Both systems are simulated at 298 K which is well below the T m of DPPC, yet well above the T m of POPC, therefore ensuring that we are not close to any phase transitions. The C36/LJ-PME with the TIPS3P water model su↵ers from characteristic issues for monolayer simulations performed with water models having too low surface tension: Negative surface pressures, corresponding to non-physical states where the absorbance of a surfactant layer increases the interfacial tension, appear above APL of 51 Å2 for DPPC and 70 Å2 for POPC in C36/LJ-PME simulations. Furthermore, stable pores appear in monolayers at an APL of 60 Å2 for DPPC (Fig. ) and 86 Å2 for POPC, which are significantly below the experimental values where the gas-L e phase coexistence begins; approximately 100-110 Å2 and 120-130 Å2 , respectively. The opening of pores at too small APLs can be explained by too low surface tension of the TIPS3P water model favoring the exposure of water surface rather than the transition of most lipids to the L e phase upon increasing APL. Notably, such pores may not appear in simulations with small box size due to finite size e↵ects, which could be the case in monolayer simulations with 36 lipids used in the optimization protocol of C36/LJ-PME model. Because C36/LJ-PME with TIPS3P showed characteristic behaviour for simulations with too low water surface tension, we set out to find a water model that reproduces the experimental surface tension with LJ-PME that could be used together with the C36/LJ-PME lipid model. To this end, we evaluated the surface tension of eight water models at three different temperatures using di↵erent Lennard-Jones cuto↵ distances and Lennard-Jones PME in Fig. . Water surface tension increases in all models with the increasing cuto↵, converging toward the values obtained with LJ-PME as expected. As shown also previously, OPC4 performs reasonably well with cuto↵s of 1.2 and 1.4 nm, but slightly overshoots the experimental value with LJ-PME. TIP4P/05 slightly undershoots water surface tension with LJ-PME, whereas other models behave poorly, with TIP3P and TIPS3P underestimating the experimental values by ⇡20 mN/m at all studied temperatures. Based on the results in Fig. , we repeated the DPPC and POPC bilayer monolayer simulations using C36/LJ-PME with OPC4 and TIP4P/05 water models that gave the best surface tension values with