Solvation energies from the linear response function of density functional theory

Starting from a local approximation to the softness kernel proposed by Vela and Gلsquez [J. Am. Chem. Soc. 112 (1990) 1490], a useful equation for the solvation energy is derived. The resulting expression contains a first order contribution representing the electrostatic solute-solvent interaction energy, and a second order term associated to the fluctuation of the reaction field potential as electrons are added to the solute system. The most relevant result of the present formalism is that the solvation energy displays a linear dependence on the global softness. This expression is tested for a series of atomic and molecular systems.