/* 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                              
*/
// Tune.java: eigenvalue solution for performace tuning

public class Tune { 

  public static void main(String[] argv) { 
		
    final int Ldim = 2051;   
    int i, j, iter = 0; 
    double [][] ham = new double [Ldim] [Ldim]; 
    double [] coef = new double [Ldim]; 
    double [] sigma = new double [Ldim];   
    double err, ener, ovlp, step = 0., time; 
		
    time = System.currentTimeMillis();             // Initialize time
		                                          // Init matrix & vector
      for ( i = 1;  i <= Ldim-1;  i++ )  {        
        for ( j=1;  j <= Ldim-1;  j++ ) { 
          if (Math.abs(j-i) >10)  ham[j][i] = 0. ;  
          else  ham[j][i] = Math.pow(0.3, Math.abs(j-i));  
        }   
        ham[i][i] = i ; 
        coef[i] = 0.;      
      } 
      coef[1] = 1.;   
      err = 1.;                                        
			iter = 0 ;  
			                                             // Start iteration
      while (iter  < 15 && err > 1.e-6)  { 
        iter = iter + 1; 
        ener = 0. ; 
        ovlp = 0.;  
				                                     // Compute E & normalize
        for ( i= 1;  i <= Ldim-1;  i++ )  {      
          ovlp = ovlp + coef[i]*coef[i] ;   
          sigma[i] = 0. ;    
          for (j= 1; j <= Ldim-1;  j++)
                 sigma[i] = sigma[i]+coef[j]*ham[j][i]; 
          ener = ener + coef[i]*sigma[i] ;    
        }  
        ener = ener/ovlp; 
        for (  i = 1;  i <= Ldim-1;  i++ )  {
          coef[i] = coef[i]/Math.sqrt(ovlp) ;   
          sigma[i] = sigma[i]/Math.sqrt(ovlp) ; 
        }    
        err = 0.;   
				                                                    // Update
        for ( i = 2;  i <= Ldim-1;  i++ )  {                    
        step = (sigma[i] - ener*coef[i])/(ener-ham[i][i]) ;  
         coef[i] = coef[i] + step  ; 
         err = err +  step*step ;        }  
        err = Math.sqrt(err) ;  
        System.out.println 
            ("iter, ener, err " + iter + ",  " + ener + ",  " + err);
      }              
      time = (System.currentTimeMillis() - time)/1000;   // Elapsed t
      System.out.println("time = " + time + "s"); 
  } 
}