anchor.diff:    end if
   
     call enforce_BCs
  +
  +  fx = 0.0_WP
  +  fy = 0.0_WP
  +  fz = 0.0_WP
   
     return
   end subroutine posvel_update
  
positive.code:       dz = pz(i) - pz(j)
        r2 = dx*dx + dy*dy + dz*dz
        h2 = h*h
        c = 45.0_WP*BULK_MODULUS*0.5/PI*mass
  
        if(r2 .ge. h2) then
          pressurekernel = 0.0_WP
        else
          pressurekernel = c / h**5 &
                          *(1.0_WP - sqrt(r2/h2))**2 &
                          /sqrt(r2/h2) &
                          *(rho(i)+rho(j)-2.0_WP*PARTICLE_DENSITY) &
                          /(rho(i)*rho(j))
        end if
  
        return
  
negative.code:   start = "particles_"
    write(buffer,"(a, i5.5)") trim(start), frame
    filename = trim(adjustl(buffer))
    fid = 0
    open(fid,file=filename,status="REPLACE",action="WRITE")
    write(fid,*) n
    do i = 1, n
  