8: Electronic Configurations

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One of the goals of quantum chemistry is to allow practicing chemists to use knowledge of the electronic states of fragments (atoms, radicals, ions, or molecules) to predict and understand the behavior (i.e., electronic energy levels, geometries, and reactivities) of larger molecules. In the preceding Section, orbital correlation diagrams were introduced to connect the orbitals of the fragments along a 'reaction path' leading to the orbitals of the products. In this Section, analogous connections are made among the fragment and product electronic states, again labeled by appropriate symmetries. To realize such connections, one must first write down N-electron wavefunctions that possess the appropriate symmetry; this task requires combining symmetries of the occupied orbitals to obtain the symmetries of the resulting states.

8.1: Orbitals Do Not Provide the Complete Picture; Their Occupancy By the N Electrons Must Be Specified
Electrons are Placed into Orbitals to Form Configurations, Each of Which Can be Labeled by its Symmetry. The Configurations May "Interact" Strongly if They Have Similar Energies
8.2: Even N-Electron Configurations are Not Mother Nature's True Energy States
Single-configuration descriptions of atomic and molecular structure do not provide fully correct or highly accurate representations of the respective electronic wavefunctions.
8.3: Mean-Field Models
The Mean-Field Model that forms the basis of Chemists' pictures of electronic structure of molecules is not very accurate
8.4: Configuration Interaction (CI) Describes the Correct Electronic States
The most commonly employed tool for introducing such spatial correlations into electronic wavefunctions is called configuration interaction (CI).
8.5: Summary
The dynamical interactions among electrons give rise to instantaneous spatial correlations that must be handled to arrive at an accurate picture of atomic and molecular structure. The simple, single-configuration picture provided by the mean-field model is a useful starting point, but improvements are often needed. Methods for treating electron correlation will be discussed in greater detail.