sentences:  In 1965, Walter L. Koltun designed and patented a simplified system with molded plastic atoms of various colours, which were joined by specially designed snap connectors; this simpler system accomplished essentially the same ends as the Corey-Pauling system, and allowed for the development of the models as a popular way of working with molecules in training and research environments.Such colour-coded, bond length-defined, van der Waal's-type space-filling models are now commonly known as CPK models, after these three developers of the specific concept.In modern research efforts, attention returned to use of data-rich crystallographic models in combination with traditional and new computational methods to provide space-filling models of molecules, both simple and complex, where added information such as which portions of the surface of the molecule were readily accessible to solvent, or how the electrostatic characteristics of a space-filling representation—which in the CPK case is almost fully left to the imagination—could be added to the visual models created.