arXiv subjects

Michal Bajdich

Publications and source records attributed to Michal Bajdich.

11 recordsLinked to original sources

arXiv2026

Molecular reference corrections for quantum Monte Carlo adsorption energies

Accurate surface thermochemistry requires balanced error cancellation between extended slabs and molecular reference states. This balance can fail whenever the electronic-structure error is not transferable across the chemically distinct species entering a thermodynamic cycle. Here we examine this problem in single-determinant fixed-node diffusion Monte Carlo (SD-FNDMC) for oxygen-reduction intermediates on Pt(111) and selected CO-reduction intermediates on Cu(111). Gas-phase thermochemistry is used to diagnose the reference-state imbalance, and a hybrid cycle is introduced to separate slab--adsorbate binding from molecular formation. The hybrid cycle keeps the surface binding term at the SD-FNDMC level, where cancellation is expected to be most favorable, and replaces the molecular formation contribution with a benchmark coupled cluster reference. For Pt(111), the resulting correction is small for O and OH but larger for OOH, while the geometry-matched refinement gives only a secondary correction. Applying the same cycle to $^\ast$CHO and $^\ast$COH adsorption on Cu(111) gives corrections of opposite sign, showing that the bias is controlled primarily by the electronic structure of the corresponding HCO and COH molecular references rather than by adsorbate geometry alone. This decomposition identifies molecular reference imbalance as a separable source of error in SD-FNDMC surface thermochemistry and reduces the corresponding bias without modifying the SD-FNDMC slab-binding contribution.

cond-mat.mtrl-sci↗

arXiv2015

Surface Energetics of Alkaline-Earth Metal Oxides: Trends in Stability and Adsorption of Small Molecules

We present a systematic theoretical investigation of the surface properties, stability and reactivity, of rock-salt type alkaline-earth metal oxides including MgO, CaO, SrO, and BaO. The accuracy of commonly used exchange-correlation density functionals (LDA, PBE, RPBE, PBEsol, BEEF-vdW and hybrid HSE) and random-phase approximation (RPA) is evaluated and compared to existing experimental values. Calculated surface energies of the four most stable surface facets under vacuum conditions: the (100) surface, the metal and oxygen terminated octopolar (111), and the (110) surfaces exhibit a monotonic increase in stability from MgO to BaO. On the MgO(100) surface, adsorption of CO, NO, CH4 is characterized by physisorption while H2O chemisorbs, which is in agreement with experimental findings. We further use the on-top metal adsorption of CO and NO molecules to map out the surface energetics of each alkaline-earth metal oxide surface. The considered functionals all qualitatively predict similar adsorption energy trends. The ordering between the adsorption energies on different surface facets can be attributed to differences in the local geometrical surface structure and the electronic structure of the metal constituent of the alkaline-earth metal oxide. The striking observation that CO adsorption strength is weaker than NO adsorption on the (100) terraces as the period of the alkaline-earth metal in the oxide increases, is analyzed in detail in terms of charge redistribution within the σ and π channels of adsorbates. Finally, we also present oxygen adsorption and oxygen vacancy formation energies in these oxide systems.

cond-mat.mtrl-sci↗

arXiv2013

Study of dipole moments of LiSr and KRb molecules by quantum Monte Carlo methods

Heteronuclear dimers are of significant interest to experiments seeking to exploit ultracold polar molecules in a number of novel ways including precision measurement, quantum computing, and quantum simulation. We calculate highly accurate Born-Oppenheimer total energies and electric dipole moments as a function of internuclear separation for two such dimers, LiSr and KRb. We apply fully-correlated, high-accuracy quantum Monte Carlo methods for evaluating these molecular properties in a many-body framework. We use small-core effective potentials combined with multi-reference Slater-Jastrow trial wave functions to provide accurate nodes for the fixed-node diffusion Monte Carlo method. For reference and comparison, we calculate the same properties with Hartree-Fock and with restricted Configuration Interaction methods, and carefully assess the impact of the recovered many-body correlations on the calculated quantities. For LiSr we find a highly nonlinear dipole moment curve, which may make this molecule's dipole moment tunable through vibrational state control.

