Please note different day and location (Thursday, McDonnell 102A).  Computational expense has long been a challenge for 3D structure determination in electron cryomicroscopy (cryo-EM).  This problem has become more pressing as resolutions have increased and the desire to separate conformational computational variations has required ever-larger datasets.  This talk presents two new algorithmic developments which, when coupled with modern GPU hardware, dramatically reduces the computational requirements for cryo-EM.  Given low-resolution starting structures, high resolution structures can now be obtained in as little as 10s of minutes on modest desktop workstations.  Further, despite the severe non-convexity of the objective function, these new refinement algorithms have shown themselves to be robust to local minima, enabling ab initio structure determination and 3D classification.  Together, these algorithms can produce ab initio high resolution structures in an hour or two.  This talk will describe the underlying algorithmic advances (stochastic optimization and branch-and-bound search) along with results from their implementation in the recently released cryoSPARC software package.