/* From: "COMPUTATIONAL PHYSICS, 2nd Ed" 
   by RH Landau, MJ Paez, and CC Bordeianu 
   Copyright Wiley-VCH, 2007.
   Electronic Materials copyright: R Landau, Oregon State Univ, 2007;
   MJ Paez, Univ Antioquia, 2007; and CC Bordeianu, Univ Bucharest, 2007.
   Support by National Science Foundation                              
*/
// Ising.java: 1-D Ising model with Metropolis algorithm

import java.io.*;                          // Location of PrintWriter
import java.util.*;                             // Location of Random

public class Ising {
  public static int N = 1000;                      // Number of atoms
  public static double B = 1.;                      // Magnetic field
  public static double mu = .33;           // Magnetic moment of atom
  public static double J = .20;                    // Exchange energy
  public static double k = 1.;                   // Boltzman constant
  public static double T = 100000000.; 
 
  public static void main(String[] argv) 
                         throws IOException, FileNotFoundException {
                           
    Random randnum = new Random(500767);     // Seed random generator
    PrintWriter q  = new 
              PrintWriter(new FileOutputStream ("ising.dat"), false);
              
    int i, j, M = 5000;                       // Number of spin flips
    double[] state = new double[N]; 
    double[] test = state; 
    double ES = energy(state), p, ET;         // State, test's energy
                                                 // Set initial state
    for ( i=0 ;  i < N ;  i++ ) state[i] = -1.;           
                                             // Change state and test
    for ( j=1 ;  j <= M ;  j++ ) {                   
      test = state; 
      i = (int)(randnum.nextDouble()*(double)N);  
                                                  // Flip random atom
      test[i] *= -1.; 
      ET = energy(test); 
      p = Math.exp((ES-ET)/(k*T));    
                                                  // Test trial state 
      if (p >= randnum.nextDouble())  {           
        state = test; 
        ES = ET; 
      }
      q.println(ES);                         // Output energy to file
    }
    q.close(); 
  }

  public static double energy (double[] S)  {   
                                               // Method calc energy
    double FirstTerm = 0., SecondTerm = 0. ; 
    int i;                                           // Sum of energy
    
    for ( i=0 ;  i <= (N-2) ;  i++ )  FirstTerm += S[i]*S[i + 1]; 
    FirstTerm *= -J; 
    for ( i=0 ;  i <= (N-1) ;  i++ )  SecondTerm += S[i];
    SecondTerm *= -B*mu; 
    return (FirstTerm + SecondTerm); 
}  }