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.

The I 2-II-IV-VI 4 quaternary chalcogenide semiconductors ͑e.g., Cu 2 ZnGeS 4 , Cu 2 ZnSnS 4 , Cu 2 ZnGeSe 4 Cu 2 CdSnSe 4 , and Ag 2 CdGeSe 4 ͒ have been studied for more than 40 years but the nature of their crystal structures has proved contentious. Literature reports exist for the stannite and kesterite mineral structures, which are zinc-blende-derived structures, and wurtzite-stannite, which is a wurtzite-derived structure. In this paper, through a global search based on the valence octet rule ͑local charge neutrality͒, we report a wurtzitederived structure corresponding to the kesterite structure, namely, wurtzite-kesterite ͑space group Pc͒, which is the ground state for some I 2-II-IV-VI 4 compounds, but is easily confused with the wurtzite-stannite ͑spacegroup Pmn2 1 ͒ structure. We show that there is a clear relationship between the properties of the wurtzitekesterite and zinc-blende-derived kesterite structures, as well as between wurtzite-stannite and stannite. Contributions from the strain and Coulomb energies are found to play an important role in determining the structural stability. The underlying trends can be explained according to the size and ionicity of the group-I,-II,-IV, and-VI atoms. Electronic-structure calculations show that the wurtzite-derived structures have properties similar to the zinc-blende-derived structures, but their band gaps are relatively larger, which has also been observed for binary II-VI semiconductors.

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Quantum Monte Carlo methods are accurate and promising many body techniques for electronic structure calculations which, in the last years, are encountering a growing interest thanks to their favorable scaling with the system size and their efficient parallelization, particularly suited for the modern high performance computing facilities. The ansatz of the wave function and its variational flexibility are crucial points for both the accurate description of molecular properties and the capabilities of the method to tackle large systems. In this paper, we extensively analyze, using different variational ansatzes, several properties of the water molecule, namely: the total energy, the dipole and quadrupole momenta, the ionization and atomization energies, the equilibrium configuration, and the harmonic and fundamental frequencies of vibration. The investigation mainly focuses on variational Monte Carlo calculations, although several lattice regularized diffusion Monte Carlo calculations are also reported. Through a systematic study, we provide a useful guide to the choice of the wave function, the pseudo potential, and the basis set for QMC calculations. We also introduce a new strategy for the definition of the atomic orbitals involved in the Jastrow -Antisymmetrised Geminal power wave function, in order to drastically reduce the number of variational parameters. This scheme significantly improves the efficiency of QMC energy minimization in case of large basis sets.

Diradical molecules are essential species involved in many organic and inorganic chemical reactions. The computational study of their electronic structure is often challenging, because a reliable description of the correlation, and in particular of the static one, requires multireference techniques. The Jastrow correlated antisymmetrized geminal power (JAGP) is a compact and efficient wave function ansatz, based on the valence-bond representation, which can be used within quantum Monte Carlo (QMC) approaches. The AGP part can be rewritten in terms of molecular orbitals, obtaining a multideterminant expansion with zero-seniority number. In the present work we demonstrate the capability of the JAGP ansatz to correctly describe the electronic structure of two diradical prototypes: the orthogonally twisted ethylene, C2H4, and the methylene, CH2, representing respectively a homosymmetric and heterosymmetric system. In the orthogonally twisted ethylene, we find a degeneracy of π and π* molecular orbitals, as correctly predicted by multireference procedures, and our best estimates of the twisting barrier, using respectively the variational Monte Carlo (VMC) and the lattice regularized diffusion Monte Carlo (LRDMC) methods, are 71.9(1) and 70.2(2) kcal/mol, in very good agreement with the high-level MR-CISD+Q value, 69.2 kcal/mol. In the methylene we estimate an adiabatic triplet-singlet (X̃(3)B1-ã(1)A1) energy gap of 8.32(7) and 8.64(6) kcal/mol, using respectively VMC and LRDMC, consistently with the experimental-derived finding for Te, 9.363 kcal/mol. On the other hand, we show that the simple ansatz of a Jastrow correlated single determinant (JSD) wave function is unable to provide an accurate description of the electronic structure in these diradical molecules, both at variational level (VMC torsional barrier of C2H4 of 99.3(2) kcal/mol, triplet-singlet energy gap of CH2 of 13.45(10) kcal/mol) and, more remarkably, in the fixed-nodes projection schemes (LRDMC torsional barrier of 97.5(2) kcal/mol, triplet-singlet energy gap of 13.36(8) kcal/mol) showing that a poor description of the static correlation yields an inaccurate nodal surface. The suitability of JAGP to correctly describe diradicals with a computational cost comparable with that of a JSD calculation, in combination with a favorable scalability of QMC algorithms with the system size, opens new perspectives in the ab initio study of large diradical systems, like the transition states in cycloaddition reactions and the thermal isomerization of biological chromophores.

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.

Although liquid water is ubiquitous in chemical reactions at roots of life and climate on the earth, the prediction of its properties by high-level ab initio molecular dynamics simulations still represents a formidable task for quantum chemistry. In this article we present a room temperature simulation of liquid water based on the potential energy surface obtained by a many-body wave function through quantum Monte Carlo (QMC) methods. The simulated properties are in good agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function, at variance of previous Density Functional Theory attempts. Given the excellent performances of QMC on large scale supercomputers, this work opens new perspectives for predictive and reliable ab-initio simulations of complex chemical systems. PACS numbers:The simulation by first principles of liquid water, the key element of human life and biological processes, has been a dream for several decades after the foundation of Density Functional theory (DFT), even within the restriction of the Born-Oppenheimer approximation for the heavy nuclei. Realistic simulations are particular important because, at the experimental level, it is not possible to clarify completely what are the relationships between the so many different and rich phases of water and the physical interactions between water molecules, determined by hydrogen bonding and weak longrange van der Waals (vdW) interactions. Moreover water is involved in many biological and chemical processes, and first principle simulations are useful to investigate and rationalize such important mechanisms.The first attempted simulations date back to the pioneer works by Car and Parrinello 1-3 , within an efficient ab-initio molecular dynamics (AIMD) based on DFT. The comparison with the experiments, at that time available, provided a pretty good agreement with the oxygen-oxygen (O-O) radial distribution function (RDF), as far as the positions of the peaks were concerned, but the overall shape given by the simulation was overstructured. After these first studies, many other works reporting standard DFT-based simulations have been published, but the agreement with the experimental data is still not satisfactory on many aspects. The equilibrium density at ambient pressure (1 atm ∼ 10 −4 GPa), is far to be consistent with the expected one (1 gr/cm 3 ) though recent DFT functionals including van der Waals substantially reduce this discrepancy 4 . The simulated diffusion 5 is much lower than what is expected from experiments 6 , and, at least in some functionals (namely, PBE and BLYP), the solidification of water occurs at a temperature which is unrealistically large (∼ 410 K), so that some of the present DFT simulations of liquid water should be considered supercooled metastable phases 6,7 .The DFT results (about which we provide a brief summary in Tab. I) appear to be substantially influenced by the choice of the functional 5,8 , but also, within a ...

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.