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

LAMMPS LAMMPS Beginners
1

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

2

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

3

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)

4

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?)

5

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.