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.

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Quantum Monte Carlo (QMC) methods have received considerable attention over past decades due to their great promise for providing a direct solution to the many-body Schrodinger equation in electronic systems. Thanks to their low scaling with the number of particles, QMC methods present a compelling competitive alternative for the accurate study of large molecular systems and solid state calculations. In spite of such promise, the method has not permeated the quantum chemistry community broadly, mainly because of the fixed-node error, which can be large and whose control is difficult. In this Perspective, we present a systematic application of large scale multideterminant expansions in QMC and report on its impressive performance with first row dimers and the 55 molecules of the G1 test set. We demonstrate the potential of this strategy for systematically reducing the fixed-node error in the wave function and for achieving chemical accuracy in energy predictions. When compared to traditional quantum chemistry methods like MP2, CCSD(T), and various DFT approximations, the QMC results show a marked improvement over all of them. In fact, only the explicitly correlated CCSD(T) method with a large basis set produces more accurate results. Further developments in trial wave functions and algorithmic improvements appear promising for rendering QMC as the benchmark standard in large electronic systems.

Quantum Monte Carlo (QMC) methods such as variational Monte Carlo and fixed node diffusion Monte Carlo depend heavily on the quality of the trial wave function. Although Slater-Jastrow wave functions are the most commonly used variational ansatz in electronic structure, more sophisticated wave functions are critical to ascertaining new physics. One such wave function is the multi-Slater-Jastrow wave function which consists of a Jastrow function multiplied by the sum of Slater determinants. In this paper we describe a method for working with these wave functions in QMC codes that is easy to implement, efficient both in computational speed as well as memory, and easily parallelized. The computational cost scales quadratically with particle number making this scaling no worse than the single determinant case and linear with the total number of excitations. Additionally, we implement this method and use it to compute the ground state energy of a water molecule.

We calculate the off-diagonal density matrix of the homogeneous electron gas at zero temperature using unbiased Reptation Monte Carlo for various densities and extrapolate the momentum distribution, and the kinetic and potential energies to the thermodynamic limit. Our results on the renormalization factor allows us to validate approximate G0W0 calculations concerning quasiparticle properties over a broad density region (1 ≤ rs 10) and show that near the Fermi surface, vertex corrections and self-consistency aspects almost cancel each other out. The uniform electron gas (jellium) is one of the most fundamental models for understanding electronic properties in simple metals and semiconductors. Knowledge of its ground state properties, and, in particular, of modifications due to electron correlation are at the heart of all approximate approaches to the many-electron problem in realistic models. Quantum Monte Carlo methods (QMC) [1] have provided the most precise estimates of the correlation energy, electron pair density and structure factor of jellium; basic quantities for constructing and parameterizing the exchange-correlation energy used in density functional theory (DFT) [2].Correlations modify the momentum distribution, n k , of electrons, and introduce deviations from the ideal Fermi-Dirac step-function. The magnitude of the discontinuity at the Fermi surface (k F ), the renormalization factor Z, quantifies the strength of a quasi-particle excitation [3] and plays a fundamental role in Fermi liquid and many-body perturbation theory (GW) for spectral quantities. Whereas the momentum distribution (as well as other spectral information) is inaccessible in current Kohn-Sham DFT formulations, the reduced singleparticle density matrix -the Fourier transform of n k in homogeneous systems -is the basic object in the so-called density-matrix functional theory [4]; these theories rely on knowledge of n k of jellium. Inelastic x-ray scattering measurement of the Compton profile of solid sodium [5] have determined n k , but experiments for elements with different electronic densities are less conclusive.In this paper, we calculate n k for the electron gas (jellium) by QMC in the density region 1 ≤ r s ≤ 10. Here, r s = (4πna 3 B /3) −3 is the Wigner-Seitz density parameter, n is the density, and a B = 2 /me 2 is the Bohr radius. In contrast to previous calculations [6], our calculations are based on more precise backflow (BF) wave functions [7], and a careful extrapolation to the thermodynamic The momentum distribution (n k ) of the unpolarized electron gas for various densities extrapolated to the thermodynamic limit. The inset shows the extrapolation of n k for rs = 5 from a system with N = 54 electrons to the thermodynamic limit, N → ∞,leading to a significant reduction of the renormalization factor Z.limit [8,9]. Similar to the worm algorithm in finite temperature path-integral and lattice Monte Carlo [10, 11], we have extended Reptation Monte Carlo (RMC) [12] to include the off-diagonal density matrix in order to o...