Properties of reactive oxygen species by quantum Monte Carlo

Abstract

The electronic properties of the oxygen molecule, in its singlet and triplet states, and of many small oxygen-containing radicals and anions have important roles in different fields of Chemistry, Biology and Atmospheric Science. Nevertheless, the electronic structure of such species is a challenge for ab-initio computational approaches because of the difficulties to correctly describe the statical and dynamical correlation effects in presence of one or more unpaired electrons. Only the highestlevel quantum chemical approaches can yield reliable characterizations of their molecular properties, such as binding energies, equilibrium structures, molecular vibrations, charge distribution and polarizabilities. In this work we use the variational Monte Carlo (VMC) and the lattice regularized Monte Carlo (LRDMC) methods to investigate the equilibrium geometries and molecular properties of oxygen and oxygen reactive species. Quantum Monte Carlo methods are used in combination with the Jastrow Antisymmetrized Geminal Power (JAGP) wave function ansatz, which has been recently shown to effectively describe the statical and dynamical correlation of different molecular systems. In particular we have studied the oxygen molecule, the superoxide anion, the nitric oxide radical and anion, the hydroxyl and hydroperoxyl radicals and their corresponding anions, and the hydrotrioxyl radical. Overall, the methodology was able to correctly describe the geometrical and electronic properties of these systems, through compact but fully-optimised basis sets and with a computational cost which scales as N 3 − N 4 , where N is the number of electrons. This work is therefore opening the way to the accurate study of the energetics and of the reactivity of large and complex oxygen species by first principles.

π-Conjugation in trans-1,3-Butadiene: Static and Dynamical Electronic Correlations Described through Quantum Monte Carlo

“…As a matter of fact, the charge fluctuations present in the JAGP can lead, even in the case of the breaking of the single bond, to an error in the estimation of the dissociation energies. We must stress here that these errors are not so dramatic, as seen in the study of the dissociation curves of the triplet state of the O 2 molecule (in the appendix of ref 48) and in that of the water dimer (in refs 35 and 42).…”

First-Principles Monte Carlo Simulations of Reaction Equilibria in Compressed Vapors

“…This very substantial increase is caused by substantially stronger NO–NO interactions compared to O 2 –O 2 and N 2 –N 2 interactions. 30 The stronger NO–NO interactions are due to NO’s permanent dipole (μ = 0.16 D) 31 and higher polarizability, 32 whereas N 2 and O 2 possess only permanent quadrupole moments. On average, the force-field based RxMC simulations of Smith and Triska 13 yield the smallest enhancement of x NO , whereas the Cheetah thermochemical code 28 , 29 yields the largest enhancement.…”

General correlated geminal ansatz for electronic structure calculations: exploiting Pfaffians in place of determinants

“…. DMC energy dispersion of the oxygen dimer with the JAGP, JsAGP [52] and JSD (with the SD obtained from DFT calculations): at large distance only the JAGP WF is size consistent. In the plot also the expectation value of the projected S 2 operator on the atoms for the JAGP that recovers at large distance the value of two isolated atoms.…”