anchor.diff:    double precision :: colloid_pos(3,2)
     type(h5md_file_t) :: hfile
     type(h5md_element_t) :: dummy_element
  -  integer(HID_T) :: fields_group
  +  integer(HID_T) :: fields_group, params_group
     type(h5md_element_t) :: rho_xy_el
     type(thermo_t) :: thermo_data
     type(particle_system_io_t) :: dimer_io
  
positive.code:   on_track = .true.
    stopped = .false.
  
    write(*,*) colloids% pos
    solvent_cells%bc = [PERIODIC_BC, SPECULAR_BC, BOUNCE_BACK_BC]
  
    write(*,*) 'Running for', N_loop, 'loops'
  
negative.code:      mass = rho * sigma_N**3 * 4 * 3.14159265/3
    end if
  
    call solvent% init(N,N_species, system_name='solvent')
  
    call colloids% init(N_colloids,2, mass, system_name='colloids') !there will be 2 species of colloids
  
    call thermo_data%init(hfile, n_buffer=50, step=N_MD_steps, time=N_MD_steps*dt)
  
    call PTkill(config)
  