Symbol g. Describing a molecular orbital of a homonuclear diatomic molecule with even parity (gerade is the German word for even). This means that during the process of inversion, the sign of the orbital is unchanged upon going from any point in the molecule through the centre of inversion to the corresponding point on the other side. The symbol g is written as a subscript. The opposite of gerade is ungerade, symbol u. The symbols g and u are used to determine selection rules for diatomic molecules. These symbols are only applicable to homonuclear diatomic molecules, as heteronuclear diatomic molecules, such as CO, do not have a centre of inversion.