First principles approach to ionicity of fragments

Abstract

a b s t r a c tWe develop a first principles approach towards the ionicity of fragments. In contrast to the bond ionicity, the fragment ionicity refers to an electronic property of the constituents of a larger system, which may vary from a single atom to a functional group or a unit cell to a crystal. The fragment ionicity is quantitatively defined in terms of the coefficients of contributing charge states in a superposition of valence configurations of the system. Utilizing the constrained density functional theory-based computations, a practical method to compute the fragment ionicity from valence electron charge densities, suitably decomposed according to the Fragment Hamiltonian (FH) model prescription for those electron densities, is presented for the first time. The adopted approach is illustrated using BeO, MgO and CaO diatomic molecules as simple examples. The results are compared and discussed with respect to the bond ionicity scales of Phillips and Pauling.

Negative piezoelectricity in low--dimensional materials

“…This "lag of Wannier center" effect provides two deductions. First, the ionicity tends to decrease with increasing bond length, which is consistent with previous study [19]. Besides, the absolute values of the Born effective charges should also decrease with increasing bond lengths (or upon a tensile strain), which conforms to this CIPS case, as shown in Table I.…”

“…Therefore, β − 0.5 describes the deviation of the Wannier center from the bond center. Here, we should emphasize that even though the expressions are different in the two models, they gauge the same physical quantity and give quite similar values [16,19]. The Hamiltonian of a diatomic molecule is expressed as [20]…”