In MD simulations, we focus on the quasi-equilibrium deformation of a periodic PAA hydrogel network with cubic topology (see Figure ). The hydrogel is applied either a uniaxial compression or stretching in the ẑ-direction by varying the strain, λ, in discrete steps (see Figure ). At each deformation ratio, the concentration of condensed counterions around the hydrogel chains, the chain conformations, pairwise-interaction energies, and pressure components are monitored to ensure a steady behavior (see Supporting Information Figure ). To distinguish condensed and free counterions, a counterion is counted as condensed if and only if the distance between the center of mass of the counterion and the nearest backbone oxygen is d ≤ 0.25 nm (see Figure ). This cut-off distance is slightly smaller than the distance between two backbone charges, ℓ 0 = 0.32 nm.