We present an improved version of RosettaHoles, a methodology for quantitative and visual characterization of protein core packing. RosettaHoles2 features a packing measure more rapidly computable, accurate and physically transparent, as well as a new validation score intended for structures submitted to the Protein Data Bank. The differential packing measure is parameterized to maximize the gap between computationally generated and experimentally determined X-ray structures, and can be used in refinement of protein structure models. The parameters of the model provide insight into components missing in current force fields, and the validation score gives an upper bound on the X-ray resolution of Protein Data Bank structures; a crystal structure should have a validation score as good as or better than its resolution.Keywords: protein folding; protein design; protein core packing; protein structure validation; crystallography Computationally generated protein structures are often incorrect, suggesting some aspect of the physical chemistry is not modeled properly. For Rosetta 1 models, bond geometry is ideal by construction, low scoring models have very few clashes, and van der waals attractive interactions are comparable to corresponding crystal structures; these models easily pass standard structure validation tests such as MolProbity 2 and Whatcheck. 3 In contrast, visual inspection suggests that computational models have more volumetric packing flaws than observed in crystal structures. This is at least in part due to some missing piece in the Rosetta energy function; crystal structures that are energy minimized in the Rosetta force field display the same packing flaws seen in decoy structures, though to a lesser extent (Supporting Information 1). A missing piece of the puzzle could be the cavity free energy, which is inherently volumetric in nature. However, the total cavity volume is not a good discriminator between computational predictions and corresponding X-ray structures (Supporting Information 2). The original version of RosettaHoles 4 showed that discrimination between computational models and crystal structures can be achieved using volumetric information. In this updated version, we seek to achieve better performance with a simplified method.To define a packing score that captures cavity free energy, we take an approach inspired by the implicit solvation model of Lazaridis and Karplus. 5