anchor.diff:    colloids% pos(:,2) = solvent_cells% edges/2.0 
     colloids% pos(1,2) = colloids% pos(1,2) + d
     
  -  write(*, *) colloids% pos  
  -
     call solvent% random_placement(solvent_cells% edges, colloids, solvent_colloid_lj)
   
     call solvent% sort(solvent_cells)
  
positive.code: 
    end do
    call main%tac()
  
    write(*,*) 'n extra sorting', n_extra_sorting
    
  
negative.code:   solvent% force_old = solvent% force
    colloids% force_old = colloids% force
  
    kin_co = (mass(1)*sum(colloids% vel(:,1)**2)+mass(2)*sum(colloids% vel(:,2)**2))/2
  
    write(*,*) 'Running for', N_loop, 'loops'
    call main%tic()
    do i = 1, N_loop
       if (modulo(i,5) == 0) write(*,'(i05)',advance='no') i
       md_loop: do j = 1, N_MD_steps
          call md_pos(solvent, dt)
  
  