Solvation in the presence of charged species

Solvation in the presence of charged species

While the solvation options described in the previous tutorial are sufficient to describe the solvation of molecules in fluids such as water, these fluids are not sufficient by themselves when the fluid contains charged species. There are a number of ways to approximate the effects of the ionic strength of fluids with charged species. Here, a modified Poisson-Boltzmann equation is used to provide a screening length for charge, providing a Debye-Huckel type of ionic screening. This should be appropriate for relatively low concentrations of ions in solution, and where the ions do not strongly interact with the species/surfaces in the explicit portion of the calculation.

To try out ionic screening, we can add the following fluid line to our calculation from the previous tutorial:

  fluid LinearPCM
  fluid-anion Na 0.1
  fluid-cation Cl 0.1

This will give us the energy of the solvated water molecule in the linear polarizable continuum model in a 0.1 M NaCl solution.

Extra practice:

1. Instead of using a water molecule, look at the solvation energy associated with an anion or a cation.
2. Calculate the pKa of an organic acid. (Tricky- need to use the vibrations tutorial as well, can compare results with the CANDLE paper)