text: LEN_UNITS = ['Bohr', 'Angstrom', 'nm', 'm']
  
  LEN_CONV = {}
  LEN_CONV['Bohr'] = {
      'Bohr': 1.0, 'Angstrom': 0.5291772106712,
      'nm': 0.05291772106712, 'm': 5.291772106712e-11
  }
  LEN_CONV['Angstrom'] = {
      'Bohr': 1.8897261254535427,
      'Angstrom': 1.0, 'nm': 1.0e-1, 'm': 1.0e-10
  }
  LEN_CONV['nm'] = {
      'Bohr': 18.897261254535427,
      'Angstrom': 10., 'nm': 1.0, 'm': 1.0e-9
  }
  LEN_CONV['m'] = {
      'Bohr': 1.8897261254535427e10,
      'Angstrom': 1.0e10, 'nm': 1.0e9, 'm': 1.0
  }
  
  units_warning = "Please only feed pbcpy quantities in atomic units (Bohr). An automatic units system might be implemented in the future"