A solvation potential replaces the problem of the conformation of a molecule in solution with the problem of the conformation of a single molecule in an external field. Practical solvation potentials for macromolecules treated with an interaction-site model must be site-pairwise additive. A key requirement for accuracy of a pairwise additive potential is the inclusion to some degree of the intramolecular screening of the solvent-mediated interactions between sites. We analyze this requirement from a diagrammatic point of view and suggest it explains the accuracy of the "Gaussian-fluctuation’; approximate potential relative to the superposition approximation. Our potential, the presentation of which is the concrete contribution here. In the following paper this new potential is tested for a model linear nonpolar polymer.