PNNL

Abstract

We present a new, novel implementation of the Many-Body Expansion (MBE) to account for the breaking of covalent bonds, thus extending the range of applications from its previous popular usage in the breaking of hydrogen bonds to molecules. A central concept of the new implementation is the in situ atomic electronic state of an atom in a molecule that casts the 1-body term as the energy required to promote it to that state from its ground state. The rest of the terms correspond to the individual diatomic, triatomic, etc. fragments. Its application to the atomization energies of the XHn series, X = C, Si, Ge, Sn and n = 1 – 4, suggests that the (negative, stabilizing) 2-B is by far the largest term in the MBE with the higher order terms oscillating between positive and negative values and decreasing dramatically in size with increasing rank of the expansion. The analysis offers an alternative explanation into the purported “first row anomaly” in the incremental Hn-1X-H bond energies seen when these energies are evaluated with respect to the lowest in energy among the states of the XHn molecules. Due to the “flipping” of the ground / first excited state between CH2 (3B1 ground state, 1A1 first excited state) and XH2, X = Si, Ge, Sn (1A1 ground state, 3B1 first excited state), the overall picture does not exhibit a “first row anomaly” when the incremental bond energies are evaluated with respect to the molecular states having the same in situ atomic states.