To quantify the mechanical response of gels to uniaxial extension, we performed a series of deformation simulations in which one dimension of the simulation cell L z was elongated at a steady rate of 10 -4 τ -1 . The other two dimensions, L x and L y , were simultaneously contracted at a rate to conserve volume, consistent with a Poisson ratio of 1/2. Strains in the range λ = L z /L z,0 ∈ [1, 4] were simulated, where L z,0 was the initial box dimension in the z direction. The engineering (or nominal) stress was computed from the diagonal components of stress tensor, σ n ≡ σ true /λ = [σ zz -0.5(σ xx + σ yy )]/λ, where σ true is the true stress. For small uniaxial deformations, the simulation data obeys the classical form of the stress-elongation relationship, σ eng = G(λ -1/λ 2 ), observed experimentally for polymer networks and predicted by network elasticity theory. Using this form, we extracted G from simulated uniaxial elongation data, averaging over 10 independent initial configurations and deformations in three independent deformation directions (x, y, and z). G and all variations of σ have units of k B T /d 3 . The images in Fig. (a), 4(e), and 4(j) were rendered using OVITO 2.9.0. 80