We have scrutinized the hydration and the potentials of mean force (PMFs) for the constituent ion pairs of tetramethylammonium chloride (TMA*Cl-) in ambient water at infinite dilution, using the blue moon ensemble method with molecular dynamics simulations, where the force fields and the simulation protocols were carefully selected because of their impacts on the resultant PMFs. As a result, it was found that (i) the PMF for the Cl--Cl- pair has two minima that correspond to the different solvent-separated solute pairs (SSSPs), (ii) the PMF for the TMA(+)-TMA(+) pair shows a minimum that corresponds to an SSSP, whereas the contact solute pair (CSP) is unfavorable unlike that for the CH4-CH4 pair, and (iii) the PMF for the TMA(+)-Cl- pair has two broad minima that correspond to a CSP and an SSSP. We have also investigated how the treatments of electrostatics impact on the resulting PMFs, by performing the simulations with a truncation method as well as the Ewald method. The results will be compared with those from other simulations, the theoretical analyses, and the experiments.