text: #: Boltzman constant in eV/K unit
  kB = 8.6173303e-5
  #: electron volt in J unit
  eV = 1.60217733e-19
  #: angstrom in meter unit
  angstrom = 1.0e-10
  #: fento second in second unit
  fs = 1e-15
  #: kcal/mol in electron volt unit
  kcal = 0.04337209302
  #: atomic mass in eV, angstrom, fs unit
  au_mass = 1.67e-27 / (eV / angstrom * fs / angstrom * fs)
  #: scaling for the Mueller-Brown potential
  escale = 0.2
  lscale = 16.0
  