Dispersion Energies with the i-DMFT Method.

The recently proposed i-DMFT method [Wang and Baerends, 013001 (2022)] has been proven to be ideally suited to recover strong static correlation in dissociating covalent bonds. Here, the application to van der Waals bonding is investigated using the prototype van der Waals systems triplet H and ground-state He. It is demonstrated that the i-DMFT orbitals are in this case essentially different from the natural orbitals, and the i-DMFT occupations differ substantially from the NO occupations.

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.

Secondary Kinetic Peak in the Kohn-Sham Potential and Its Connection to the Response Step.

We consider a prototypical 1D model Hamiltonian for a stretched heteronuclear molecule and construct individual components of the corresponding KS potential, namely, the kinetic, the - 1, and the conditional potentials. These components show very special features, such as peaks and steps, in regions where the density is drastically low. Some of these features are quite well-known, whereas others, such as a secondary peak in the kinetic potential or a second bump in the conditional potential, are less or not known at all.

Self-Consistent-Field Method for Correlated Many-Electron Systems with an Entropic Cumulant Energy.

A self-consistent field method is presented within density matrix functional theory. The computational cost for a correlated many-electron calculation is reduced to that of the self-consistent-field Hartree-Fock method, while the accuracy still reaches that of sophisticated configuration interaction based methods. In this method, the two-electron cumulant energy is measured with an information entropy associated with the Fermi-Dirac distribution of the occupation numbers.

Origin of the Enhanced Binding Capability toward Axial Nitrogen Bases of Ni(II) Porphyrins Bearing Electron-Withdrawing Substituents: An Electronic Structure and Bond Energy Analysis.

Axial coordination to metalloporphyrins is important in many biological and catalytic processes. Experiments found the axial coordination of nitrogenous bases to nickel(II) porphyrins to be strongly favored by electron-withdrawing substituents such as perfluorophenyls at the meso carbon positions. Careful analysis of the electronic structure reveals that the natural explanation in terms of density change of the nickel(II) porphyrin system (in particular the metal), does not apply.

Density functional approximations for orbital energies and total energies of molecules and solids.

The relation of Kohn-Sham (KS) orbital energies to ionization energies and electron affinities is different in molecules and solids. In molecules, the local density approximation (LDA) and generalized gradient approximations (GGA) approximate the exact ionization energy (I) and affinity (A) rather well with self-consistently calculated (total energy based) I and A, respectively. The highest occupied molecular orbital (HOMO) energy and lowest unoccupied molecular orbital (LUMO) energy, however, differ significantly (by typically 4-6 eV) from these quantities, ϵ(mol)>-I(mol)≈-I(mol), ϵ(mol)<-A(mol)≈-A(mol).

Light-induced water splitting by titanium-tetrahydroxide: a computational study.

Water oxidation by Ti(OH)4 in the ground and excited states was investigated using density functional (ΔSCF, TDDFT) methods gauged by the coupled cluster (CCSD, CCSD(T)) calculations. O2 and H2 are generated in a reaction sequence that starts with Ti(OH)4 reacting with H2O. This reaction can proceed by either nucleophilic attack by H2O or by H-atom abstraction from H2O.

Real-space representation of electron correlation in π-conjugated systems.

π-electron conjugation and aromaticity are commonly associated with delocalization and especially high mobility of the π electrons. We investigate if also the electron correlation (pair density) exhibits signatures of the special electronic structure of conjugated systems. To that end the shape and extent of the pair density and derived quantities (exchange-correlation hole, Coulomb hole, and conditional density) are investigated for the prototype systems ethylene, hexatriene, and benzene.

The importance of large-amplitude motions for the interpretation of mid-infrared vibrational absorption and circular dichroism spectra: 6,6'-dibromo-[1,1'-binaphthalene]-2,2'-diol in dimethyl sulfoxide.

Using the 6,6'-dibromo-[1,1'-binaphthalene]-2,2'-diol molecule and its vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra measured in deuterated dimethyl sulfoxide as example, we present a first detailed study of the effects induced in VCD spectra by the large-amplitude motions of solvent molecules loosely bound to a solute molecule. We show that this type of perturbation can induce significant effects in the VA and VCD spectra. We also outline a computational procedure that can effectively model the effects induced in the spectra and at the same time provide detailed structural information regarding the relative orientations of moieties involved in a solute-solvent molecular complex.

