The quantum chemical calculations, based on density functional theory, have been implemented to explore the corrosion inhibition mechanism and the corresponding inhibition effectiveness of quinazolinone and pyrimidinone compounds, viz., 6-chloroquinazolin-4(3H)-one (Q1A); 2,3-dihydro-3-phenethyl-2-thioxopyrido[2,3-d]pyrimidin-4(1H)-one (Q1B) and 6-chloro-2,3-dihydro-3-phenethyl-2-thioxoquinazolin-4(1H)-one (Q1C) for mild steel in acidic solution. Global reactivity of the molecules related to the quantum chemical parameters such as E_HOMO, E_LUMO, energy gap (ΔE), softness (S), hardness (η) and fraction of electron transferred (ΔN) between the inhibitor molecule and the metal surface atom have been calculated and explored. In order to describe the reactive sites of the inhibitor molecules Fukui indices analysis has been performed. To mimic the real environment of corrosion inhibition, molecular dynamic (MD) simulations have also been modelled consisting of all concerned species (inhibitor molecule, H₂O, H₃O⁺ ion, SO₄^2 − ion and Fe surface) and thereby simulated by the consistent-valence force field (CVFF).