Boiling down the Schrodinger equation

Trial wavefunctions are usually constructed by linear combination of Gaussian error functions that are convenient to integrate. The results can be of predictive value and such calculations have become everyday tools for chemists in all branches of chemistry, to guide experiments and not least to rule out untenable hypotheses. This is a remarkable achievement that seemed to be out of reach a few decades ago. Still, simple qualitative models that are amenable to perturbation theory are required to understand and predict trends in a series of related compounds. Our goal here is to describe the minimal quantum mechanical models that can still provide a useful qualitative description of electronically excited states, their electronic stmcture and their reactivity. Such models also provide a language to convey the results of state-of-the-art, but essentially black-box ab initio calculations. 

Photochemistry of Organic Compounds From Concepts to Practice Petr Klan and Jakob Wirz 2009 P. Klan and J. Wirz. ISBN 978-1-405-19088-6 

Before we proceed, recall the Aufbau principle Having defined a set of orbitals for a given molecule, we fill the electrons successively into the orbitals of lowest energy, 

The expression configuration used in stereochemistry has a different meaning. 

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