MO H2O as an example

The set of orbitals that we have found for the E representations, e(1) and e(2), is not unique. Any pair of independent linear combinations of these two orbitals also constitutes a basis for this representation, and the same is true for the 2px and 2py orbitals on the central atom. The e(1) and e(2) orbitals shown here are, however, the set that is used most frequently. The first is transformed in the same way as 2px by all the symmetry operations of the Csv point group and the second like 2py. They are therefore often written ex and ey, respectively. 

The problem of allowing AO, or SALCO of these orbitals, to interact to form molecular orbitals (MO), is simplified considerably if the symmetry properties of the system are taken into account. The use of symmetry allows us to identify rapidly those interactions which are exactly zero, and therefore to consider only those which really do contribute to the formation of the MO. 

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