!	Program to simulate thin film spinglasses.
!
!	Ge Nieuwenhuys, June, 2002,      Written as Ising Metropolis program
!                   October 2005,    Rewritten as Heisenberg Zero-Temperature 
!					October 17, 2005 Bug in direct access file "removes" by 
!                                    oversizing the recordlength
!
!	October 12: periodic boundary conditions in y- z plane 
!	October 14: random number start randomly (based on clock) for
!				batch calculations.
!	October 14: output-file-names are automatically indexed.
!
!	Spins are located on a fcc lattice
!
!	nspin	number of spins
!	nsp		number of spins asked
!	d		thickness
!	a		lattice constant
!	ah		half of lattice constant
!
	Use DFPORT			! library only needed for obtaining CPU-time
	Use DFLIB
!
	parameter ( max_spins = 100000 )
!
	structure /spin/
	  integer*4 x,y,z
	  real*8    c(3)
	  end structure
!
	structure /inter/
	  integer  si
	  real*8   val
	  end structure	
!
	integer*4     j,k,l,m,n, nsp, nspin, nat, steps_per_spin, mh, ix, iy, iz
	integer*4     l_calc, ifile, iseed 
	record        /spin/  s(max_spins)
	record        /inter/ i(100,max_spins)
	real*8        d, concentration, c, dd(max_spins), rkky, norm, p(3), k_F
	real*8        b_ext(3), b_ext_K(3), moment, T_glass, mag(3)
	character*1   answer
	character*80  calculation, line
	logical       open_inp
	character*8   dddd, tttt, zone
	character*4   file_index
	integer       dt(8)
	real*4        runtime(2), start_time, end_time
!
!	initialization
!
	val      = 0
	a        = .407			! nm, lattice parameter for Au
	k_F      = 12.0			! 1/nm, Fermi wavevector for Au
	moment   = 2.2			! mu_B, moment of the impurity spins
	b_ext(1) = 0.01
	b_ext(2) = 0.0
	b_ext(3) = 0.0			! extrenal field of 100 gauss in x-direction
	T_glass  = 15.0			! glass_temperature in Kelvin
	ah	     = a / 2.0
	iseed    = 1234567
!
	Write(6,*) '-----------------------------------------------------'
	Write(6,*) '|       SPIN-GLASS GROUNDSTATE SIMULATION           |'
	Write(6,*) '|           Version October 17, 2005                |'
	Write(6,*) '|                                                   |'
	Write(6,*) '|  This program simulates a spin-glass groundstate  |'
	Write(6,*) '|         using the method as described by          |'
	Write(6,*) '|  Walstedt and Walker, Phys. Rev. B22 (1980) 3816  |'
	Write(6,*) '|                                                   |'
	Write(6,*) '|  The program can be used in batch mode by         |'
	Write(6,*) '|  supplying the name of the calculation            |'
	Write(6,*) '|  (Enter will start the online mode)               |'
	Write(6,*) '|                                                   |'
	Write(6,*) '| In de batch-mode the parameters will be read from |'
	Write(6,*) '| <calculation>.inp                                 |'
	Write(6,*) '| A comment on the first line and then on           |'
	Write(6,*) '| each line of this file                            |'
	Write(6,*) '| number of spins,                                  |'
	Write(6,*) '| thickness of sample [nm],                         |'
	Write(6,*) '| lattice parameter [nm],                           |'
	Write(6,*) '| Fermi wave number [nm^-1]                         |'
	Write(6,*) '| concentration of spins [at.%],                    |'
	Write(6,*) '| glass temperature [K],                            |'
	Write(6,*) '| magnetic moment   [mu_B],                         |'
	Write(6,*) '| external magnetic field (3 components) [tesla]    |'
	Write(6,*) '| number of iterations                              |'
	Write(6,*) '|                                                   |'
	Write(6,*) '| <calculation> can be entered as a                 |'
	Write(6,*) '|                   commandline parameter           |'
	Write(6,*) '-----------------------------------------------------' 
!
!	Ask name of calculation (and read if in batch mode )
!
!
!	files :
!
	open(9,file='u:\simulations\counter.his',status='old')
	read(9,*) ifile								! initialize outputfile counter									  
!
	IF ( iargc() .GT. 0 ) THEN
	  call getarg(1, calculation)
	  Write(6,*) ' Calculation taken from commandline > ',calculation
	  ELSE
777	  write(6,778)
778	  format(' '/' Give name of the calculation     > ', \)
	  read(5,'(a80)') calculation
	  END IF
!
