pressures of both of the phases are equal. The first condition is automatically fulfilled by the G-RxMC algorithm. To meet the second condition, we utilize the pressurecomposition protocol. We carry out a series of simulations of the interpolyelectrolyte complex phase at various polymer volume fractions, determined by setting the simula-tion box length. In each simulation we measure the osmotic pressure difference between the interpolyelectrolyte complex and the supernatant, ∆P (ϕ mon ) = P pec (ϕ mon ) -P sup , as a function of the polymer volume fraction. Then we fit the data using a smooth function to find the polymer volume fraction, ϕ 0 mon , such that ∆P (ϕ 0 mon ) ≈ 0, as illustrated in Figure ).