A typical example of the pressure-composition protocol is represented by the cyan dataset in Figure ), corresponding to c sup NaCl ≈ 156mM. We observe that ∆P (ϕ mon ) ≫ 0 at high polymer volume fractions, mainly due to the steric repulsion. As the density of the PEC phase decreases, the ∆P (ϕ mon ) dependence eventually crosses the zeropressure baseline, indicating that the conditions for phase coexistence of supernatant and interpolyelectrolyte complex can be fulfilled at a certain volume fraction, ϕ 0 mon , shown on the snapshot in Figure ) . While volume fractions at ϕ mon > ϕ 0 mon correspond to the states of interpolyelectrolyte complex under compression, we have to be cautious about interpreting the compositions of ϕ mon < ϕ 0 mon , because they no longer correspond to a single homogeneous phase. At ϕ mon < ϕ 0 mon , the system can undergo a phase separation within the simulation cell, forming a set of polymer-rich and polymerpoor domains, as exemplified in Figure ). In such cases, the actual monomer density of the polymer-rich domains is not equal to the mean monomer density. Naturally, such a separation within the box is affected by massive finite-size effects and leads to large fluctuations of pressure. Therefore, we fit the P (ϕ mon ) dependence only in the range where ∆P ≳ 0, in order to avoid the region of possible phase separation and simultaneously obtain a reliable estimate of ϕ 0 . We employ a phenomenological fitting function, ∆P (ϕ) = a 0 + a 1 / tan(ϕ -a 2 ) where a 0 , a 1 , a 2 are the adjustable parameters.