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Our recently developed optimized potentials for liquid simulations-virtual site ionic liquid (OPLS-VSIL) force field has been shown to provide accurate bulk phase properties and local ion-ion interactions for a wide variety of imidazolium-based ionic liquids. The force field features a virtual site that offloads negative charge to inside the plane of the ring with careful attention given to hydrogen bonding interactions. In this study, the Diels-Alder reaction between cyclopentadiene and methyl acrylate was computationally investigated in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF₆], as a basis for the validation of the OPLS-VSIL to properly reproduce a reaction medium environment. Mixed ab initio quantum mechanics and molecular mechanics (QM/MM) calculations coupled to free energy perturbation and Monte Carlo sampling (FEP/MC) that utilized M06-2X/6-31G(d) and OPLS-VSIL gave activation free energy barriers of 14.9 and 16.0 kcal/mol for the <i>endo-cis</i> and <i>exo-cis</i> Diels-Alder reaction pathways, respectively (exptl. &#916;<i>H</i>^&#8225; of 14.6 kcal/mol). The <i>endo</i> selectivity trend was correctly predicted with a calculated 73% <i>endo</i> preference. The rate and selectivity enhancements present in the <i>endo</i> conformation were found to arise from preferential hydrogen bonding with the exposed C4 ring hydrogen on the BMIM cation. Weaker electronic stabilization of the <i>exo</i> transition state was predicted. For comparison, our earlier &#177;0.8 charge-scaled OPLS-2009IL force field also yielded a &#916;<i>G</i>^&#8225; of 14.9 kcal/mol for the favorable <i>endo</i> reaction pathway but did not adequately capture the highly organized solvent interactions present between the cation and Diels-Alder transition state.