def get_mode_from_operator_identifier(identifier):
    """Get the Fmm mode from the operator identifier."""

    descriptor = identifier.split("_")[0]

    if descriptor == "laplace":
        return "laplace"
    elif descriptor == "helmholtz":
        return "helmholtz"
    elif descriptor == "modified":
        return "modified_helmholtz"
    elif descriptor == "maxwell":
        return "helmholtz"
    else:
        raise ValueError("Unknown identifier string.")