Green’s functions

The gfs class of TRIQS contains objects representing Green functions over real or imaginary times, real or imaginary frequencies… that can be easily manipulated at the C++ level. Here are a couple of simple examples showing the basic use of this class. Learn more in the full reference.

Matsubara Green’s functions

Creation of a simple Green’s function \(G(i\omega)\)

In this example, we show how to initialize the following Green’s functions:

\[G(i\omega) = \frac{1}{i\omega -3}\]

#include <triqs/gfs.hpp>
using namespace triqs::gfs;
using triqs::clef::placeholder;

int main() {

  double beta = 1;                                       //inverse temperature
  int nw      = 100;                                     //number of Matsubara frequencies
  auto g      = gf<imfreq>{{beta, Fermion, nw}, {1, 1}}; //{1,1} : 1x1 Green's function

  //the shortest way to fill a gf
  placeholder<0> w_;
  g(w_) << 1 / (w_ - 3);
  std::cout << g(0) << std::endl;

  g() = 0.0;
  //an equivalent way
  for (auto const &w : g.mesh()) g[w] = 1 / (w - 3);
  std::cout << g(0) << std::endl;

  //an incorrect way : throws exception as expected
  //g(w) returns a const_view: () are to be used for interpolation, see bottom of the page
  //for(auto const & w : g.mesh())  g(w) = 1/(w-3);
}

Two-frequency Green’s function \(G(i\omega,i\nu)\)

In this example, we show how to initialize the following Green’s functions:

\[G(i\omega, i\nu) = \frac{1}{i\omega + i\nu -4}\]

#include <triqs/gfs.hpp>
using namespace triqs::gfs;
using triqs::clef::placeholder;

int main() {

  double beta = 1;
  int nw      = 100;
  auto g2     = gf<cartesian_product<imfreq, imfreq>>{{{beta, Fermion, nw}, {beta, Fermion, nw}}, {1, 1}};

  //the shortest way to fill a gf
  placeholder<0> w_;
  placeholder<1> nu_;
  g2(w_, nu_) << 1 / (w_ + nu_ - 4);
  std::cout << g2(0, 0) << std::endl;

  g2() = 0.0;
  //equivalent way
  for (auto const &w : std::get<0>(g2.mesh().components()))
    for (auto const &nu : std::get<1>(g2.mesh().components())) g2[{w, nu}] = 1 / (w + nu - 4);
  std::cout << g2(0, 0) << std::endl;
}

Imaginary-time Green’s functions \(G(\tau)\)

#include <triqs/gfs.hpp>
using namespace triqs::gfs;

int main() {
  double beta = 10;  // the time interval
  int n_times = 100; // we will have 100
  auto g      = gf<imtime, scalar_valued>{{beta, Fermion, n_times}};
}

Real-time Green’s functions \(G(t)\)

Here we create a GF defined on the time interval from tmin to tmax. If we want the value of the GF at any time to be a scalar, we use:

#include <triqs/gfs.hpp>
using namespace triqs::gfs;

int main() {
  double tmin = 0, tmax = 10; // the time interval
  int n_times = 100;          // we will have 100 points
  auto g      = gf<retime, scalar_valued>{{tmin, tmax, n_times}};
}

If we need a matrix of size n by m, we use:

#include <triqs/gfs.hpp>
using namespace triqs::gfs;

int main() {
  double tmin = 0, tmax = 10;            // the time interval
  const int n = 2, m = 2, n_times = 100; // we will have 100 points
  auto g = gf<retime, matrix_valued>{{tmin, tmax, n_times}, {n, m}};
}

Or a tensor!

#include <triqs/gfs.hpp>
using namespace triqs::gfs;

int main() {
  double tmin = 0, tmax = 10; // the time interval
  double beta = 1;
  int n_times = 100; // we will have 100 pointspoints
  auto g      = gf<cartesian_product<retime, imtime>, tensor_valued<3>>{{{tmin, tmax, n_times}, {beta, Fermion, n_times}}, {2, 2, 2}};
}

Creation of a two real time GF \(G(t,t')\)

#include <triqs/gfs.hpp>
using namespace triqs::gfs;
int main() {
  double tmin = 0, tmax = 1.0;
  int nt = 100;
  auto g = gf<cartesian_product<retime, retime>, scalar_valued>{{{tmin, tmax, nt}, {tmin, tmax, nt}}};
}

How to fill a GF with placeholders

#include <triqs/gfs.hpp>
using namespace triqs::gfs;
int main() {
  double tmin = 0, tmax = 1.0;
  int nt = 100;
  auto g = gf<cartesian_product<retime, retime>, scalar_valued>{{{tmin, tmax, nt}, {tmin, tmax, nt}}};
  triqs::clef::placeholder<0> t1_;
  triqs::clef::placeholder<1> t2_;
  g(t1_, t2_) << 2 * t1_;
}

How to interpolate the GF value at a point of the domain

You simply have to call the GF with the coordinates of the point:

#include <triqs/gfs.hpp>
using namespace triqs::gfs;
int main() {
  double tmin = 0, tmax = 1.0;
  int nt = 100;
  auto g = gf<cartesian_product<retime, retime>, scalar_valued>{{{tmin, tmax, nt}, {tmin, tmax, nt}}};
  triqs::clef::placeholder<0> t1_;
  triqs::clef::placeholder<1> t2_;
  g(t1_, t2_) << 2. * t1_;
  std::cout << g(0.24, 0.36) << std::endl;
}

Learn more in the full reference, see C++ documentation