We describe the adaptation of VMD, a popular molecular visualization and analysis tool, to exploit the Tesla K20X GPU for acceleration of large scale molecular visualization runs on Cray XK7 petascale supercomputers such as Blue Waters and Titan. We will describe ray tracing performance benefits and memory efficiency optimizations achieved through the use of custom geometric primitives and triangle mesh formats, and relate our experiences adapting the Tachyon CPU-based ray tracing engine used by VMD, to NVIDIA's OptiX GPU ray tracing framework. We will present performance data for large visualization runs on the Cray XK7, discuss our approach to integrating OptiX into VMD, and describe avenues for further improvement.