table

std::unordered_map<ThreeSpins, double, ThreeSpinHash> smash::ClebschGordan::lookup_table

inlinestaticprivate

Tabulation of Clebsch-Gordan coefficients.

The C++ code to produce this member declaration can be found in the "tabulate" unit test of this file.

Definition at line 193 of file clebschgordan_lookup.h.

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The documentation for this class was generated from the following files:

• src/include/smash/clebschgordan_lookup.h
• src/clebschgordan_lookup.cc