Metropolis Code in a Message Passing Language

C----------------------------------------------------------------------C
C                                                                      C
C       Metropolis Monte Carlo update of the Ising model               C
C       in Fortran with message passing (e.g. MPI).                    C
C                                                                      C
C   As long as size is >=2, don't have to do a red/black checkerboard  C
C   update, since sites being updated simultaneously on different      C
C   processors will not be neighbors.                                  C
C                                                                      C
C----------------------------------------------------------------------C


	SUBROUTINE metupdate(iterations)
	INCLUDE "common.h"
	INTEGER iterations
	INTEGER n,i,j
	INTEGER old_spin, new_spin
	INTEGER spin_sum, old_energy, new_energy, energy_diff
	INTEGER spin1,spin2,spin3,spin4
	INTEGER shift
	REAL random    
	
	DO n=1,iterations

	  DO i=1,size
	  DO j=1,size

	    old_spin = spin(i,j)
	    new_spin = -old_spin
C
C --------  Get neighboring spins.
C           shift is a function defined to handle the
C           periodic boundary conditions and passing of
C           data between processors.
C
            spin1 = shift(i-1,j)
            spin2 = shift(i+1,j)
            spin3 = shift(i,j-1)
            spin4 = shift(i,j+1)
C
C --------  Sum neighboring spins to get energy.
C
	    spin_sum = spin1 + spin2 + spin3 + spin4
	    old_energy = old_spin * spin_sum
	    new_energy = - old_energy
	    energy_diff = new_energy - old_energy
C
C --------  Metropolis accept/reject step.
C
	    IF ( ( energy_diff.LE.0 ) .OR.
     &           ( EXP(-beta*energy_diff).GT.random() ) ) THEN
	      spin(i,j) = new_spin
	    ENDIF

	  ENDDO
	  ENDDO

	ENDDO

	RETURN 
	END

Paul Coddington, Northeast Parallel Architectures Center at Syracuse University, paulc@npac.syr.edu