anchor.diff:    wall_v = 0
     do i = 1, 200
        call simple_mpcd_step(solvent, solvent_cells, mt)
  -     call mpcd_stream(solvent, solvent_cells, 0.1d0,[0d0, 0d0, 0d0])
  +     call mpcd_stream_periodic(solvent, solvent_cells, tau)
        solvent_cells% origin(1) = genrand_real1(mt) - 1
        solvent_cells% origin(2) = genrand_real1(mt) - 1
        solvent_cells% origin(3) = genrand_real1(mt) - 1
  
positive.code:   call e_solvent_v% create_time(solvent_group, 'velocity', solvent% vel, ior(H5MD_TIME, H5MD_STORE_TIME))
    call e_solvent_spec% create_time(solvent_group, 'species', solvent% species, ior(H5MD_TIME, H5MD_STORE_TIME))
    call e_solvent_id% create_time(solvent_group, 'id', solvent% id, ior(H5MD_TIME, H5MD_STORE_TIME))
  
    call solvent% sort(solvent_cells)
    call solvent_cells%count_particles(solvent% pos)
  
negative.code:   type(h5md_file_t) :: datafile
    type(h5md_element_t) :: elem
    type(h5md_element_t) :: e_solvent, e_solvent_v, e_solvent_spec, e_solvent_id
    type(h5md_element_t) :: e_solvent_image
    integer(HID_T) :: box_group, solvent_group
  
    integer, parameter :: N = 2000
  