Set the number of atoms for the simulation, and populate the simulation lattice.

        Args:
            n (Int): Number of atoms for this simulation.
            selected_sites (:obj:(List|Set|String), optional): Selects a subset of site types to be populated with atoms. Defaults to None.

        Returns:
            None

def set_number_of_atoms( self, n, selected_sites=None ):
        """
        Set the number of atoms for the simulation, and populate the simulation lattice.

        Args:
            n (Int): Number of atoms for this simulation.
            selected_sites (:obj:(List|Set|String), optional): Selects a subset of site types to be populated with atoms. Defaults to None.

        Returns:
            None
        """
        self.number_of_atoms = n
        self.atoms = species.Species( self.lattice.populate_sites( self.number_of_atoms, selected_sites=selected_sites ) )