On the link between the reaction force constant and conceptual DFT.

Context: The reaction force constant ( ), introduced by Professor Alejandro Toro-Labbé, plays a pivotal role in characterizing the reaction pathway by assessing the curvature of the potential energy profile along the intrinsic reaction coordinate. This study establishes a novel link between and the reactivity descriptors of conceptual density functional theory (c-DFT). Specifically, we derive expressions that relate the reaction force constant to nuclear softness and variations in chemical potential.

Clustering Methods for Vibro-Acoustic Sensing Features as a Potential Approach to Tissue Characterisation in Robot-Assisted Interventions.

This article provides a comprehensive analysis of the feature extraction methods applied to vibro-acoustic signals (VA signals) in the context of robot-assisted interventions. The primary objective is to extract valuable information from these signals to understand tissue behaviour better and build upon prior research. This study is divided into three key stages: feature extraction using the Cepstrum Transform (CT), Mel-Frequency Cepstral Coefficients (MFCCs), and Fast Chirplet Transform (FCT); dimensionality reduction employing techniques such as Principal Component Analysis (PCA), t-Distributed Stochastic Neighbour Embedding (t-SNE), and Uniform Manifold Approximation and Projection (UMAP); and, finally, classification using a nearest neighbours classifier.

DFT exchange: sharing perspectives on the workhorse of quantum chemistry and materials science.

In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022.

Electronegativity under Confinement.

The electronegativity concept was first formulated by Pauling in the first half of the 20th century to explain quantitatively the properties of chemical bonds between different types of atoms. Today, it is widely known that, in high-pressure regimes, the reactivity properties of atoms can change, and, thus, the bond patterns in molecules and solids are affected. In this work, we studied the effects of high pressure modeled by a confining potential on different definitions of electronegativity and, additionally, tested the accuracy of first-order perturbation theory in the context of density functional theory for confined atoms of the second row at the Hartree-Fock level.

Vascular Auscultation of Carotid Artery: Towards Biometric Identification and Verification of Individuals.

Background: Biometric sensing is a security method for protecting information and property. State-of-the-art biometric traits are behavioral and physiological in nature. However, they are vulnerable to tampering and forgery.

Links among the Fukui potential, the alchemical hardness and the local hardness of an atom in a molecule.

This paper presents a brief summary of the difficulty that resides in the definition of the elusive concept of local chemical hardness. We argue that a definition of local hardness should be useful to a reactivity principle and not just as a mere definition. We then continue with a formal discussion about the benefits and difficulties of using the Fukui potential, which is interpreted as an alchemical derivative (alchemical hardness), as descriptor of local hardness of molecules.

Coulomb Explosion of Multi-charged Atomic Alkaline Metal Clusters.

In the present work, a computational study of the Coulomb explosions of atomic metal clusters of the type X was carried out, X = (Li-Cs). The work was done within the Kohn-Sham methodology using the Born-Oppenheimer molecular dynamics approximation. The dominant fission channels were established and the electron bonding patterns were analyzed with the help of the Electron Localization Function (ELF).

Predicting Deprotonation Sites Using Alchemical Derivatives.

An alchemical transformation is any process, physical or fictitious, that connects two points in the chemical space. A particularly important transformation is the vanishing of a proton, whose energy can be linked to the proton dissociation enthalpy of acids. In this work we assess the reliability of alchemical derivatives in predicting the proton dissociation enthalpy of a diverse series of mono- and polyprotic molecules.

Cardiotocograph Data Classification Improvement by Using Empirical Mode Decomposition.

This work proposes to study the fetal heart rate (FHR) signal based on information about its dynamics as a signal resulting from the modulation by the autonomic nervous system. The analysis is performed using the complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) technique. The main idea is to extract a set of signal features based on that technique and also conventional time-domain features proposed in the literature in order to study their performance by using a support vector machine (SVM) as a classifier.

Electron Localization Function in Excited States: The Case of the Ultrafast Proton Transfer of the Salicylidene Methylamine.

The physical characterization of the chemical bond in the ground state has been a central theme to theoretical chemistry. Among many techniques, quantum chemical topology (QCT) has emerged as a robust technique to understand the features of the chemical bond and electron organization within molecules. One consolidate tool within QCT is the topological analysis of the electron localization function (ELF).

The Pauli principle and the confinement of electron pairs in a double well: Aspects of electronic bonding under pressure.

It has become recently clear that chemical bonding under pressure is still lacking guiding principles for understanding the way electrons reorganize when their volume is constrained. As an example, it has recently been shown that simple metals can become insulators (aka electrides) when submitted to high enough pressures. This has lead to the general believe that "a fundamental yet empirically useful understanding of how pressure alters the chemistry of the elements is lacking" [R.

Local electrophilicity.

In this work some possibilities for deriving a local electrophilicity are studied. First, we consider the original definition proposed by Chattaraj, Maiti, and Sarkar (J Phys Chem A 107:4973, 2003), in which the local electrophilicity is given by the product of the global electrophilicity, and the Fukui function for charge acceptance is derived by two different approaches, making use of the chain rule for functional derivatives. We also modify the proposals based on the electron density so as to have a definition with the same units of the original definition, which also introduces a dependence in the Fukui function for charge donation.

Proposal of a simple and effective local reactivity descriptor through a topological analysis of an orbital-weighted fukui function.