quant-ph↗

arXiv2011

Simple Impurity Embedded in a Spherical Jellium: Approximations of Density Functional Theory compared to Quantum Monte Carlo Benchmarks

We study the electronic structure of a spherical jellium in the presence of a central Gaussian impurity. We test how well the resulting inhomogeneity effects beyond spherical jellium are reproduced by several approximations of density functional theory (DFT). Four rungs of Perdew's ladder of DFT functionals, namely local density approximation (LDA), generalized gradient approximation (GGA), meta-GGA and orbital-dependent hybrid functionals are compared against our quantum Monte Carlo (QMC) benchmarks. We identify several distinct transitions in the ground state of the system as the electronic occupation changes between delocalized and localized states. We examine the parameter space of realistic densities ($1 \le r_s\le 5$) and moderate depths of the Gaussian impurity ($Z<7$). The selected 18 electron system (with closed-shell ground state) presents $1d \to 2s$ transitions while the 30 electron system (with open-shell ground state) exhibits $1f \to 2p$ transitions. For the former system, the accuracy for the transitions is clearly improving with increasing sophistication of functionals with meta-GGA and hybrid functionals having only small deviations from QMC. However, for the latter system, we find much larger differences for the underlying transitions between our pool of DFT functionals and QMC. We attribute this failure to treatment of the exact exchange within these functionals. Additionally, we amplify the inhomogeneity effects by creating the system with spherical shell which leads to even larger errors in DFT approximations.

cond-mat.mtrl-sci↗

arXiv2010

Electronic structure quantum Monte Carlo

Quantum Monte Carlo (QMC) is an advanced simulation methodology for studies of manybody quantum systems. In this review, we focus on the electronic structure QMC, i.e., methods relevant for systems described by the electron-ion Hamiltonians. Some of the key QMC achievements include direct treatment of electron correlation, accuracy in predicting energy differences and favorable scaling in the system size. Calculations of atoms, molecules, clusters and solids have demonstrated QMC applicability to real systems with hundreds of electrons while providing 90-95% of the correlation energy and energy differences typically within a few percent of experiments. Advances in accuracy beyond these limits are hampered by the so-called fixed-node approximation which is used to circumvent the notorious fermion sign problem. Many-body nodes of fermion states and their properties have therefore become one of the important topics for further progress in predictive power and efficiency of QMC calculations. Some of our recent results on the wave function nodes and related nodal domain topologies will be briefly reviewed. This includes analysis of few-electron systems and descriptions of exact and approximate nodes using transformations and projections of the highly-dimensional nodal hypersurfaces into the 3D space. Studies of fermion nodes offer new insights into topological properties of eigenstates such as explicit demonstrations that generic fermionic ground states exhibit the minimal number of two nodal domains. Recently proposed trial wave functions based on Pfaffians with pairing orbitals are presented and their nodal properties are tested in calculations of first row atoms and molecules. Finally, backflow "dressed" coordinates are introduced as another possibility for capturing correlation effects and for decreasing the fixed-node bias.

cond-mat.other↗

arXiv2010

Quantum Monte Carlo Calculations of Dihydrogen Binding Energetics on Ca Cations: an Assessment of Errors in Density Functionals for Weakly Bonded Systems

We investigate the binding of single and quadruple hydrogen molecules on a positively charged Ca ion. By comparing with benchmark quantum Monte Carlo (QMC) calculations we demonstrate wide variability in other more approximate electronic structure methods including common density functionals. Single determinant QMC calculations find no binding at short range by approximately 0.1 eV for the quadruple hydrogen molecule case, for a fixed hydrogen bond length of 0.77 Angstrom. Density functional calculations using common functionals such a LDA and B3LYP differ substantially from the QMC binding curve. We show that use of full Hartree-Fock exchange and PBE correlation(HFX+PBEC) obtains close agreement with the QMC results, both qualitatively and quantitatively. These results both motivate the use and development of improved functionals and indicate that caution is required applying electronic structure methods to weakly bound systems such as hydrogen storage materials based on metal ion decorated nanostructures.

