Abstract: Quantum Monte Carlo (QMC) methods, specifically projector Monte Carlo such as diffusion Monte Carlo (DMC), offer a unique path toward high-accuracy calculations for a broad range of electronic systems, from molecules to solids. Unlike other electronic structure methods, QMC uses a many-body wavefunction, explicitly including effects such as localization, van der Waals interactions, and strong electronic correlations, which are often approximated by other methods. An exact mapping between the Schrodinger equation and an equivalent stochastic process is used to both represent and obtain the actual solution by stochastic methods. Electrons and ions are represented in the continuum rather than on a lattice, allowing QMC to address a much broader range of materials phenomena. The stochastic representation allows the compute-intensive parts of the algorithms to be parallelized along several domains (random walkers domain, basis and orbital domain, k-point domain, etc.) independently and efficiently, making it an ideal method for high-performance computing. With a limited number of controlled approximations, the method allows many systems to reach the golden chemical accuracy (~1 KCal/mol), allowing a link between experiment and theory.

Recent theoretical and algorithmic development and ever-growing computing powers have increased the productivity of DMC by many orders of magnitude and opened up realistic opportunities to compute and predict materials properties. This talk will briefly describe the QMC theory implemented in the QMCPACK code and some examples of applications in condensed matter and quantum chemistry.