	IF ( calculation(1:1) .NE. ' ' ) THEN
	  l_calc = index( calculation, ' ') - 1
	  open(1,file=calculation(1:l_calc)//'.inp', status='old', action='read', err=777)
	  open(2,file=calculation(1:l_calc)//'.out', status='unknown', action='write')
	  END IF
!
    inquire(1,opened=open_inp)
	IF (open_inp) read(1,'(a80)') comment
!
	open(9,file='u:\simulations\counter.sgl',status='old')
	read(9,*) ifile
!
888	IF (open_inp) THEN                            ! new calculation
889	  ifile = ifile + 1							  ! increase outputfile number
	  rewind(9)
	  write(9,*) ifile                            ! store for next program
	  write(file_index,'(''_'',i3)') ifile		  ! generate file_name
	  DO j = 2, 4 
	    IF (file_index(j:j) .EQ. ' ' ) file_index(j:j) = '0'
		END DO
!
!	Although the length of the record /spin/ is 3*4 + 3*8 = 36, the length had to be
!	set to 40, otherwise the writing went wrong (s.c(3) always, except for the last
!	equal to zero
!
      open(3,file=calculation(1:l_calc)//file_index//'.sgl',status='unknown',   &
 &		   access='direct',form='binary',recl=40)
!
890	  read(1,'(a80)',end=999) line
	  IF ( line(1:1) .EQ. '!' ) THEN
	    write(6,'(a)') line
		GOTO 890
		ELSE
	    read(line,*,err=998,end=999) nsp, d, a, k_F, concentration,             &
 &                                         T_glass, moment, b_ext,              &
 &                                         steps_per_spin
	    END IF
!
!	  Initialize randomnumber generator "randomly" 
!
	  call date_and_time( dddd, tttt, zone, dt )
	  DO j = 1, dt(8)								! number milliseconds on the clock
	    dummy = rand(iseed) 
		END DO
!		                                  
	  ELSE
!
!	Ask size of the system
!	   	        
1	  write(6,2)
2	  format( '  How many spins             ?  '\)
	  read(5,*,err=1) nsp
	  IF ( nsp .LE. 0         ) STOP '  Programm terminated by operator '
	  IF ( nsp .GT. max_spins ) GOTO 1
!
3	  write(6,4)
4	  format( '  What thickness [nm]        ?  '\)
	  read(5,*,err=3) d
!
5	  write(6,6) 
6	  format( '  Which concentration [at.%] ?  '\)
	  read(5,*,err=5) concentration

7	  write(6,8) 
8	  format('   Give the T_glass and the magnetic moment > '\)
	  read(5,*,err=7) T_glass, moment
	  END IF
!
!	end of getting all parameters
!
	c = concentration / 100
!
!	Calculate the magnetic field energy in Kelvin
!
	b_ext_K = moment * (0.927 / 1.38) * b_ext
!
	start_time = dtime(runtime)		! record the starttime
!
!	Calculate the 'rounded' number of spins for a lattice n*m*m for
!	the given concentration
!
	n     = floor( d / ah )
	nat	  = floor( nsp / c )
	m     = floor( sqrt( 2.0 * float(nat) / float(n) ) )
	mh    = m / 2
	nat   = m * m * n / 2
	nspin = floor( nat * c )
!
	write(6,*)  ' n  = ', n,'   m = ', m
	write(6,*)  ' nat   = ', nat
	write(6,*)  ' nspin = ', nspin
!
	write(2,*)  ' n  = ', n,'   m = ', m
	write(2,*)  ' nat   = ', nat
	write(2,*)  ' nspin = ', nspin
!
!	Place the spins on the lattice
!
	nspin = 0
	DO j = 0, n-1
	DO k = 0, m-1
	DO l = 0, m-1
	  IF ( mod(j+k+l,2) .EQ. 0 ) THEN
	    IF ( ran(iseed) .LT. c ) THEN
		  nspin = nspin + 1
		  s(nspin).x = j
		  s(nspin).y = k
		  s(nspin).z = l
		END IF 
	  END IF 
	END DO
	END DO
	END DO
!
!	Calculate the 100 shortest distances
!
	write(6,*) ' '
	write(6,*) ' '
!
	DO j = 1, nspin
	  write(6,9) j, char(13)
9	  format('  Considering spin ', i5, a1,\)
	  DO k = 1, nspin
	    IF ( k .EQ. j ) THEN
		  dd(k) = 1e10
		  ELSE
	      ix    = s(j).x - s(k).x
		  iy    = s(j).y - s(k).y
		  IF ( iy .LT. -mh ) iy = iy + m		! periodic boundary along y-axis
		  IF ( iy .GT.  mh ) iy = iy - m 
		  iz    = s(j).z - s(k).z
		  IF ( iz .LT. -mh ) iz = iz + m		! periodic boundary along z-axis
		  IF ( iz .GT.  mh ) iz = iz - m 
	      dd(k) = (ix*ix + iy*iy + iz*iz)
		  END IF
	    END DO
!	
