Rational Design of Nonbonded Point Charge Models for Monovalent Ions with Lennard-Jones 12–6 Potential

Ions are of central importance in
nature, and a variety of potential
models was proposed to model ions in different phases for an in-depth
exploration of ion-related systems. Here, we developed point charge
models of 14 monovalent ions with the traditional 12–6 Lennard-Jones
(LJ) potential for use in conjunction with 11 water models of TIP3P,
OPC3, SPC/E, SPC/Eb, TIP3P-FB, a99SB-disp, TIP4P-Ew, OPC, TIP4P/2005, TIP4P-D, and TIP4P-FB. The designed
models reproduced the real hydration free energy (HFE) of ions and
the ion-oxygen distance (IOD) in the first hydration shell accurately
and simultaneously, a performance similar to the previously reported
12–6–4 LJ-type ion models (12–6 LJ plus an attractive C4 term for cations or a repulsive one for anions).
This work, along with our previous work on di-, tri-, and tetravalent
metal cations (J. Chem. Inf. Model. 2021, 61, 4031–4044; J. Chem. Inf. Model. 2021, 61, 4613–4629), demonstrates the feasibility
of the simple 12–6 LJ potential in ion modeling. In order for
the 12–6 LJ potential to reproduce both the HFE and IOD, the
LJ R parameters need to be close to Shannon’s
ionic radii for the highly charged cations and to the Stokes’s
van der Waals (vdW) radii for the monovalent ions. With an additional C4 term, the R parameters of
12–6–4 LJ ion models agree well with the Stokes’s
vdW radii and have a more physical meaning. It appears that the C4 term can be merged into the 12–6 LJ
potential by a rational tuning of R and the LJ well
depth. Simulations of the osmotic coefficients of alkali chloride
solutions and the properties of gaseous and solid alkali halides indicate
the necessity of further optimizing ion–ion interactions via,
for instance, targeting more properties or using a more physical (polarizable)
model.