29 research outputs found
Quantum Monte Carlo method modeling supported metal catalysis: Ni(111) converting adsorbed formyl 'en route' to hydrogen
Hydrogen production as a clean, sustainable replacement for fossil fuels is
gathering pace. Doubling the capacity of Paris-CDG airport has been halted,
even with the upcoming Olympic Games, until hydrogen-powered planes can be
used.
It is thus timely to work on catalytic selective hydrogen production and
optimise catalyst structure. Over 90 % of all chemical manufacture uses a solid
catalyst. This work describes the dissociation of a C-H bond in formyl
radicals, chemisorbed at Ni(111) that stabilises the ensuing Ni-H linkage. As
part of this mechanistic step, gaseous hydrogen is given off.
Many chemical reactions involve bond-dissociation. This process is often the
key to rate-limiting reaction steps at solid surfaces.
Since bond-breaking is poorly described by Hartree-Fock and DFT methods, our
embedded active site approach is used. This work demonstrates Quantum Monte
Carlo (QMC) methodology using a very simple monolayer Ni(111) surface model.Comment: arXiv admin note: substantial text overlap with arXiv:2202.00542,
arXiv:2004.1056
Quantum Monte Carlo calculations of electronic excitation energies: the case of the singlet (CO) transition in acrolein
We report state-of-the-art quantum Monte Carlo calculations of the singlet (CO) vertical excitation energy in the acrolein molecule, extending
the recent study of Bouab\c{c}a {\it et al.} [J. Chem. Phys. {\bf 130}, 114107
(2009)]. We investigate the effect of using a Slater basis set instead of a
Gaussian basis set, and of using state-average versus state-specific
complete-active-space (CAS) wave functions, with or without reoptimization of
the coefficients of the configuration state functions (CSFs) and of the
orbitals in variational Monte Carlo (VMC). It is found that, with the Slater
basis set used here, both state-average and state-specific CAS(6,5) wave
functions give an accurate excitation energy in diffusion Monte Carlo (DMC),
with or without reoptimization of the CSF and orbital coefficients in the
presence of the Jastrow factor. In contrast, the CAS(2,2) wave functions
require reoptimization of the CSF and orbital coefficients to give a good DMC
excitation energy. Our best estimates of the vertical excitation energy are
between 3.86 and 3.89 eV.Comment: 6 pages, 1 figure, 2 tables, to appear in Progress in Theoretical
Chemistry and Physic
Quantum Monte Carlo facing the Hartree-Fock symmetry dilemma: The case of hydrogen rings
When using Hartree-Fock (HF) trial wave functions in quantum Monte Carlo
calculations, one faces, in case of HF instabilities, the HF symmetry dilemma
in choosing between the symmetry-adapted solution of higher HF energy and
symmetry-broken solutions of lower HF energies. In this work, we have examined
the HF symmetry dilemma in hydrogen rings which present singlet instabilities
for sufficiently large rings. We have found that the symmetry-adapted HF wave
function gives a lower energy both in variational Monte Carlo and in fixed-node
diffusion Monte Carlo. This indicates that the symmetry-adapted wave function
has more accurate nodes than the symmetry-broken wave functions, and thus
suggests that spatial symmetry is an important criterion for selecting good
trial wave functions.Comment: 6 pages, 3 figures, 2 tables, to appear in "Advances in Quantum Monte
Carlo", AC