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The molecular dissociation energy has often been explained and discussed in terms of singlet bonds, formed by bounded pairs of valence electrons. In this work we use a highly correlated resonating valence bond ansatz, providing a consistent paradigm for the chemical bond, where spin fluctuations are shown to play a crucial role. Spin fluctuations are known to be important in magnetic systems and correspond to the zero point motion of the spin waves emerging from a magnetic broken symmetry state. Recently, in order to explain the excitation spectrum of the carbon dimer, an unusual quadruple bond has been proposed. Within our ansatz, a satisfactory description of the carbon dimer is determined by the magnetic interaction of two Carbon atoms with antiferromagnetically ordered S = 1 magnetic moments. This is a first step that, thanks to the highly scalable and efficient quantum Monte Carlo technique, may open the way for understanding challenging complex systems containing atoms with large spins (e.g. transition metals).

We propose here a single Pfaffian correlated variational ansatz, that dramatically improves the accuracy with respect to the single determinant one, while remaining at a similar computational cost. A much larger correlation energy is indeed determined by the most general two electron pairing function, including both singlet and triplet channels, combined with a many-body Jastrow factor, including all possible spin-spin spin-density and density-density terms. The main technical ingredient to exploit this accuracy is the use of the Pfaffian for antisymmetrizing an highly correlated pairing function, thus recovering the Fermi statistics for electrons with an affordable computational cost. Moreover the application of the Diffusion Monte Carlo, within the fixed node approximation, allows us to obtain very accurate binding energies for the first preliminary calculations reported in this study: C$_2$, N$_2$ and O$_2$ and the benzene molecule. This is promising and remarkable, considering that they represent extremely difficult molecules even for computationally demanding multi-determinant approaches, and opens therefore the way for realistic and accurate electronic simulations with an algorithm scaling at most as the fourth power of the number of electrons.

TurboRVB is a computational package for {\it ab initio} Quantum Monte Carlo (QMC) simulations of both molecular and bulk electronic systems. The code implements two types of well established QMC algorithms: Variational Monte Carlo (VMC), and Diffusion Monte Carlo in its robust and efficient lattice regularized variant. A key feature of the code is the possibility of using strongly correlated many-body wave functions. The electronic wave function (WF) is obtained by applying a Jastrow factor, which takes into account dynamical correlations, to the most general mean-field ground state, written either as an antisymmetrized geminal product with spin-singlet pairing, or as a Pfaffian, including both singlet and triplet correlations. This wave function can be viewed as an efficient implementation of the so-called resonating valence bond (RVB) ansatz, first proposed by L. Pauling and P. W. Anderson in quantum chemistry and condensed matter physics, respectively. The RVB ansatz implemented in TurboRVB has a large variational freedom, including the Jastrow correlated Slater determinant as its simplest, but nontrivial case. Moreover, it has the remarkable advantage of remaining with an affordable computational cost, proportional to the one spent for the evaluation of a single Slater determinant. The code implements the adjoint algorithmic differentiation that enables a very efficient evaluation of energy derivatives, comprising the ionic forces. Thus, one can perform structural optimizations and molecular dynamics in the canonical NVT ensemble at the VMC level. For the electronic part, a full WF optimization is made possible thanks to state-of-the-art stochastic algorithms for energy minimization. The code has been efficiently parallelized by using a hybrid MPI-OpenMP protocol, that is also an ideal environment for exploiting the computational power of modern GPU accelerators., 41 pages, 21 figures, 3 tables

Accurate and predictive computations of the quantum-mechanical behavior of many interacting electrons in realistic atomic environments are critical for the theoretical design of materials with desired properties, and they require solving the grand-challenge problem of the many-electron Schrödinger equation. An infinite chain of equispaced hydrogen atoms is perhaps the simplest realistic model for a bulk material, embodying several central themes of modern condensed-matter physics and chemistry while retaining a connection to the paradigmatic Hubbard model. Here, we report a combined application of cutting-edge computational methods to determine the properties of the hydrogen chain in its quantum-mechanical ground state. Varying the separation between the nuclei leads to a rich phase diagram, including a Mott phase with quasi-long-range antiferromagnetic order, electron density dimerization with power-law correlations, an insulator-to-metal transition, and an intricate set of intertwined magnetic orders.

Partiendo de los edificios del Departamento de Ciencias de la Administración, UNS, como unidades de análisis, se reconoce que las condiciones mínimas de habitabilidad dependen de un significativo insumo de energía auxiliar; registrando importantes costos económicos y ambientales. Consecuentemente, basados en estudios previos y un Proyecto de Grupos de Investigación (PGI) UNS 2018-2020, se infiere que el mejoramiento de la calidad tecnológica en la envolvente de la edificación existente, asegura beneficios higro-térmicos y ambientales en forma inmediata y beneficios económicos en períodos mediatos. A tal fin, en el presente artículo se describe: el marco de actuación; los objetivos de la investigación; la metodología de trabajo; y el desarrollo de los principales análisis y resultados de la investigación. Como conclusión, se reconoce que la aplicación de las alternativas tecnológico-constructivo en la edilicia existente presenta beneficios para todos los actores involucrados: el Estado, los usuarios y la salud del edificio.

We report a quantum Monte Carlo (QMC) study, on a very simple but nevertheless very instructive model system of four hydrogen atoms, recently proposed in Ref. 1. We find that the Jastrow correlated Antisymmetrized Geminal Power (JAGP) is able to recover most of the correlation energy even when the geometry is symmetric and the hydrogens lie on the edges of a perfect square. Under such conditions the diradical character of the molecule ground state prevents a single determinant ansatz to achieve an acceptable accuracy, whereas the JAGP performs very well for all geometries. Remarkably, this is obtained with a similar computational effort. Moreover we find that the Jastrow Factor is fundamental in promoting the correct resonances among several configurations in the JAGP, that cannot show up in the pure Antisymmetrized Geminal Power (AGP). We also show the extremely fast convergence of this approach in the extension of the basis set. Remarkably only the simultaneous optimization of the Jastrow and the AGP part of our variational ansatz is able to recover an almost perfect nodal surface, yielding therefore state of the art energies, almost converged in the complete basis set limit (CBS), when the so called Diffusion Monte Carlo is applied., Comment: Accepted on Journal of Chemical Physics