anchor.diff:       call solvent% sort(solvent_cells)
        call solvent_cells%count_particles(solvent% pos)
        call e_solvent% append(solvent% pos, i, i*tau)
  +     call e_solvent_image% append(solvent% image, i, i*tau)
        call e_solvent_v% append(solvent% vel, i, i*tau)
        call e_solvent_spec% append(solvent% species, i, i*tau)
        call e_solvent_id% append(solvent% id, i, i*tau)
  
positive.code:   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])
       solvent_cells% origin(1) = genrand_real1(mt) - 1
       solvent_cells% origin(2) = genrand_real1(mt) - 1
       solvent_cells% origin(3) = genrand_real1(mt) - 1
  
negative.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 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)
  