Aufbau principle applications, 23

Standard wavefunction methods (i.e., other than DFT), which have been extensively applied both to the computation of vertical (i.e., at ground state equilibrium geometry) excitation energies and excited state reaction paths are the current preferred method for applications in this field. Wavefunction methods that are used in studying photochemical mechanisms are limited to those that can describe excited states correctly. Unfortunately, standard methods for the evaluation of the ground state PES such as SCF and DFT cannot describe excited states because they are restricted to the aufbau principle. 

The original aufbau principle of Dias [16] (see also above) is not applicable to protrusive benzenoids. Apart from this, it has been formulated for the even-carbon atom systems, but can also be extended immediately to the odd-carbon atom systems. As such this incomplete principle works in most of the relevant cases of Figs. 6 and 7 only C38H16 and C40H16 are counterexamples therein. 

The mentioned complications may cause serious difficulties under an application of the present aufbau procedure in general. But anyhow it is a great advantage that the systems under (c ) are considerably smaller than those under (c) in the fundamental aufbau principle. 

When working with the final model potentials of equations (10) and (11) in molecular applications, we are interested in valence properties and, in fact, do not need to include core orbitals in our wavefunction any more. This is so, since core-valence interaction has been accounted for implicitly in equation (10), and the sum of energies of individual cores with fixed orbitals is a constant, anyway. However, in order to get an aufbau principle for the valence wavefunction, core orbitals > have to be shifted from energies c to energies above those of the (occupied) valence ones, and this level-shift is done by... 

Figure 2.60 Electrons in energy levels or orbitals to show the application of the Aufbau or building-up principle... 

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