Quantum chemical calculations based on density functional theory (DFT) methods were performed on indigo blue (IB), methylene blue (MB), and crystal violet (CV) molecules as inhibitors for iron corrosion in acid media. DFT calculations were performed on the molecular structures to describe electronic parameters which are associated with inhibition efficiency such as the values ？4.981？eV, ？4.518？eV, and ？3.872？eV which increased in the order IB > MB > CV while values were ？3.73？eV, ？3.63？eV, and ？2.87？eV for IB, MB, and CV, respectively. Quench molecular dynamics simulations performed at metal/vacuum interface were applied to find the equilibrium adsorption configurations and calculate the minima interaction energy between inhibitor molecules and iron surface Fe(110). The theoretical order of inhibition efficiency of these dye molecules had a linear relationship with experimentally observed inhibition efficiency on iron corrosion in acid media. The electronic structures as well as reactivity elucidate parameters which could be practical in designing novel high-efficiency, cheap, and eco-friendly inhibitors by quantitative structure-activity relationship (QSAR) method. 1. Introduction A dye is a coloured substance that has an affinity to the substrate to which it is being applied. Dyes are obtained from animal, vegetable, or mineral origin and appear to be coloured because they absorb some wavelengths of light. Dyes are in our ancient science and effects of dyes are known in the medical industry, textile industry, and cellulose industries and in recent times as corrosion inhibitors of metals both in acidic and alkaline aggressive environments, as redox indicator in analytical chemistry, photosensitizer used to create singlet oxygen when exposed to both oxygen and light, to examine RNA or DNA gel electrophoresis, and also in a number of different staining procedures such as Wright's stain and Jenner's stain [1–5]. The corrosion inhibition characteristics of dye are attributed to adsorption of the dye molecule on the metal surface, hence reducing the surface area susceptible to attacks by the corrosive media. This might be by algebraic blocking of active sites on the metal surface or by polarizing the individual metal atoms to which they are adsorbed thereby influencing the intrinsic reactivity of the metal [6]. Many researchers report that the inhibition effect mainly depends on some physicochemical and electronic properties of the organic inhibitor which relate to its functional groups, steric effects, electronic density of donor atoms, orbital character
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