ELECTRONIC DELOCALIZATION: A QUANTITATIVE STUDY FROM MODERN AB INITIO VALENCE BOND THEORY

An ab initio spin-free valence bond code called Xiamen-99 has been developed based on an efficient algorithm called paired-permanent-determinant approach, where Hamiltonian and overlap matrix elements are expressed in terms of paired-permanent-determinants. With this tool, we probed the electronic delocalization phenomenon in a few typical examples including benzene, formamide and ethane. Our computations revealed that ab initio valence bond methods are able to estimate the energetic contribution from the delocalization effect to the stabilization of molecules, thus pave the way to illuminate the resonance theory at the quantitative level. In particular, we analyzed the cyclic electronic delocalization in benzene and showed that different understandings on the resonance may originate from the different usage of one-electron orbitals in the valence bond theory. Our investigation into the hyperconjugative interaction in ethane demonstrated that the hyperconjugation effect is not the dominating factor in the preference of the staggered conformer of ethane.