= Solution
\b[A <symmetry orbital> is a <molecular orbital> when it is also an <eigenfunction> of the effective one-electron <Hamiltonian operator>.] If an <irreducible representation> occurs once in a complete invariant model space, a symmetry-invariant <Hamiltonian> acts as a scalar on that copy by <Schur's lemma>. Its basis functions are then degenerate <eigenfunctions>. When the same symmetry occurs in several copies, those copies can mix; one must diagonalize the corresponding block. Merely having a one-dimensional symmetry label is insufficient if other orbitals have that same label.
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