program fcc
      implicit none
c     place NP particles of diameter=1
c     in a FCC lattice of size Lbox

      real Lbox(3)
      integer NP,m
      real posx,posy,posz,fracvol
      real sx(4),sy(4),sz(4)
      real sh,pi
      data sx /0.0,0.5,0.5,0.0/
      data sy /0.0,0.5,0.0,0.5/
      data sz /0.0,0.0,0.5,0.5/
      data sh /0.01/

      real a
      integer n
      integer i,j,k,l
      real box,dens

      write(*,*) "INTRODUCE: fracvol,lado de la caja"
      read(*,*) fracvol,box

      pi=4.0*atan(1.0)
      dens=(6.0/pi)*fracvol

      np=dens*box**3

      a=(4.0/dens)**(1.0/3.0)
      n=(np/4)**(1.0/3.0)+0.001
      np=4.0*n**3

      do k=1,3
         lbox(k)=n*a
      end do
      open(21,file='fcc.rst')
      write(21,*) np,lbox,   " !np box_sizes"
      write(21,*) 0.2,0.2,   " !rPOL/rCOL mupoly"
      write(21,*) 0.1,      " !amplitud salto MC"

      m=1
      do i=1,n
         do j=1,n
            do k=1,n
               do l=1,4
              posx=(i-1+sx(l)+sh)*a -lbox(1)/2.0
              posy=(j-1+sy(l)+sh)*a -lbox(1)/2.0
              posz=(k-1+sz(l)+sh)*a -lbox(1)/2.0
              m=m+1
              write(21,*) posx,posy,posz
               end do
            end do
         end do
      end do
      write(*,*) 'number of particles',m
      pi=4.0*atan(1.0)
      write(*,*) 'frac-volume',(pi/6)*real(m)/lbox(1)**3
      end