The Diels-Alderase ribozyme, an in vitro-evolved ribonucleic acid enzyme, accelerates the formation of carbon-carbon bonds between an anthracene diene and a maleimide dienophile in a [4 + 2] cycloaddition, a reaction with broad application in organic chemistry. Here, the Diels-Alderase ribozyme is examined via molecular dynamics (MD) simulations in both crystalline and aqueous solution environments. The simulations indicate that the catalytic pocket is highly dynamic. At low Mg2+ ion concentrations, inactive states with the catalytic pocket closed dominate. Stabilization of the enzymatically active, open state of the catalytic pocket requires a high concentration of Mg2+ ions (e.g., 54 mM), with cations binding to specific phosphate sites on the backbone of the residues bridging the opposite strands of the pocket. The free energy profile for pocket opening at high Mg2+ cation concentration exhibits a double minimum, with a barrier to opening of similar to 5.5 kJ/mol and the closed state similar to 3 kJ/mol lower than the open state. Selection of the open state on substrate binding leads to the catalytic activity of the ribozyme. The simulation results explain structurally the experimental observation that full catalytic activity depends on the Mg2+ ion concentration.