My source is Walter A. Harrison:"Applied Quantum Mechanics" Section 5.4 p.83. When studying diatomic molecules such as N_2 one may make use of the high symmetry of the molecule with relfection planes σx,y,z. In Harrison it is said that each molecular orbital can be chosen to be even or odd under each reflection. As a idiot I do not understand what he means with that and it seems to have relevance to the rest of the text. Thanks for any help.