The activation strain or distortion/interaction model is a tool used to analyze activation barriers that determine reaction rates. For bimolecular reactions, the activation energy is the sum of the energy required to distort the reactants into transition state geometries plus the interaction energy between the distorted molecules. The energy required to distort the molecules is called the activation strain or distortion energy, which is the primary contributor to the activation barrier. The transition state occurs when this activation strain is overcome by the stabilizing interaction energy. This model has been applied to various reactions in organic and inorganic chemistry, including substitutions, eliminations, cycloadditions, and organometallic reactions. The model is an extension of the original FMO (frontier molecular orbital) theory, which approximates the interaction between molecules but often neglects the distortion that occurs during reaction. The distortion/interaction model accounts for the energy penalties associated with the deformations of reactants as the reaction progresses, providing a more comprehensive understanding of reactivity. The activation energy of a reaction is decomposed into two contributions: the reaction strain (or distortion energy) and the interaction energy. The reaction strain is determined by the rigidity of the reactants and the type of reaction mechanism, while the interaction energy depends on the electronic structure and mutual orientation of the reactants. The interplay between these two energies determines the height of the activation barrier. The model has been applied to various reactions, including E2 and S_N2 elimination and substitution, nucleophilic additions to alkenes and alkynes, Diels-Alder cycloadditions, dehydro-Diels-Alder reactions, bioorthogonal cycloadditions, homogeneous catalysis, organocatalysis, and arynes additions and cycloadditions. It provides insights into reactivity trends, regioselectivity, and the role of steric and electronic factors. The model also explains the linear free energy relationships between activation energies and reaction energies, as well as the Hammond postulate. It has been compared to Marcus theory, which relates the activation energy to thermodynamic parameters, and has been shown to be complementary in understanding reactivity. The effect of solvation on reaction rates can be accounted for by computing the reaction strain and interaction energies for solvated reactants, providing a more accurate prediction of reaction profiles in solution.The activation strain or distortion/interaction model is a tool used to analyze activation barriers that determine reaction rates. For bimolecular reactions, the activation energy is the sum of the energy required to distort the reactants into transition state geometries plus the interaction energy between the distorted molecules. The energy required to distort the molecules is called the activation strain or distortion energy, which is the primary contributor to the activation barrier. The transition state occurs when this activation strain is overcome by the stabilizing interaction energy. This model has been applied to various reactions in organic and inorganic chemistry, including substitutions, eliminations, cycloadditions, and organometallic reactions. The model is an extension of the original FMO (frontier molecular orbital) theory, which approximates the interaction between molecules but often neglects the distortion that occurs during reaction. The distortion/interaction model accounts for the energy penalties associated with the deformations of reactants as the reaction progresses, providing a more comprehensive understanding of reactivity. The activation energy of a reaction is decomposed into two contributions: the reaction strain (or distortion energy) and the interaction energy. The reaction strain is determined by the rigidity of the reactants and the type of reaction mechanism, while the interaction energy depends on the electronic structure and mutual orientation of the reactants. The interplay between these two energies determines the height of the activation barrier. The model has been applied to various reactions, including E2 and S_N2 elimination and substitution, nucleophilic additions to alkenes and alkynes, Diels-Alder cycloadditions, dehydro-Diels-Alder reactions, bioorthogonal cycloadditions, homogeneous catalysis, organocatalysis, and arynes additions and cycloadditions. It provides insights into reactivity trends, regioselectivity, and the role of steric and electronic factors. The model also explains the linear free energy relationships between activation energies and reaction energies, as well as the Hammond postulate. It has been compared to Marcus theory, which relates the activation energy to thermodynamic parameters, and has been shown to be complementary in understanding reactivity. The effect of solvation on reaction rates can be accounted for by computing the reaction strain and interaction energies for solvated reactants, providing a more accurate prediction of reaction profiles in solution.