Molecular surfaces at atomic and subatomic scales are inherently ill-defined. In many computational chemistry problems, boundaries are better represented as volumetric regions than as discrete surfaces. Molecular structure of a system at equilibrium is given by the self-consistent field, commonly interpreted as a scalar field of electron density. While experimental measurements such as chemical bond and van der Waals radii do not spatially define the interface, they can serve as useful indicators of chemical and inert interactions, respectively. Rather than using these radial values to directly determine surface geometry, we use them to map an uncertainty interval in the electron density distribution, which then guides classification of volume data. This results in a new strategy for representing, analyzing, and rendering molecular boundaries that is agnostic to the type of interaction.