Systematic Parameterization of Monovalent Ions Employing the Nonbonded Model. 
Monovalent ions play fundamental roles in many biological processes in organisms.  Modeling these ions in molecular simulations continues to be a challenging problem.  The 12-6 Lennard-Jones (LJ) nonbonded model is widely used to model monovalent ions in classical molecular dynamics simulations.  A lot of parameterization efforts have been reported for these ions with a number of experimental end points.  However, some reported parameter sets do not have a good balance between the two Lennard-Jones parameters (the van der Waals (VDW) radius and potential well depth), which affects their transferability.  In the present work, via the use of a noble gas curve we fitted in former work (J. Chem.  Theory Comput.  2013, 9, 2733), we reoptimized the 12-6 LJ parameters for 15 monovalent ions (11 positive and 4 negative ions) for three extensively used water models (TIP3P, SPC/E, and TIP4P(EW)).  Since the 12-6 LJ nonbonded model performs poorly in some instances for these ions, we have also parameterized the 12-6-4 LJ-type nonbonded model (J. Chem.  Theory Comput.  2014, 10, 289) using the same three water models.  The three derived parameter sets focused on reproducing the hydration free energies (the HFE set) and the ion-oxygen distance (the IOD set) using the 12-6 LJ nonbonded model and the 12-6-4 LJ-type nonbonded model (the 12-6-4 set) overall give improved results.  In particular, the final parameter sets showed better agreement with quantum mechanically calculated VDW radii and improved transferability to ion-pair solutions when compared to previous parameter sets.