The prediction of reactivity is one of the long-standing objectives of chemistry, contributing to enforce the link between theory and experiment. In particular, the regioselectivity of aromatic molecules has motivated the proposal of different reactivity descriptors based on foundational theories, like Frontier Molecular Orbital (FMO) theory and density functional theory, to predict and rationalize such regioselectivity. This article examines cases where reactivity descriptors, based on FMO theories, are known to have failed, specifically on electrophilic aromatic substitution reactions, through a simple but effective new reactivity model: the Orbital-weighted Fukui function ( fw-(r)) and its topological analysis.

Ferromagnetic bond of Li10 cluster: An alternative approach in terms of effective ferromagnetic sites.

In this work, a model to explain the unusual stability of atomic lithium clusters in their highest spin multiplicity is presented and used to describe the ferromagnetic bonding of high-spin Li10 and Li8 clusters. The model associates the (lack of-)fitness of Heisenberg Hamiltonian with the degree of (de-)localization of the valence electrons in the cluster. It is shown that a regular Heisenberg Hamiltonian with four coupling constants cannot fully explain the energy of the different spin states.

Lewis Acidity/Basicity Changes in Imidazolium Based Ionic Liquids Brought About by Impurities.

We herein report on the effect that water molecules, present as impurities, in the vicinity of an ionic liquid model structure, may induce on the Lewis acidity/basicity patterns normally observed in these materials. Depending on the position and orientation of water, the Lewis acidity/basicity pattern changes from "normal distribution" (i.e.

Regional electrophilic and nucleophilic Fukui functions efficiently highlight the Lewis acidic/basic regions in ionic liquids.

The origin of catalysis and selectivity induced by room temperature ionic liquids in several organic reactions has putatively been associated with the concept of cation effect (hydrogen bond donor ability of the ionic liquids) or anion effect (hydrogen bond accepting ability of the ionic liquids). We show that there may be cases where this a priori classification may not be correctly assigned. Cations may concentrate both Lewis acidity and basicity functions in one fragment of the ionic liquid: an effect we tentatively call bifunctional distribution of the molecular Lewis acidity/basicity.

An information-theoretic resolution of the ambiguity in the local hardness.

The ambiguity of the local hardness is resolved by using information theory to select definitions of the local hardness that are as close as possible to a well-defined approximate formula for the local hardness. A condensed local hardness is derived by using the atomic hardnesses as a reference distribution; a pointwise local hardness is derived by using the uniform electron gas as a reference distribution. This information-theoretic condensed local hardness is tested by examining electrophilic attack on some substituted pyridines.

How to Compute the Fukui Matrix and Function for Systems with (Quasi-)Degenerate States.

A system in a spatially (quasi-)degenerate ground state responds in a qualitatively different way to a change in the external potential. Consequently, the usual method for computing the Fukui function, namely, taking the difference between the electron densities of the N- and N ± 1 electron systems, cannot be applied directly. It is shown how the Fukui matrix, and thus also the Fukui function, depends on the nature of the perturbation.

Atomic Charges and the Electrostatic Potential Are Ill-Defined in Degenerate Ground States.

A system in a spatially degenerate ground state responds in a qualitatively different way to positive and negative point charges. This means that the molecular electrostatic potential is ill-defined for degenerate ground states due to the ill-defined nature of the electron density. It also means that it is impossible, in practice, to define fixed atomic charges for molecular mechanics simulations of molecules with (quasi-)degenerate ground states.

Comment on "Going beyond the frozen core approximation: development of coordinate-dependent pseudopotentials and application to Na2(+)" [J. Chem. Phys. 138, 054110 (2013)].

The new coordinate-dependent pseudopotential for Na2(+) by Kahros and Schwartz [J. Chem. Phys.

Effect of unsaturation on the absorption of ethane and ethylene in imidazolium-based ionic liquids.

The influence of the presence of imidazolium side chain unsaturation on the solubility of ethane and ethylene was studied in three ionic liquids: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide-saturated alkyl side-chain in the cation; 1-methyl-3-(buten-3-yl)imidazolium bis(trifluorosulfonyl)imide-double bond in the side-chain of the cation; and 1-methyl-3-benzylimidazolium bis(trifluorosulfonyl)imide-benzyl group in the side-chain of the cation. The solubility of both gases decreases when the side-chain of the cations is functionalized with an unsaturated group. This can be explained by a less favorable enthalpy of solvation.

On the exponential model for energy with respect to number of electrons.

Using an exponential model for the variation in energy with respect to the number of electrons it is shown that, within the model, the hardness, softness, electrophilicity and other global parameters connected to higher order derivatives follow an equalization principle after a molecule is formed from two separated species. Two generalizations of the model are also discussed, one of which presents discontinuity of the chemical potential at integer values of N.

A new isomer of C20 and a way to a new C240.

Here we show that the dynamic simulation of a molecular collision can give insight into new molecular species. In this way, a new stable isomer of C(20) (IV) has been found. It is planar with pentagonal form.

Assembling Small Silicon Clusters Using Criteria of Maximum Matching of the Fukui Functions.

In this work, we present a methodology inspired by criteria of "maximum matching" between the Fukui functions to predict the best interaction between small silicon clusters to form larger ones. The model is based on the topological analysis of the Fukui functions. We tested the methodology in the formation of Si4-Si8 using a set of small Si2-Si6 clusters as building blocks in ground state structures in singlet and triplet multiplicities.

A computationally efficient and reliable bond order measure.

Bond order indexes are useful measures that connect quantum mechanical results with chemical understanding. One of these measures, the natural bond order index, based on the natural resonance theory procedure and part of the natural bond orbital analysis tools, has been proved to yield reliable results for many systems. The procedure's computational requirements, nevertheless, scales so highly with the number of functions in the basis set and the delocalization of the system, that the calculation of this bond order is limited to small or medium size molecules.