xy.cpp

#include <sstream>
#include <iostream>
#include <string>
#include <math.h>
 
#define NDIM 2
 
// Include the lattice field definition
#include "plumbing/field.h"
 
// Define some parameters for the simulation
double beta = 0.1;
int n_measurements = 100;
int n_updates_per_measurement = 10;
long seed = 123456;
CoordinateVector nd = {64, 64};
int VOLUME = nd[0] * nd[1];
 
int main(int argc, char **argv) {
 
    // Basic setup
    hila::initialize(argc, argv);
    lattice.setup(nd);
    // Define a field
    Field<double> spin;
 
    hila::seed_random(seed);
 
    // Set to 1
    spin[ALL] = 0;
 
    // Run update-measure loop
    for (int i = 0; i < n_measurements; i++) {
 
        // Run a number of updates, starting with EVEN sites
        // and alternating between parities
        Parity p = EVEN;
        for (int j = 0; j < 2 * n_updates_per_measurement; j++) {
            onsites(p) {
                double deltaS;
                double tspin = spin[X];
                double tnspin = tspin + M_PI * (1. - 2. * hila::random());
                deltaS = cos(spin[X + e_x] - tspin) + cos(spin[X - e_x] - tspin) +
                         cos(spin[X + e_y] - tspin) + cos(spin[X - e_y] - tspin);
                deltaS -= cos(spin[X + e_x] - tnspin) + cos(spin[X - e_x] - tnspin) +
                          cos(spin[X + e_y] - tnspin) + cos(spin[X - e_y] - tnspin);
 
                if (deltaS < 0 || hila::random() < exp(-beta * deltaS)) {
                    if (tnspin < -M_PI) {
                        tnspin += 2.0 * M_PI;
                    } else if (tnspin > M_PI) {
                        tnspin -= 2.0 * M_PI;
                    }
                    spin[X] = tnspin;
                }
            }
            if (hila::random() < 0.5) {
                p = opp_parity(p);
            }
        }
 
        // Measure magnetisation
        double M = 0;
        onsites(ALL) { M += cos(spin[X]); }
        hila::out0 << "Magnetisation " << M / VOLUME << "\n";
    }
 
    hila::finishrun();
    return 0;
}