program LennardJonesLiquid
       implicit none
       integer L,N,MCS,i,j,k,d, sim_time, check_time, m
       parameter (L=35, MCS=230000, sim_time = 30000, check_time = 100)
       real r_c, x_step, y_step,r_step, ran1, P, dr, ro, PI, T
       parameter (r_c = 2.5, x_step = 0.01,y_step = 0.01,r_step = 0.01,
     &                  dr = 0.1 ,PI = 3.141592653, ro = 0.82, T = 0.7)
       ! 0.8 - liquid & 0.93 - solid
       real xnew, ynew, dx, dy, r_ij2, Rnew2, dU, r, Probability, Rmax
       real dx_new, dy_new
       integer counter, r_size, mcs_size
       real, allocatable:: x(:), y(:), Prob(:,:)
       d =-1
       Rmax = L/2.0 - 1.0
       !--------
       r_size = int( Rmax/r_step + 1.0 )
       mcs_size = int( (mcs-sim_time)/check_time + 1.0)
       !---------- matrix places
       N = int( ro*L**2 + 0.0)
       write(*,*) 'N = ', N
       allocate (x(N), y(N), Prob(r_size,mcs_size) )
       call Liquid(L,N,x,y)
       do i=1, r_size
         do j=1, mcs_size
            Prob(i,j) = 0.0
         enddo
       enddo
       write(*,*) '----------------------------------'
       write(*,*) 'Probability', '     ', 'r'
        counter = 1
        P = 0
        do k=1, MCS
          do i=1, N
             dU = 0
             xnew = x(i) + (ran1(d)-0.5)*x_step
             ynew = y(i) + (ran1(d)-0.5)*y_step
             if(xnew.gt.L-0.5) xnew = xnew - L + 0.5
             if(ynew.gt.L-0.5) ynew = ynew - L + 0.5
             if(xnew.lt.0.5) xnew = xnew + L-1.0
             if(ynew.lt.0.5) ynew = ynew + L-1.0
              do j=1, N
               if(j.ne.i) then
                  dx = abs( x(i) - x(j) )
                  dy = abs( y(i) - y(j) )
                  if(dx.gt.L/2.) dx = L - dx
                  if(dy.gt.L/2.) dy = L - dy
                  r_ij2 = dx**2 + dy**2

                  dx_new = abs( xnew-x(j) )
                  dy_new = abs( ynew-y(j) )
                  if(dx_new.gt.L/2.) dx_new = L - dx_new
                  if(dy_new.gt.L/2.) dy_new = L - dy_new
                  Rnew2 = dx_new**2 + dy_new**2
                  if(Rnew2.le.1.0) goto 7
                  !r^2<=1 then r<=1, sqrt() not needed

                  if(r_ij2.le.r_c.and.Rnew2.le.r_c) then
                     dU = dU + 4.0*(1/Rnew2**6 - 1/r_ij2**6)
                  endif
               endif
              enddo
              if(ran1(d).le.min(1.0,exp(-dU/T))) then
                  x(i) = xnew
                  y(i) = ynew
              endif
7           continue
          enddo
          if(k.ge.sim_time.and.mod(k,check_time).eq.0) then
             r = 0.0
             m = 1
33           continue
                Prob(m,counter) = Prob(m,counter) +
     &                   Probability(r,dr,N,ro,x,y) !probabilistic matrix
                r = r + r_step
                m = m + 1
             if(r.le.Rmax) goto 33
             counter = counter + 1
          endif
          write(*,*) k
        enddo
        call save_prob(Prob,r_size,mcs_size,r_step)
        pause
        !call showmatrix(N,x,y)

       end program

       function Probability(r,dr,N,ro,x,y)
        integer i, N, j, M
        real dr, deltaR2, r, x(N), y(N)
        real P, Probability, PI, ro
        PI = 3.141592653
        P = 0
        Probability = P
        if(r.le.dr) return
        do j=1, N
          M = 0
          do i=j+1, N
           deltaR2 = sqrt( (x(i)-x(j))**2 + (y(i)-y(j))**2 )
           if(deltaR2.ge.r-dr.and.deltaR2.le.r) then
                M = M + 1
           endif
          enddo
          P = P + M/(2.0*PI*r*dr*ro)
          !ring width: r-dr to r
        enddo
        Probability = P/(2*N+0.0)
        return
        end

       subroutine Liquid(L,N,x,y)
        integer L, N, M, k
        real x(N), y(N), d
        x(1) = 0.5
        y(1) = 0.5
        M = (L-1)*(L-1)
        k = int(N/2.)
        if(N.ge.M) then
           d = 1.0
        else
           d = 1.0 + 1./L
        endif
        write (*,*) 'd = ',d
        do i=2, k
          y(i) = y(i-1)
          x(i) = x(i-1) + d
          if(x(i).gt.L-0.5) then
             y(i) = y(i) + d
             x(i) = 0.5
          endif
        enddo
        y(k) = L - 0.5
        x(k) = L - 0.5
        do i=k+1, N
          y(i) = y(i-1)
          x(i) = x(i-1) - d
          if(x(i).lt.0.5) then
             y(i) = y(i) - d
             x(i) = L - 0.5
          endif
        enddo
       end

       subroutine save_prob(Prob, A, B, dr)
        integer A,B, i, j
        real Prob(A,B), P
        open(13,file='prob-gas.txt')
        do i=1, A
           P = 0.0
           do j=1, B
              P = P + Prob(i,j)
           enddo
           P = P/B
           write(13,*) i*dr, P
           write(*,*) i*dr, P
        enddo
        close(13)
       end

       FUNCTION ran1(idum)
      INTEGER idum,IA,IM,IQ,IR,NTAB,NDIV
      REAL ran1,AM,EPS,RNMX
      PARAMETER (IA=16807,IM=2147483647,AM=1./IM,IQ=127773,IR=2836,
     *NTAB=32,NDIV=1+(IM-1)/NTAB,EPS=1.2e-7,RNMX=1.-EPS)
      INTEGER j,k,iv(NTAB),iy
      SAVE iv,iy
      DATA iv /NTAB*0/, iy /0/
      if (idum.le.0.or.iy.eq.0) then
        idum=max(-idum,1)
        do 11 j=NTAB+8,1,-1
          k=idum/IQ
          idum=IA*(idum-k*IQ)-IR*k
          if (idum.lt.0) idum=idum+IM
          if (j.le.NTAB) iv(j)=idum
11      continue
        iy=iv(1)
      endif
      k=idum/IQ
      idum=IA*(idum-k*IQ)-IR*k
      if (idum.lt.0) idum=idum+IM

      j=1+iy/NDIV
      iy=iv(j)
      iv(j)=idum
      ran1=min(AM*iy,RNMX)
      return
      END