Ungerade parity

In point groups with a center of symmetry, x, y, and z are of ungerade parity (u) that is, they change sign upon inversion. Therefore, one and only one of the two states and has to be of ungerade parity if the transition moment is to be nonzero. Thus, within the electric dipole approximation... 

Figure B A qualitative molecular orbital diagram for ferrocene. The subscripts g and u refer to the parity of the orbitals g (German gerade, even) indicates that the orbital (or orbital combination) is symmetric with respect to inversion, whereas the subscript u (ungerade, odd) indicates that it is antisymmetric with respect to inversion. Only orbitals with the same parity can combine.

The first selection rule, known as the Laporte rule, states that the only allowed transitions are those with a change of parity gerade to ungerade and ungerade... 

The various combinations are named according to the degree of degeneracy and to the g ( gerade , i.e. even) or u ( ungerade , i.e. odd) parity (see page 75) the aig and the two Cg combinations have an even number of nodes, whereas each of the three tiu combinations have just one node. 

---