def from_string(cls, contents):
        """
        Creates QcInput from a string.

        Args:
            contents: String representing a QChem input file.

        Returns:
            QcInput object
        """
        mol = None
        charge = None
        spin_multiplicity = None
        params = dict()
        lines = contents.split('\n')
        parse_section = False
        section_name = None
        section_text = []
        ghost_atoms = None
        for line_num, line in enumerate(lines):
            l = line.strip().lower()

            if len(l) == 0:
                continue
            if (not parse_section) and (l == "$end" or not l.startswith("$")):
                raise ValueError("Format error, parsing failed")
            if parse_section and l != "$end":
                section_text.append(line)
            if l.startswith("$") and not parse_section:
                parse_section = True
                section_name = l[1:]
                available_sections = ["comment", "molecule", "rem"] + \
                    sorted(list(cls.optional_keywords_list))
                if section_name not in available_sections:
                    raise ValueError("Unrecognized keyword " + line.strip() +
                                     " at line " + str(line_num))
                if section_name in params:
                    raise ValueError("duplicated keyword " + line.strip() +
                                     "at line " + str(line_num))
            if parse_section and l == "$end":
                func_name = "_parse_" + section_name
                if func_name not in QcTask.__dict__:
                    raise Exception(func_name + " is not implemented yet, "
                                    "please implement it")
                parse_func = QcTask.__dict__[func_name].__get__(None, QcTask)
                if section_name == "molecule":
                    mol, charge, spin_multiplicity, ghost_atoms = parse_func(section_text)
                else:
                    d = parse_func(section_text)
                    params[section_name] = d
                parse_section = False
                section_name = None
                section_text = []
        if parse_section:
            raise ValueError("Format error. " + section_name + " is not "
                             "terminated")
        jobtype = params["rem"]["jobtype"]
        title = params.get("comment", None)
        exchange = params["rem"].get("exchange", "hf")
        method = params["rem"].get("method", None)
        correlation = params["rem"].get("correlation", None)
        basis_set = params["rem"]["basis"]
        aux_basis_set = params["rem"].get("aux_basis", None)
        ecp = params["rem"].get("ecp", None)
        optional_params = None
        op_keys = set(params.keys()) - {"comment", "rem"}
        if len(op_keys) > 0:
            optional_params = dict()
            for k in op_keys:
                optional_params[k] = params[k]
        return QcTask(molecule=mol, charge=charge,
                      spin_multiplicity=spin_multiplicity,
                      jobtype=jobtype, title=title,
                      exchange=exchange, correlation=correlation,
                      basis_set=basis_set, aux_basis_set=aux_basis_set,
                      ecp=ecp, rem_params=params["rem"],
                      optional_params=optional_params,
                      ghost_atoms=ghost_atoms,
                      method=method)