Solvent induced enhancement of enantiomeric excess: a case study of the Henry reaction with cinchona thiourea as the catalyst.

Enantiomeric excess (ee) in asymmetric catalysis may be strongly dependent on the solvent. The reaction product may range from an almost racemic mixture to an ee of over 90% for different solvents. We study this phenomenon for the C-C coupling reaction between nitromethane and benzaldehyde (the Henry reaction) with cinchona thiourea as the catalyst, where solvents that are strong Lewis bases induce a high ee.

A frontier orbital study with ab initio molecular dynamics of the effects of solvation on chemical reactivity: solvent-induced orbital control in FeO-activated hydroxylation reactions.

Solvation effects on chemical reactivity are often rationalized using electrostatic considerations: the reduced stabilization of the transition state results in higher reaction barriers and lower reactivity in solution. We demonstrate that the effect of solvation on the relative energies of the frontier orbitals is equally important and may even reverse the trend expected from purely electrostatic arguments. We consider the H abstraction reaction from methane by quintet [EDTAH(n)·FeO]((n-2)+), (n = 0-4) complexes in the gas phase and in aqueous solution, which we examine using ab initio thermodynamic integration.

Assessment of density functional methods for reaction energetics: iridium-catalyzed water oxidation as case study.

We investigate basis set convergence for a series of density functional theory (DFT) functionals (both hybrid and nonhybrid) and compare to coupled-cluster with single and double excitations and perturbative triples [CCSD(T)] benchmark calculations. The case studied is the energetics of the water oxidation reaction by an iridium-oxo complex. Complexation energies for the reactants and products complexes as well as the transition state (TS) energy are considered.

Understanding solvent effects in vibrational circular dichroism spectra: [1,1'-binaphthalene]-2,2'-diol in dichloromethane, acetonitrile, and dimethyl sulfoxide solvents.

We present a combined experimental and computational investigation of the vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of [1,1'-binaphthalene]-2,2'-diol. First, the sensitive dependence of the experimental VA and VCD spectra on the solvent is demonstrated by comparing the experimental spectra measured in CH(2)Cl(2), CD(3)CN, and DMSO-d(6) solvents. Then, by comparing calculations performed for the isolated solute molecule to calculations performed for molecular complexes formed between solute and solvent molecules, we identify three main types of perturbations that affect the shape of the VA and VCD spectra when going from one solvent to another.

On the equivalence of conformational and enantiomeric changes of atomic configuration for vibrational circular dichroism signs.

We study systematically the vibrational circular dichroism (VCD) spectra of the conformers of a simple chiral molecule, with one chiral carbon and an "achiral" alkyl substituent of varying length. The vibrational modes can be divided into a group involving the chiral center and its direct neighbors and the modes of the achiral substituent. Conformational changes that consist of rotations around the bond from the next-nearest neighbor to the following carbon, and bond rotations further in the chain, do not affect the modes around the chiral center.

Hydroxylation catalysis by mononuclear and dinuclear iron oxo catalysts: a methane monooxygenase model system versus the Fenton reagent Fe(IV)O(H2O)5(2+).

Hydroxylation of aliphatic C-H bonds is a chemically and biologically important reaction, which is catalyzed by the oxidoiron group FeO(2+) in both mononuclear (heme and nonheme) and dinuclear complexes. We investigate the similarities and dissimilarities of the action of the FeO(2+) group in these two configurations, using the Fenton-type reagent [FeO(2+) in a water solution, FeO(H(2)O)(5)(2+)] and a model system for the methane monooxygenase (MMO) enzyme as representatives. The high-valent iron oxo intermediate MMOH(Q) (compound Q) is regarded as the active species in methane oxidation.

Cu(bipy)2+/TEMPO-catalyzed oxidation of alcohols: radical or nonradical mechanism?

In the oxidation of alcohols with TEMPO as catalyst, the substrate has alternatively been postulated to be oxidized but uncoordinated TEMPO(+) (Semmelhack) or Cu-coordinated TEMPO(•) radical (Sheldon). The reaction with the Cu(bipy)(2+)/TEMPO cocatalyst system has recently been claimed, on the basis of DFT calculations, to not be a radical reaction but to be best viewed as electrophilic attack on the alcohol C-H(α) bond by coordinated TEMPO(+). This mechanism combines elements of the Semmelhack mechanism (oxidation of TEMPO to TEMPO(+)) and the Sheldon proposal ("in the coordination sphere of Cu").

