where Here, an orbit implies a cluster of sites, which for example, can be a point, a pair, or a triplet, as depicted in Fig. . σ is the occupation variable of a given site within a cluster, whose value depends on the basis set used. For example, σ can take the value of -1 or +1 to indicate the presence or absence of an atom at a given site. To account for local interactions, orbits are usually truncated at finite distances from a given site. In Eq. 9, the terms V 0 and V orbit are the kinetic effective cluster interactions (KECIs). The values of the KECIs are determined by fitting Eq. 9 a set of NEB-calculated E KRA .