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Revista de la Sociedad Química de México

von Neumann Entropies Analysis in Hilbert Space for the Dissociation Processes of Homonuclear and Heteronuclear Diatomic Molecules

Nelson Flores-Gallegos, Rodolfo O. Esquivel

Published Mar 1, 2008• 11 citations
Publisher

Abstract

Quantum Information Theory is a new field with potential implications for the conceptual foundations of Quantum Mechanics through density matrices. In particular, information entropies in Hilbert space representation are highly advantageous in contrast with the ones in phase space representation since they can be easily calculated for large systems. In this work, novel von Neumann conditional, mutual, and joint entropies are employed to analyze the dissociation process of small molecules, Cl2 and HCl, by using the spectral decomposition of the first reduced density matrix in natural atomic orbital-based representation which allows us to assure rotational invariance, N- and v-representability in the Atoms-in-Molecules (AIM) scheme. Quantum information entropies permit to analyze the dissociation process through quantum mechanics concepts such as electron correlation and entanglement, showing interesting critical points which are not present in the energy profile, such as charge depletion and accumulation, along with bond breaking regions.

DOI
10.29356/jmcs.v52i1.1042

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Quantum Information Theory is a new field with potential implications for the conceptual foundations of Quantum Mechanics through density matrices. In particular, information entropies in Hilbert space representation are highly advantageous in contrast with the ones in phase space representation since they can be easily calculated for large systems. In this work, novel von Neumann conditional, mutual, and joint entropies are employed to analyze the dissociation process of small molecules, Cl2 and HCl, by using the spectral decomposition of the first reduced density matrix in natural atomic orbital-based representation which allows us to assure rotational invariance, N- and v-representability in the Atoms-in-Molecules (AIM) scheme. Quantum information entropies permit to analyze the dissociation process through quantum mechanics concepts such as electron correlation and entanglement, showing interesting critical points which are not present in the energy profile, such as charge depletion and accumulation, along with bond breaking regions.
The accurate calculation of the (differential) correlation energy is central to the quantum chemical description of bond-formation and bond-dissociation processes. In order to estimate the quality of single- and multireference approaches for this purpose, various diagnostic tools have been developed. In this work, we elaborate on our previous observation [J. Phys. Chem. Lett.2012, 3, 3129] that one- and two-orbital-based entanglement measures provide quantitative means for the assessment and classification of electron correlation effects among molecular orbitals. The dissociation behavior of some prototypical diatomic molecules features all types of correlation effects relevant for chemical bonding. We demonstrate that our entanglement analysis is convenient to dissect these electron correlation effects and to provide a conceptual understanding of bond-forming and bond-breaking processes from the point of view of quantum information theory.
By using quantum information theory metrics, we have studied the relationship between quantum correlation and bond dissociation in hydrogen fluoride. Von Neumann entropy, concurrence and quantum discord are calculated as the bond length of H-F is stretched. These are analyzed in comparison with classical descriptor like dipole moment of H-F and Mulliken charge fluorine. Laplacian contour maps also visualize electron concentration and depletion during bond stretching. At equilibrium bond length, quantum correlations and bond polarization both are optimally balanced. As the bond length increases, charge localize on Fluorine but entanglement reduces leading to bond dissociation. At large separations, quantum correlation which are still residual persist near dissociation. These findings provides a critical role of quantum correlations that are involve in stabilizing strongly polar bond. It gives quantitative insights into interplay electronic structure and quantum information theory properties during bond dissociation.
Entanglement is the fundamental difference between classical and quantum systems and has become one of the guiding principles in the exploration of high-and low-energy physics.The calculation of entanglement entropies in interacting quantum field theories, however, remains challenging.Here, we present the first method for the explicit computation of reduced density matrices of interacting quantum field theories using truncated Hamiltonian methods.The method is based on constructing an isomorphism between the Hilbert space of the full system and the tensor product of Hilbert spaces of subintervals.This naturally enables the computation of the von Neumann and arbitrary Rényi entanglement entropies as well as mutual information, logarithmic negativity, and other measures of entanglement.Our method is applicable to equilibrium states and nonequilibrium evolution in real time.It is model independent and can be applied to any Hamiltonian truncation method that uses a free basis expansion.We benchmark the method on the free Klein-Gordon theory finding excellent agreement with the analytic results.We further demonstrate its potential on the interacting sine-Gordon model, studying the scaling of von Neumann entropy in ground states and real-time dynamics following quenches of the model.
Pair density Shannon information measures and their NNN-dependent behaviour in diatomic molecular systems
Pair density Shannon entropies and mutual information in position and in momentum space are calculated for two series of homonuclear and heteronuclear diatomic molecules using Hartree-Fock wave functions. The pair density entropy sum increases with the quality of the basis set. Mutual information, a measure of statistical correlation, is seen to be smaller in momentum space as compared to position space. The interpretation is that the momentum pair density is closer than the position pair density to a Hartree-like reference. The NNN-dependent behaviours of the entropy and mutual information sums are examined by fitting the data to three different model behaviours. Results show that these models are capable of representing the molecular data in differing degrees. The parameters obtained from a particular model are relatively constant across different chemical series. This adds evidence to the argument of a universal NNN-dependent behaviour of the entropy sums.
The dynamics and processes involved in particle-molecule scattering, including nuclear dynamics, are described and analyzed using various quantum information quantities throughout the different stages of the scattering. The main process studied and characterized with the information quantities is the interatomic Coulombic electronic capture, an inelastic process that can lead to dissociation of the target molecule. The analysis focuses on a one-dimensional transversely confined NeHe molecule model used to simulate the scattering between an electron ${e}^{\ensuremath{-}}(\text{particle})$ and a NeHe+{\text{NeHe}}^{+}NeHe+ ion (molecule). The time-independent Schr\"odinger equation is solved using the finite element method with a self-developed Julia package FEMTISE to compute potential energy curves and the parameters of the interactions between particles. The time-dependent Schr\"odinger equation is solved using the multiconfiguration time-dependent Hartree algorithm. The time-dependent electronic and nuclear probability densities are calculated for different electron incoming energies, evidencing elastic and inelastic processes that can be correlated to changes in von Neumann entropy, von Neumann mutual information, and Shannon mutual information. The expectation value of the position of the particles, as well as their standard deviations, are analyzed along the whole dynamics and related to the entanglement during the collision and after the process is over, thus highlighting the dynamics of entanglement generation. It is shown that the correlations generated in the collision are partially retained only when the inelastic process is active.