Severe Underestimation of Density at High Ethanol Concentration in TIP4P + OPLS-UA Simulations

ethanol.index (291 Bytes)
ethanol.mol (423 Bytes)
ethanol.parm (547 Bytes)
npt_3000_0.7.log (34.2 KB)
npt_3000_0.7.txt (847 Bytes)
water_ethanol_box_3000_0.7.data (1.9 MB)

Dear LAMMPS users,

I am performing NPT simulations of ethanol-water mixtures using the TIP4P model for water and the OPLS-UA model for ethanol. The total number of molecules is fixed at 3000, and I varied the ethanol mole fraction across five values: 0.05, 0.10, 0.15, 0.20, and 0.70.

For mole fractions up to 0.20, the final density obtained after equilibration agreed well with experimental values (within 1–2% error). However, for the highest ethanol mole fraction of 0.70, the simulated density was 0.75 g/cm³, which is significantly lower than the experimental value of ~0.85 g/cm³.

The following files are attached:

[ethanol.index|attachment](upload://8eCnjOBgJFikGNDdyiV4W6UYpNK.index) (291 Bytes) [ethanol.mol|attachment](upload://8sTHMbfmZS8XryWsIMwbcT443r8.mol) (423 Bytes) [ethanol.parm|attachment](upload://iyiIZh1VFtgJDkiuSuSLideWDs7.parm) (547 Bytes) [npt_3000_0.7.log|attachment](upload://wiAT4lVSO1xyTcGZ3YtE5891JeD.log) (34.2 KB) [npt_3000_0.7.txt|attachment](upload://ojylSu2BIqcz1I1OIzDPWMNIpGR.txt) (847 Bytes) [water_ethanol_box_3000_0.7.data|attachment](upload://2TyCqd0ORjDhAcDn0Jsa9BJAVYQ.data) (1.9 MB)
  • water_ethanol_box_3000_0.7.data – Structure file for 3000 molecules at 70% ethanol
  • npt_3000_0.7.txt – LAMMPS input script for NPT equilibration at 315 K, 1 atm
  • npt_3000_0.7.log – Log file showing temperature, pressure, and density over time
  • ethanol.mol – Ethanol molecule template
  • ethanol.parm – Parameter file for ethanol (pair, bond, angle, dihedral definitions)

Why does the density decreases?

Thank you very much in advance for your support.

asas

Hello,

I don’t see anything that seem wrong in your LAMMPS input, so my main interrogation is: are you sure that united-atom models are expected to reproduce the bulk density of water-ethanol mixture? Have you seen some publications where the authors use your models ?

Simon

Hello, Simon

Thank you very much for your reply.

I want to study diffusion behavior based on temperature gradients, and this script is a preparatory step. The following paper discusses thermal diffusion using TIP4P water molecules and the OPLS-UA model of ethanol.

Nieto-Draghi et al., J. Chem. Phys. 122, 114504 (2005):
" Computing the Soret coefficient in aqueous mixtures using
boundary driven nonequilibrium molecular dynamics"

TIP4P water was referenced from the LAMMPS official website, and OPLS-UA ethanol parameters were referenced from oplsaa.lt.

oplsaa.lt (335.7 KB)

OK, but do they explore the water-ethanol mixtures density in the same range as you?

What I would try to do in your position is to replace the UA model by the AA one for the ethanol. I know that this one reproduces the density for all water-ethanol compositions. If you still find a difference with experiments, then there must be something wrong in your simulation or in the way you are measuring quantities (density, mol fraction?)

No, their paper only simulated alcohol solutions with concentrations of 20% or less. As you pointed out, it was not guaranteed that the same model could be used for higher concentrations.

As you said, I would like to try simulating with the AA model. Thank you for your kind guidance.