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Physical Review A

Information entropy as a measure of the correlation energy associated with the cumulant

Yu Wang, Peter J. Knowles, Jian Wang

Published Jun 15, 2021• 19 citations
Publisher

Abstract

A linear relation is found between Jaynes entropy and the correlation energy associated with the cumulant for a correlated molecular system, as demonstrated in ${\mathrm{H}}_{2}$ and a water molecule. The relation modifies the Collins conjecture, in which the correlation energy is defined as the total energy difference with reference to the Hartree-Fock counterpart. The linear behavior is system dependent and shifts slightly on a different basis set. This correlation energy is extensive. For a system whose wave function can be expressed as the antisymmetrized product of orthogonal groups, the correlation energy is just the sum of the individual groups. The finding offers a convenient way to construct an approximate correlation energy functional in reduced density-matrix-functional theory, which may be used in large-scale molecular dynamics simulations and potential energy surface calculations where the molecular geometry needs to be frequently updated.

DOI
10.1103/physreva.103.062808

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A linear relation is found between Jaynes entropy and the correlation energy associated with the cumulant for a correlated molecular system, as demonstrated in H2{\mathrm{H}}_{2}H2​ and a water molecule. The relation modifies the Collins conjecture, in which the correlation energy is defined as the total energy difference with reference to the Hartree-Fock counterpart. The linear behavior is system dependent and shifts slightly on a different basis set. This correlation energy is extensive. For a system whose wave function can be expressed as the antisymmetrized product of orthogonal groups, the correlation energy is just the sum of the individual groups. The finding offers a convenient way to construct an approximate correlation energy functional in reduced density-matrix-functional theory, which may be used in large-scale molecular dynamics simulations and potential energy surface calculations where the molecular geometry needs to be frequently updated.
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 Jaynes entropy of the one-particle reduced density matrix is examined within the context of the convex set of N-representable one-matrices. The monotonic relationship of the Jaynes entropy with distance from the hull of the convex set is highlighted, and we show that the Jaynes entropy is monotonically related to the density-functional definition of the correlation energy, thus qualifying a conjecture of Collins which assumed a simple proportionality. It is further suggested that maximization of the Jaynes entropy subject to the constraints of fixed density and kinetic energy picks out the true one-matrix.
Accurately treating strong electron correlation in quantum chemistry typically requires multireference wave-function methods with steep computational scaling. The recently proposed i-DMFT method promises near configuration-interaction accuracy at mean-field cost by invoking an empirical linear relation between correlation energy and entropy (Collins' conjecture), whose validity remains unclear. We systematically assess this relation across a range of di- and polyatomic molecules, including diverse bond types, third-row elements, different types of geometric distortions, and excited states. We find that the conjectured linearity holds for bond-breaking processes dominated by electron redistribution within orbital pairs, but breaks down for heterolytic dissociation and excited states. In simple molecules, i-DMFT provides a reasonable description of total energies, but does not reliably reproduce reduced density matrices or individual energy components. It further degrades in more complex cases such as ethylene. Based on these results, we formulate criteria for the validity of the conjecture and outline implications for entropy-based reduced density matrix functionals.
Abstract In the present work, we present an investigation of Collins Conjecture, a hypothesis made by D. M. Collins in 1993 relating correlation energy and entanglement entropy, by calculating the ground state and singly‐excited triplet‐spin 1s2s 3S and 1s3s 3S state energies of the helium iso‐electronic sequence, with Z = 2–15. By using extensive orthonormal configuration interaction (CI) type wave functions with B‐Spline basis up to about 6000 terms, linear entropy and von Neumann entropy for the abovementioned atomic systems are determined. Together with the Hartree‐Fock energies obtained following a self‐consistent field theory, we have found that there exist linearly proportionalities between the renormalized correlation energies and entanglement entropies of both linear and von Neumann, showing a support for Collins Conjecture applicable to the ground and singly‐excited triplet‐spin states in the helium sequence, for a range of finite Z‐values.
The sums of position- and momentum-space mutual information measures are used to examine the pairwise and higher-order statistical correlation in the ground states of N-particle coupled oscillators. Analytical expressions for these measures are shown to be related to the logarithmic interaction energies of these states, plus those of mirror states where the intensities of the one- and two-body potentials are interchanged, and the nature of the attractive or repulsive interaction is opposite to that in the original state. The measures separate the contributions from the interactions and those from the effective interactions due to marginalization into different terms. The pairwise mutual information sum is linearly related to the Shannon entropy sum in two particle systems, while the total correlation sum exhibits a similar relationship in three particle ones. In the latter instance, the interaction information sum can be related to entropy differences. This illustrates how entropy sums are connected to correlation measure sums in these systems. All measures approach zero with large N, when the magnitudes of the one- and two-body potentials are fixed. The pair mutual information and total correlation sums decay monotonically with N in the presence of an attractive potential and monotonically increase with a repulsive potential. On the other hand, the interaction information sum exhibits a minimum at small N, with an attractive potential. This is a consequence of the higher-order correlations governing behavior at smaller N while the pairwise ones dominate at larger N. Results are presented when the magnitude of the one-body potential is set to N.
In this work, we investigate the reliability of information-theoretic measures based on the electron-density and shape-function, specifically Shannon and Rényi entropies, as descriptors of electronic correlation. By establishing a rigorous decomposition of these entropic measures into additive and nonadditive contributions, supported on a Mulliken-like atomic partition of molecules, we systematically analyze the asymptotic behavior of the entropies at the infinite-internuclear-distance limit to assess the problem of static correlation and extensivity. Our algebraic and numerical analysis reveals several flaws in the use of these density-based descriptors. We demonstrate that for minimal-basis and different theoretical levels, the Shannon and Rényi entropies fail to encode the amount of static correlation conveyed by the underlying wavefunction. Conversely, shape-function Shannon entropies and Rényi entropies (for α≠1α\neq 1α=1) violate extensivity. In larger basis sets, uncorrelated Hartree-Fock densities consistently overestimate entropy compared to sufficiently correlated (e.g., full-valence-CAS) densities. Moreover, the entropies for insufficiently correlated methods violate extensivity. These findings indicate that electron-density-based measures are insufficient for capturing static correlation, suggesting that robust entropic descriptors should be constructed from higher-dimensional Hilbert-space objects.