Charge, mass, formula
*********************

These are properties of the ChemicalSystem:

property ChemicalSystem.charge: float

   The total charge of the system in atomic units (i.e. in units of
   elementary charge).

ChemicalSystem.total_mass() -> float

   Total mass in atomic mass units (dalton).

To set or get the mass of individual atoms, see Atom Attributes.

ChemicalSystem.formula() -> str

   Chemical formula in Hill notation (e.g. H2O, CH4):

   * If Carbon is present, the order is: C, H, alphabetical.

   * If Carbon is not present all the elements (including Hydrogen)
     are listed alphabetically.

   * Single-letter elements come before 2-letters elements (e.g. 'K'
     comes before 'Kr', 'B' before 'Be')
