The UBI–QEP treatment of polyatomic molecules without bond-energy partitioning

Bond-energy partitioning in polyatomic molecules is a mathematical construct realizing intuitive chemical reasoning but having no physical justification. The bond order conservation–Morse potential method [1,2] and its generalization as the unity bond index–quadratic exponential potential (UBI–QEP) method [3,4] treat polyatomic molecules as quasi-diatomic and often use some bond-energy partitioning, which creates conceptual and numerical uncertainty. We present a new UBI–QEP formalism to calculate binding energies of polyatomic molecules without bond-energy partitioning. We discuss the types of molecules best suited for the new treatment and the nature and values of the relevant parameters. Examples include both symmetric molecules such as ethylene, acetylene and hydrazine and non-symmetric molecules such as nitrous oxide on various metal fcc(1 1 1) surfaces. The calculated binding energies were compared with those obtained by the previous UBI–QEP formulas (using some bond-energy partitioning). The older and new UBI–QEP values typically were found to be close. The scarcity of appropriate experimental data prevents a systematic comparison in order to decide where each approximation can be used most efficiently. One thing is certain, however, that the new UBI–QEP formalism, conceptually superior and quantitatively more reasonable than the older ones, expands opportunities of the practitioners.