Where q i is the donor orbital occupancy, ε i and ε j are the diagonal elements (orbital energies) and F(i,j) is the interaction element between donor and acceptor orbitals and is known as diagonal NBO Fock matrix element. In this case, the electronic wave function is interpreted in terms of a set of occupied Lewis and a set of unoccupied non-Lewis localized orbital and the delocalization effects can be identified by means of off-diagonal elements of the Fock matrix. The forces of these delocalization interaction, E(2) (kcal/mol), are estimated by second order perturbation theory . When electrons are shared via the Y lp →σ* C-H (Y = N, O, or F) bonding and antibonding interaction, this is equivalent to transferring electron density from the lone pair of N and/or O atoms into the σ* C-H orbital. Typically Y n(lp) →σ* X-H is strongest for a linear or near linear H-bond. In particular, the H-bond strength has been related to the charge transfer component E (2) n→σ* of the NBO analysis. E (2) n→σ* is proportional to the amount of electron density (qi) donated from the filled donor lone-pair orbital into the empty σ* orbital, moderated by the energy difference between these two fragment orbitals (Δε), and thus the NBO analysis on the DFT optimized structure allows the analysis of intermolecular donor-acceptor orbitals interactions .