Collaborative Research: Petascale Simulations of Quantum Systems by Stochastic Methods

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TECHNICAL SUMMARY This award supports developing, testing, and optimizing a new generation of simulation codes for calculating electronic states on petascale computers. The codes will be based on quantum Monte Carlo methods. The effort will combine recent advances in computer science, such as Global Arrays, fault-tolerant parallelism, Graphics Processing Units and task-dynamical parallelism, with recent disciplinary advances that can treat correlated electronic systems and study physical effects currently out of reach. The goal is to establish production-quality quantum Monte Carlo codes that can exploit Track I and follow-on hardware to achieve scientific breakthrough calculations. Successful use of petascale computers will likely be very challenging. Scaling up the quantum Monte Carlo codes by more than two orders of magnitude will require pursuing new avenues for algorithm organization and parallelization; for example, utilizing the multicore/shared memory nature of the nodes. Attention to fault tolerance and load balance will lead to reliable and efficient petascale codes. A key thrust will be to couple quantum Monte Carlo methods for electrons with classical simulation methods for the ions enabling realistic simulations on virtually any system. New predictive capabilities and insights will result, particularly into dynamics and finite temperature phenomena using a full quantum mechanical description of the electrons. Another thrust involves applying quantum Monte Carlo to strongly correlated electronic systems - among the most important challenges in condensed matter physics. The PIs plan to implement and test new correlated wavefunctions such as pfaffians with backflow, and new quantum Monte Carlo algorithms, such as the dynamical coupling of classical and quantum degrees of freedom. These developments will be applied to study water with a full quantum mechanical description of all the relevant degrees of freedom such as electrons and protons at nonzero temperatures without any empirical or mean-field inputs. Quantum Monte Carlo calculations will also be used to understand transition metal compounds, in particular, the magnetic states and metal-insulator transitions at high pressures in transition metal oxide materials. Despite decades of studies these systems remain inadequately understood and require exceedingly high accuracy to reveal the origins of a variety of many-body phenomena and experimentally observed phenomena. The developed computational tools will be available to the scientific community through the open source projects QMCPACK, and QWalk. The Global Arrays toolkit will provide a high-level and scalable programming environment based on the global address space programming model. This research and development effort will also support training of graduate students and postdocs in the area of high performance computing. The PIs will organize a Summer School, "Petaflop Quantum Monte Carlo Methods," to train students, postdocs and researchers in the developed methodologies and enable them to open new frontiers with high performance computing at the petascale and beyond. NONTECHNICAL SUMMARY This award supports developing, testing, and tuning the performance of a new generation of computer codes targeted for the most powerful computers. These new codes will be based on quantum mechanics and create a "virtual microscope" that probe materials at the atomic scale. These codes will provide an accurate determination of the forces between atoms enabled by an accurate quantum mechanical description of the electrons and their motions which are the ultimate sources of these forces. The resulting code will be applied to study water, perhaps the most important component of life but also one of the most challenging liquids and solids to understand. The accurate description of electrons afforded by the PIs codes will enable the most accurate computational study of materials which involve electrons that interact strongly with each other leading to new states of matter. Understanding these materials may lead to the discovery of new states of matter that may lead to new materials and device technologies. The codes that are developed will be made available to the broader computational community of scientists and empower them to utilize the most powerful computers to open new frontiers. This research and development effort will also support training of graduate students and postdocs in the area of high performance computing. The PIs will organize a Summer School, "Petaflop Quantum Monte Carlo Methods," to train students, postdocs and researchers in the developed methodologies and enable them to open new frontiers with high performance computing at the petascale.