symmetry of an MO is to rotate the orbital about the axis joining the two nuclei and then rotate the orbital about a line perpendicular to the axis. If the sign of the lobes remains the same, the orbital is gerade, and if the sign changes, the orbital is ungerade. Wigner-Witmer correlation rules In 1928 Eugene Wigner and E.E. Witmer proposed rules to determine the possible term symbols for diatomic molecular states formed by the combination of a pair of atomic states with given atomic term symbols. For example, two like atoms in identical 3S states can form a diatomic molecule in 1Σg+, 3Σu+, or 5Σg+ states. For one like atom in a 1Sg state and one in a 1Pu state, the possible diatomic states are 1Σg+, 1Σu+, 1Πg and 1Πu. The parity of an atomic term is g if the sum of the individual angular momentum is even, and u if the sum is odd. Alternative empirical notation Electronic states are also often identified by an empirical single-letter label. The ground state is labelled X, excited states of the same multiplicity (i.e., having the same spin quantum number) are labelled in ascending order of energy with capital letters A, B, C...; excited states having different multiplicity than the ground state are labelled with lower-case letters a, b, c... In polyatomic molecules (but not in diatomic) it is customary to add a tilde (e.g. , ) to these empirical labels to prevent possible confusion with symmetry labels based on group representations. See also Molecular orbital theory References Molecular physics Quantum chemistry Atomic physics Spectroscopy