Returns a z-matrix representation of the molecule.

def get_zmatrix(self):
        """
        Returns a z-matrix representation of the molecule.
        """
        output = []
        outputvar = []
        for i, site in enumerate(self._mol):
            if i == 0:
                output.append("{}".format(site.specie))
            elif i == 1:
                nn = self._find_nn_pos_before_site(i)
                bondlength = self._mol.get_distance(i, nn[0])
                output.append("{} {} B{}".format(self._mol[i].specie,
                                                 nn[0] + 1, i))
                outputvar.append("B{}={:.6f}".format(i, bondlength))
            elif i == 2:
                nn = self._find_nn_pos_before_site(i)
                bondlength = self._mol.get_distance(i, nn[0])
                angle = self._mol.get_angle(i, nn[0], nn[1])
                output.append("{} {} B{} {} A{}".format(self._mol[i].specie,
                                                        nn[0] + 1, i,
                                                        nn[1] + 1, i))
                outputvar.append("B{}={:.6f}".format(i, bondlength))
                outputvar.append("A{}={:.6f}".format(i, angle))
            else:
                nn = self._find_nn_pos_before_site(i)
                bondlength = self._mol.get_distance(i, nn[0])
                angle = self._mol.get_angle(i, nn[0], nn[1])
                dih = self._mol.get_dihedral(i, nn[0], nn[1], nn[2])
                output.append("{} {} B{} {} A{} {} D{}"
                              .format(self._mol[i].specie, nn[0] + 1, i,
                                      nn[1] + 1, i, nn[2] + 1, i))
                outputvar.append("B{}={:.6f}".format(i, bondlength))
                outputvar.append("A{}={:.6f}".format(i, angle))
                outputvar.append("D{}={:.6f}".format(i, dih))
        return "\n".join(output) + "\n\n" + "\n".join(outputvar)