def as_ppmodel(cls, obj):
        """
        Constructs an instance of PPModel from obj.

        Accepts obj in the form:
            * PPmodel instance
            * string. e.g "godby:12.3 eV", "linden".
        """
        if isinstance(obj, cls):
            return obj

        # obj is a string
        if ":" not in obj:
            mode, plasmon_freq = obj, None
        else:
            # Extract mode and plasmon_freq
            mode, plasmon_freq = obj.split(":")
            try:
                plasmon_freq = float(plasmon_freq)
            except ValueError:
                plasmon_freq, unit = plasmon_freq.split()
                plasmon_freq = units.Energy(float(plasmon_freq), unit).to("Ha")

        return cls(mode=mode, plasmon_freq=plasmon_freq)
Constructs an instance of PPModel from obj.

        Accepts obj in the form:
            * PPmodel instance
            * string. e.g "godby:12.3 eV", "linden".