SEMIEMPIRICAL HAMILTONIANS FOR SPATIALLY CONFINED PI-ELECTRON SYSTEMS

A study of pi-electron systems confined by impenetrable surfaces is pr esented. The study results in a nonempirical-based approach to obtain confinement-adapted semiempirical pi-Hamiltonians including repulsive terms (PPP or Hubbard). The impenetrable surface confinement of a phys ical system involves changes in the boundary conditions that the eigen vectors of its differential Hamiltonian operator have to fulfill, whil e the Hamiltonian itself remains unchanged. However, if this Hamiltoni an is written in second quantization language, then confinement only i nvolves changes of the Hamiltonian scalar factors (integrals). Semiemp irical Hamiltonian integrals are replaced by parameters; therefore, co nfinement involves only changes of these parameters. It is shown that confinement changes Coulomb (alpha(i)) and exchange (beta(ij)), while repulsion (gamma(ij)) parameters remain unaffected. Next, the influenc e of confinement upon the electron correlation of (i) pi-electron mole cular systems, (ii) atoms, and (iii) an electron gas is discussed. The behaviour of the correlation energy vs. the confinement size is found to be different for each type of system. A neat explanation of this v ariety is given in terms of the Coulomb attractive fields of the syste ms. Some chemical confinement effects such as an increase in the react ivity of pi-electron systems is also outlined. (C) 1996 John Wiley & S ons, Inc.