The calculation of the first electronic transition of an argon-atom impurity in a neon lattice is carried out in the Heitler-London scheme. The energy of this transition is taken to be the energy of the argon atomic transition, plus the correction to the SCF 4s orbital energy due to the presence of the crystal. The modification to the SCF 4s orbital is accomplished by the addition of charge-transfer functions on neighboring neon atoms. It is shown that the choice of the SCF atomic function is a very bad starting point for these systems. A general method for choosing basis functions for bound systems is presented. It is shown that for many cases a suitably chosen "model Hamiltonian" may be appropriate to a more complex system, and the case of a dielectrically screened hydrogenic "model Hamiltonian" is worked out for Ar-Ne.