“…Accordingly, DFT-based and quantum computations were performed to validate the proposed mechanism of adsorption of BPUA on the surface of the C-steel that was presented. In principle, the frontier molecular orbital (FMO) theory of chemical reactivity proposes that the orbital energy gap (ΔE) is a crucial chemical parameter that may be used to characterize various physicochemical properties of materials [ [55] , [56] , [57] , [58] ]. Accordingly, the adsorption of inhibitor molecules on metallic surfaces can be assessed based on the FMO, where this phenomenon is attributed to the possibility of interaction between the HOMO and LUMO states of reacting species.…”