Returns:
            set representing the chemical system, e.g., {"Li", "Fe", "P", "O"}

def chemsys(self) -> set:
        """
        Returns:
            set representing the chemical system, e.g., {"Li", "Fe", "P", "O"}
        """
        chemsys = set()
        for e in self.entries:
            chemsys.update([el.symbol for el in e.composition.keys()])
        return chemsys