We will use the FBH-based definition for the remainder of this work, and our main interest is in understanding how various energy components compare when the ion is in a bulk versus an interfacial environment. As noted in the examples presented above, the magnitude of E CT can depend strongly on the method that is used to count electrons. This observation suggests that in other applications of constrained DFT , which is the more common form of charge-constrained SCF calculation (in contrast to the constrained HF calculations employed here), the results should be checked carefully to ensure that conclusions are robust with respect to the details of how the constraints are implemented.