anchor.diff:    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 e_solvent_image% create_time(solvent_group, 'image', solvent% image, ior(H5MD_TIME, H5MD_STORE_TIME))
   
     call solvent% sort(solvent_cells)
     call solvent_cells%count_particles(solvent% pos)
  
positive.code:      else 
          solvent% species(i) = 2
       end if 
    end do 
    do i = 1, 200
       call simple_mpcd_step(solvent, solvent_cells, mt)
       v_com = sum(solvent% vel, dim=2) / size(solvent% vel, dim=2)
  
       call mpcd_stream(solvent, solvent_cells, tau,[0d0, 0d0, 0d0])
  
       solvent_cells% origin(1) = genrand_real1(mt) - 1
       solvent_cells% origin(2) = genrand_real1(mt) - 1
  
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
  