MD simulations were conducted using the LAMMPS software (Sandia National Laboratories, Albuquerque, NM, USA) , and the SPICA force field was employed for the POPC bilayer systems containing ethanol. The CG representation of each molecule is shown in Figure . Ethanol represented as a single CG site is introduced as an amino acid side chain analogue in the SPICA force field. We set up lamellar membrane systems with different ethanol concentrations. For each simulated system, a short MD run using an NVT ensemble was conducted to thermalize the system for 100 ps after energy minimization. Subsequently, 1.1 µs NPT MD production runs were conducted, and the trajectories in the last 100 ns were used in the structural analyses. The temperature was controlled at 310 K using the Nosé-Hoover thermostat , and the pressure was controlled at 1 atm using the Parrinello-Rahman semi-isotropic coupling barostat. The Lennard-Jones-type interaction for a nonbonding interaction was simply truncated at 1.5 nm, although the Coulombic interactions among the lipid headgroups were calculated via the P 3 M method. The size of the time step used to integrate the equation of motion was 10 fs.