Electron pair density in the lowest 1Σ(u)(+) and 1Σ(g)(+) states of H2.

We demonstrate and advocate the use of observable quantities derived from the two-electron reduced density matrix - pair densities, conditional densities, and exchange-correlation holes--as signatures of the type of electron correlation in a chemical bond. The prototype cases of the lowest (1)Σ(u)(+) and (1)Σ(g)(+) states of H(2), which exhibit large variation in types of bonding, ranging from strongly ionic to covalent, are discussed. Both the excited (1)Σ(g)(+) and (1)Σ(u)(+) states have been interpreted as essentially consisting of (natural) orbital configurations with an inner electron in a contracted 1sσ(g) orbital and an outer electron in a diffuse (united atom type, Rydberg) orbital.

On the origin dependence of the angle made by the electric and magnetic vibrational transition dipole moment vectors.

The concept of robustness of rotational strengths of vibrational modes in a VCD spectrum has been introduced as an aid in assignment of the absolute configuration with the help of the VCD spectrum. The criteria for robustness have been based on the distribution around 90° of the angles ξ(i) between electric and magnetic transition dipoles of all the modes i of a molecule. The angles ξ(i) (not, of course, the rotational strengths) are, however, dependent on the choice of origin.

An abiotic analogue of the diiron(IV)oxo "diamond core" of soluble methane monooxygenase generated by direct activation of O2 in aqueous Fe(II)/EDTA solutions: thermodynamics and electronic structure.

We study the generation of a dinuclear Fe(IV)oxo species, [EDTAH·FeO·OFe·EDTAH](2-), in aqueous solution at room temperature using Density Functional Theory (DFT) and Ab Initio Molecular Dynamics (AIMD). This species has been postulated as an intermediate in the multi-step mechanism of autoxidation of Fe(II) to Fe(III) in the presence of atmospheric O(2) and EDTA ligand in water. We examine the formation of [EDTAH·FeO·OFe·EDTAH](2-) by direct cleavage of O(2), and the effects of solvation on the spin state and O-O cleavage barrier.

Counterpoise correction is not useful for short and Van der Waals distances but may be useful at long range.

This article investigates the errors in supermolecule calculations for the helium dimer. In a full CI calculation, there are two errors. One is the basis set superposition error (BSSE), the other is the basis set convergence error (BSCE).

Signatures of counter-ion association and hydrogen bonding in vibrational circular dichroism spectra.

We study the effect of counter-ion complexation on the example of Cl(-) ions interacting with the [Co(en)(3)](3+) complex. The H-bonding of the N-H groups of the ethylenediamine (en) ligands with the Cl(-) ions may lead to giant enhancement of the VCD intensity for the N-H stretches, but may also lead to VCD sign changes in the finger print region of N-H wagging, twisting and scissoring motions. Such sign changes should not be mistaken for signatures of the presence of the other enantiomer.

Counterintuitive Coulomb hole around the bond midplane.

The Coulomb hole does not have its largest depth around an electron in or near the bond midplane. It splits into two parts, localized on both nuclear sites forming the bond. Even counterintuitive positive values of the "hole" around such a position may be observed.

Is [FeO](2+) the active center also in iron containing zeolites? A density functional theory study of methane hydroxylation catalysis by Fe-ZSM-5 zeolite.

Arguments are put forward that the active alpha-oxygen site in the Fe-ZSM-5 catalyst consists of the FeO(2+) moiety. It is demonstrated that this zeolite site for FeO(2+) indeed obeys the design principles for high reactivity of the FeO(2+) moiety proposed earlier: a ligand environment consisting of weak equatorial donors (rather oxygen based than nitrogen based) and very weak or absent trans axial donor. The alpha-oxygen site would then owe its high reactivity to the same electronic structure features that lends FeO(2+) its high activity in biological systems, as well as in the classical Fenton chemistry.

A new analytical potential energy surface for the adsorption system CO/Cu(100).

Electronic structure data and analytical representations of the potential energy surface for the adsorption of carbon monoxide on a crystalline copper Cu(100) substrate are reviewed. It is found that a previously published and widely used analytical hypersurface for this process [J. C.