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.In recent years much progress has been made in the definition of variational wave functions (WF) capable to describe rather accurately the electron correlation. To this purpose two strategies have been employed: i) the use of multi-determinant wave functions 2-4 or ii) exploiting the large variational freedom that can be achieved by applying a correlation term, dubbed Jastrow factor (JF), to a generic pairing function 5-7 . Even if the latter approach cannot be systematically improved, it may open the way to deal with large systems, thanks to the moderate scaling with the number of electrons. Indeed, the corresponding correlated WF, can be simulated efficiently within a statistical method, based on quantum Monte Carlo 8 . Thanks to well established advances 9,10 in this field, it is possible nowadays to compute the total energy of a given correlated ansatz and to optimize several variational parameters with a computational effort scaling at most with the fourth power of the number of electrons.A good variational ansatz allows a good description of the ground state by energy optimization. Moreover an even better characterization can be obtain by applying the so called diffusion Monte Carlo (DMC) method with the Fixed Node approximation (FNA) 11,12 . Within this projection method it is possible to obtain the lowest energy state constrained to have the same signs of a chosen trial WF, in the configuration space where electron positions and spins are given. The connected regions of space with the same sign are called nodal pockets and the surface determining this pockets, satisfying WF= 0, the nodal surface. Usually the energy optimization, implemented here, has been shown to be very successful to determine the nodal surface of the WF as we will show also in the present study. In this w...
The recently synthesized hydrogen boride monolayer in the Cmmm phase is a promising superconductor due to its similarity to MgB2 and the large hydrogen content in its structure. Making use of first-principles calculations based on density functional theory, we study its electronic, vibrational, and superconducting properties and conclude that, despite the expectations, hydrogen boride does not have a sizable superconducting critical temperature. The presence of hydrogen in the system alters the boron-boron bonding, weakening the electron-phonon interaction. We have studied the effect of enhancing the critical temperature by doping the system, but the inclusion of electrons or holes reveals ineffective. We attribute the small critical temperature of this system to the vanishing hydrogen character of the states at the Fermi level, which are dominated by boron p states. Our results hint at a possible relation between the presence of a large proportion of hydrogen-like states at the Fermi level and a large superconducting critical temperature in hydrogenated monolayers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.