143 research outputs found
Natural orbitals and sparsity of quantum mutual information
Natural orbitals, defined in electronic structure and quantum chemistry as
the (molecular) orbitals diagonalizing the one-particle reduced density matrix
of the ground state, have been conjectured for decades to be the perfect
reference orbitals to describe electron correlation. In the present work we
applied the Wavefunction-Adapted Hamiltonian Through Orbital Rotation (WAHTOR)
method to study correlated empirical ans\"atze for quantum computing. In all
representative molecules considered, we show that the converged orbitals are
coinciding with natural orbitals. Interestingly, the resulting quantum mutual
information matrix built on such orbitals is also maximally sparse, providing a
clear picture that such orbital choice is indeed able to provide the optimal
basis to describe electron correlation. The correlation is therefore encoded in
a smaller number of qubit pairs contributing to the quantum mutual information
matrix.Comment: 11 pages + supplementary meteria
Properties of Reactive Oxygen Species by Quantum Monte Carlo
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 highest-level 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
, where 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
Early-stage formation of (hydr)oxo bridges in transition-metal catalysts for photosynthetic processes
Ab initio simulations have been used to assess reaction pathways for the formation of M–(hydr)oxo–M (M = Co, Mn, Ni) bridges from M(ii) aqueous solutions, as early-stage building blocks of transition-metal catalysts for oxygen evolution
Ab initio molecular dynamics simulation of liquid water by quantum Monte Carlo
Although liquid water is ubiquitous in chemical reactions at roots of life and climate on the earth, the prediction of its properties by high-level ab initio molecular dynamics simulations still represents a formidable task for quantum chemistry. In this article, we present a room temperature simulation of liquid water based on the potential energy surface obtained by a many-body wave function through quantum Monte Carlo (QMC) methods. The simulated properties are in good agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function, at variance of previous density functional theory attempts. Given the excellent performances of QMC on large scale supercomputers, this work opens new perspectives for predictive and reliable ab initio simulations of complex chemical systems
Complexation of halide ions to tyrosine: role of non-covalent interactions evidenced by IRMPD spectroscopy
The binding motifs in the halide adducts with tyrosine ([Tyr + X]-, X = Cl, Br, I) have been investigated
and compared with the analogues with 3-nitrotyrosine (nitroTyr), a biomarker of protein nitration, in a
solvent-free environment by mass-selected infrared multiple photon dissociation (IRMPD) spectroscopy
over two IR frequency ranges, namely 950–1950 and 2800–3700 cm-1. Extensive quantum chemical
calculations at B3LYP, B3LYP-D3 and MP2 levels of theory have been performed using the 6-311++G(d,p)
basis set to determine the geometry, relative energy and vibrational properties of likely isomers and
interpret the measured spectra. A diagnostic carbonyl stretching band at B1720 cm-1 from the intact
carboxylic group characterizes the IRMPD spectra of both [Tyr + X]- and [nitroTyr + X]-, revealing that
the canonical isomers (maintaining intact amino and carboxylic functions) are the prevalent structures.
The spectroscopic evidence reveals the presence of multiple non-covalent forms. The halide complexes
of tyrosine conform to a mixture of plane and phenol isomers. The contribution of phenol-bound
isomers is sensitive to anion size, increasing from chloride to iodide, consistent with the decreasing
basicity of the halide, with relative amounts depending on the relative energies of the respective
structures. The stability of the most favorable phenol isomer with respect to the reference plane
geometry is in fact 1.3, -2.1, -6.8 kJ mol-1, for X = Cl, Br, I, respectively. The change in p-acidity by ring
nitration also stabilizes anion–p interactions yielding ring isomers for [nitroTyr + X]-, where the anion is
placed above the face of the aromatic ring
Many-body study of the photoisomerization of the minimal model of the retinal protonated Schiff base
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