| Abstract: | The CI space Xn generated by n electrons moving over 2n spin orbitals is considered. It is shown that the transitions between different eigenstates ψi ? Xn of alternant and weakly alternant Hamiltonians are governed by some special selection rules. These selection rules are characteristic to alternant systems, and they do not apply to nonalternant systems. The set of all such selection rules can be easily derived from the splitting theorem. In particular, the selection rules associated with spin independent alternant systems are considered. As an example, the PPP Hamiltonian ?p describing netural alternant hydrocarbons is treated. In the case of electron dipole transitions between eigenstates ψi ? Xn of the Hamiltonian ?p, the selection rules obtained are in agreement with the selection rules derived previously by Pariser and McLachlan. |