	  DO ii = 1, 100
	    dd_min = 1E+10
	    DO k = 1, nspin
	      IF ( dd(k) .LT. dd_min ) THEN
	        l      = k
		    dd_min = dd(k)
	      END IF 
	    END DO
	    i(ii,j).si  = l
	    i(ii,j).val = ah * sqrt(dd(l))
	    dd(l)       = 1e10
	    END DO
!
!		translate distance into interaction strength
!
	  DO ii = 1, 100
	    i(ii,j).val = rkky( i(ii,j).val, k_F, T_glass )
	    END DO
!
	  END DO
!
	end_time = dtime(runtime)
	write(6,*) ' Finally, nspin = ', nspin,' in ', end_time - start_time,' seconds ' 
	write(2,*) ' Finally, nspin = ', nspin,' in ', end_time - start_time,' seconds ' 
!
!	Initialize the spins
!
	start_time =  dtime(runtime)
!
	DO j = 1, nspin
	  DO k = 1, 3
	    s(j).c(k) = 2.0*ran(iseed) - 1.0
		END DO
		norm   = sqrt( sum( s(j).c * s(j).c ) )
		s(j).c = s(j).c / norm 
	  END DO  
!
!	Calculated the energy and magnetization
!
97	e    = 0.0
	mag  = 0.0 
!
	DO j = 1, nspin	
	  p = 0.0
	  DO ii = 1, 100
	    p		= p + i(ii,j).val * s(i(ii,j).si).c
	    END DO
	  e   = e + sum( p * s(j).c ) + sum( b_ext_K * s(j).c )
	  mag = mag + s(j).c
	  END DO
!
	end_time = dtime(runtime)
!
	Write(6,971)  mag / float(nspin)
	Write(2,971)  mag / float(nspin)
971	format(' Magnetization = ',3F8.4)
	Write(6,972) e / float(nspin), end_time - start_time
	Write(2,972) e / float(nspin), end_time - start_time
972	format(	' Energy = ', E14.4, ' after ', f8.2, ' seconds ' )		  
!
!	Now start the serious running 
!
91	IF ( .NOT. open_inp ) THEN	
98	  write(6,99) 
99	  format('  Give the number of steps per spin [0: new glass] > '\)
	  read(5,*,err=98) steps_per_spin
	  IF ( steps_per_spin .LE. 0 ) GOTO 777
	  END IF
!
	start_time = dtime(runtime)		! record the starttime
	write(6,*) ' '
	Write(6,*) ' '					! to make space for the hashes
!
!	Now comes the real hard work !!!!!!!!!!!
!
	DO mon = 1, steps_per_spin
	  DO j = 1, nspin
	    e = 0.0
		p = 0.0
		DO ii = 1, 100
		  p		  = p + i(ii,j).val * s(i(ii,j).si).c
		  END DO
		p       = p + b_ext_K 					
		norm    = sqrt( sum( p * p ) )
		p       = p / norm
		s(j).c  = p
	    END DO
	  IF ( mod(mon,100) .EQ. 0 ) idummy = putc('#')
	  END DO
!
876	write(6,*) ' '
	write(6,*) ' '
	write(3,rec=1) n,m,nspin,a,moment,T_glass
	write(3,rec=2) concentration,b_ext,steps_per_spin
	DO ispin = 1, nspin
	  write(3,rec=ispin+2) s(ispin)
	  END DO
	close(3)
!		  			  
	IF ( open_inp ) THEN
	  DO j = 1, nspin	
	    p = 0.0
	    DO ii = 1, 100
	      p		= p + i(ii,j).val * s(i(ii,j).si).c
	      END DO
	    e   = e + sum( p * s(j).c ) + sum( b_ext_K * s(j).c )
	    mag = mag + s(j).c
	    END DO
!
	  end_time = dtime(runtime)
!
	  Write(6,971)  mag / float(nspin)
	  Write(2,971)  mag / float(nspin)
	  Write(6,972) e / float(nspin), end_time - start_time
	  Write(2,972) e / float(nspin), end_time - start_time		  
	  GOTO 888
	  ELSE
	  GOTO 97
	  END IF
!
998	STOP 'ERROR IN INPUT FILE '
!
999 STOP 'stopped at end of input'
!
	END
!
!
	REAL*8 function RKKY(x, k_F, T_glass)
	real*8 x, k_F, T_glass, xx
!
!	  calculates the RKKY interaction, 
!	  Te factor of one thousand makes takes care that the RKKY still only 
!	  has to be multiplied by the glass-temperature.
!
	  xx     = 2.0 * k_F * x	 	
	  rkky   = 1.0D+03 * T_glass * ( xx * cos(xx) - sin(xx) ) /(xx*xx*xx*xx)
!
	  RETURN
	END