cond-mat.mtrl-sci↗

arXiv2010

Systematic reduction of sign errors in many-body calculations of atoms and molecules

The self-healing diffusion Monte Carlo algorithm (SHDMC) [Phys. Rev. B {\bf 79}, 195117 (2009), {\it ibid.} {\bf 80}, 125110 (2009)] is shown to be an accurate and robust method for calculating the ground state of atoms and molecules. By direct comparison with accurate configuration interaction results for the oxygen atom we show that SHDMC converges systematically towards the ground-state wave function. We present results for the challenging N$_2$ molecule, where the binding energies obtained via both energy minimization and SHDMC are near chemical accuracy (1 kcal/mol). Moreover, we demonstrate that SHDMC is robust enough to find the nodal surface for systems at least as large as C$_{20}$ starting from random coefficients. SHDMC is a linear-scaling method, in the degrees of freedom of the nodes, that systematically reduces the fermion sign problem.

cond-mat.str-el↗

arXiv2007

Generalized Pairing Wave Functions and Nodal Properties for Electronic Structure Quantum Monte Carlo

The quantum Monte Carlo (QMC) is one of the most promising many-body electronic structure approaches. It employs stochastic techniques for solving the stationary Schr\" odinger equation and for evaluation of expectation values. The key advantage of QMC is its capability to use the explicitly correlated wave functions, which allow the study of many-body effects beyond the reach of mean-field methods. The most important limit on QMC accuracy is the fixed-node approximation, which comes from necessity to circumvent the fermion sign problem. The size of resulting fixed-node errors depends on the quality of the nodes (the subset of position space where the wave function vanishes) of a used wave function. In this dissertation, we analyze the nodal properties of the existing fermionic wave functions and offer new types of variational wave functions with improved nodal structure.

cond-mat.other↗

arXiv2007

QWalk: A Quantum Monte Carlo Program for Electronic Structure

We describe QWalk, a new computational package capable of performing Quantum Monte Carlo electronic structure calculations for molecules and solids with many electrons. We describe the structure of the program and its implementation of Quantum Monte Carlo methods. It is open-source, licensed under the GPL, and available at the web site http://www.qwalk.org

physics.comp-ph↗

arXiv2005

Approximate and exact nodes of fermionic wavefunctions: coordinate transformations and topologies

A study of fermion nodes for spin-polarized states of a few-electron ions and molecules with $s,p,d$ one-particle orbitals is presented. We find exact nodes for some cases of two electron atomic and molecular states and also the first exact node for the three-electron atomic system in $^4S(p^3)$ state using appropriate coordinate maps and wavefunction symmetries. We analyze the cases of nodes for larger number of electrons in the Hartree-Fock approximation and for some cases we find transformations for projecting the high-dimensional node manifolds into 3D space. The node topologies and other properties are studied using these projections. We also propose a general coordinate transformation as an extension of Feynman-Cohen backflow coordinates to both simplify the nodal description and as a new variational freedom for quantum Monte Carlo trial wavefunctions.

cond-mat.other↗

arXiv2003

Coherent "metallic" resistance and medium localisation in a disordered 1D insulator

It is believed, that a disordered one-dimensional (1D) wire with coherent electronic conduction is an insulator with the mean resistance <ρ> \simeq e^{2L/ξ} and resistance dispersion Δ_ρ \simeq e^{L/ξ}, where L is the wire length and ξis the electron localisation length. Here we show that this 1D insulator undergoes at full coherence the crossover to a 1D "metal", caused by thermal smearing and resonant tunnelling. As a result, Δ_ρ is smaller than unity and tends to be L/ξ- independent, while <ρ> grows with L/ξfirst nearly linearly and then polynomially, manifesting the so-called medium localisation.

cond-mat.mes-hall↗