Constructing SALCs

The acronym SALC stands for Symmetry Adapted Linear Combinations of atomic orbitals. In small molecules, SALCs can be identical to the one-electron orbitals found by Huckel calculations, but in larger molecules they must be linearly combined to make Huckel orbitals. But their utility goes far beyond the Huckel approximation. By using SALCs as basis functions, the most sophisticated of quantum calculations can greatly reduce the size of the matrices that need to be diagonalized. The reduction of matrix size goes hand in hand with a higher level of understanding of the problem. If you are interested in understanding molecules (as opposed to just calculating their properties) you can hardly understand them at all without SALCs.