QMCPACK is an open source quantum Monte Carlo package for ab initio electronic structure calculations. It supports calculations of metallic and insulating solids, molecules, atoms, and some model Hamiltonians. Implemented real space quantum Monte Carlo algorithms include variational, diffusion, and reptation Monte Carlo. QMCPACK uses Slater-Jastrow type trial wavefunctions in conjunction with a sophisticated optimizer capable of optimizing tens of thousands of parameters. The orbital space auxiliary-field quantum Monte Carlo method is also implemented, enabling cross validation between different highly accurate methods. The code is specifically optimized for calculations with large numbers of electrons on the latest high performance computing architectures, including multicore central processing unit and graphical processing unit systems. We detail the program's capabilities, outline its structure, and give examples of its use in current research calculations. The package is available at http://qmcpack.org.

We review recent advances in the capabilities of the open source ab initio Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for greater efficiency and reproducibility. The auxiliary field QMC (AFQMC) implementation has been greatly expanded to include k-point symmetries, tensor-hypercontraction, and accelerated graphical processing unit (GPU) support. These scaling and memory reductions greatly increase the number of orbitals that can practically be included in AFQMC calculations, increasing the accuracy. Advances in real space methods include techniques for accurate computation of bandgaps and for systematically improving the nodal surface of ground state wavefunctions. Results of these calculations can be used to validate application of more approximate electronic structure methods, including GW and density functional based techniques. To provide an improved foundation for these calculations, we utilize a new set of correlation-consistent effective core potentials (pseudopotentials) that are more accurate than previous sets; these can also be applied in quantum-chemical and other many-body applications, not only QMC. These advances increase the efficiency, accuracy, and range of properties that can be studied in both molecules and materials with QMC and QMCPACK.

ABSTRACT:We have applied the many-body ab-initio diffusion quantum Monte Carlo (DMC) method to study Zn and ZnO crystals under pressure, and the energetics of the oxygen vacancy, zinc interstitial and hydrogen impurities in ZnO. We show that DMC is an accurate and practical method that can be used to characterize multiple properties of materials that are challenging for density functional theory approximations. DMC agrees with experimental measurements to within 0.3 eV, including the band-gap of ZnO, the ionization potential of O and Zn, and the atomization energy of O 2 , ZnO dimer, and wurtzite ZnO. DMC predicts the oxygen vacancy as a deep donor with a formation energy of 5.0(2) eV under O-rich conditions and thermodynamic transition levels located between 1.8 and 2.5 eV from the valence band maximum. Our DMC results indicate that the concentration of zinc interstitial and hydrogen impurities in ZnO should be low under n-type, and Zn-and H-rich conditions because these defects have formation energies above 1.4 eV under these conditions. Comparison of DMC and hybrid functionals shows that these DFT approximations can be parameterized to yield a general correct qualitative description of ZnO. However, the formation energy of defects in ZnO evaluated with DMC and hybrid functionals can differ by more than 0.5 eV. a) Electronic mail: reboredofa@ornl.gov 2

In view of the continuous theoretical efforts aimed at an accurate microscopic description of the strongly correlated transition metal oxides and related materials, we show that with continuum quantum Monte Carlo (QMC) calculations it is possible to obtain the value of the spin superexchange coupling constant of a copper oxide in a quantitatively excellent agreement with experiment. The variational nature of the QMC total energy allows us to identify the best trial wave function out of the available pool of wave functions, which makes the approach essentially free from adjustable parameters and thus truly ab initio. The present results on magnetic interactions suggest that QMC is capable of accurately describing ground state properties of strongly correlated materials. 02.70.Ss,75.30.Et,75.47.Lx For decades, transition metal oxides have been amongst the most intriguing materials due to the complex correlated behavior of the 3d or 4d electrons of a transition metal ion. In particular, strong electronic correlations often give rise to non-trivial magnetism, such as quantum spin-liquid states in low-dimensional Mott insulating oxides. High-temperature superconductivity in copper oxides (cuprates) is also believed to originate from magnetic spin excitations that bind Cooper pairs [1-3]. Electronic correlations, however, also make this class of materials among the most difficult to describe theoretically, both from model as well as ab initio perspectives.One of the practical challenges of ab initio electronic structure theory is to accurately predict the strength of magnetic coupling between localized spins of transition metal ions [4]. For solids, a natural method of choice is periodic density functional theory (DFT). DFT gives access to the system's ground state energy corresponding to different configurations of localized spins, which can be mapped onto the eigenstates of the a spin model to extract the magnetic couplings J. This approach relies on the accuracy of the description of the ground state. Unfortunately, the presently available approximations to the exchange-correlation functional in DFT either poorly account for the exchange and correlation effects [local density approximation (LDA)] or depend on empirical input parameters (LDA+U, hybrid functionals). Often, the only way to find an appropriate approximation for the system of interest is by comparing theoretical calculations with experiment, compromising the predictive nature of such calculations.This problem is generic to a broad class of transition metal oxides. Let us exemplify the aforementioned limitations of DFT by considering the case of the Mott insulators Ca 2 CuO 3 and Sr 2 CuO 3 . These systems are one of the best realizations of the one-dimensional (1D) spin-1 2 antiferromagnetic Heisenberg chain model, demonstrat-ing spin-liquid behavior and separation of spin and orbital degrees of freedom [11]. The crystal structure of Ca 2 CuO 3 and Sr 2 CuO 3 is similar to that of the superconducting two-dimensional cuprates, with the difference that ...

Titanium dioxide, TiO 2 , has multiple applications in catalysis, energy conversion and memristive devices because of its electronic structure. Most of these applications utilize the naturally existing phases: rutile, anatase and brookite. Despite the simple form of TiO 2 and its wide uses, there is longstanding disagreement between theory and experiment on the energetic ordering of these phases that has never been resolved. We present the first analysis of phase stability at zero temperature using the highly accurate many-body fixed node diffusion Quantum Monte Carlo (QMC) method. We also include the effects of temperature by calculating the Helmholtz free energy including both internal energy and vibrational contributions from density functional perturbation theory based quasi harmonic phonon calculations. Our QMC calculations find that anatase is the most stable phase at zero temperature, consistent with many previous mean-field calculations. However, at elevated temperatures, rutile becomes the most stable phase. For all finite temperatures, brookite is always the least stable phase.