Analysis of chemical phenomena by solving constrained hartree‐fock equation. I. Method and application to resonance energy in linear polyenes

The authors proposed a general method to analyze chemical phenomena in such static states as resonance and aromaticity, without changing the variational space of the trial wave function. Solution of the Hartree–Fock–Roothaan ( HFR ) equation for a molecular system gives the ground‐state electronic structure and its energy. One can impose constraint conditions, which have chemical meanings, on the HFR equation (the constrained HF equation). The presented method is to analyze the energy differences between usual and constrained HF solutions. This method has an advantage over conventional methods in that it neither varies the trial wave function nor the geometry. The method was applied to interpretation of resonance energy in unsaturated linear hydrocarbons to show that both vertical and adiabatic resonance energies are produced proportionally to the number of ethylene units and that they originate by the lowering of the kinetic energy of π electrons, namely, resonance releases the kinetic‐energy pressure of π electrons. This is consistent with the previous investigations © 1994 John Wiley & Sons, Inc.