Electronic correlation and effective interactions in small alkali clusters

The crucial importance of correlation effects versus delocalization, and their nature in small Alkali clusters is analysed from an ab-initio point of view through a detailed investigation of the Li₂ dimer. The role of the external correlation (provided by extended basis sets and large Configuration Interaction calculations) is shown to lower the energy of ionic configurations and to increase their weight in the electronic wavefunction, increasing simultaneously the importance of delocalization versus internal correlation within thes-band. Effective interactions are determined from accurate diabatic calculations on dimers and transfered to clusters via an effective hamiltonian spanned bys orthogonal orbitals. Although not including explicitely thep-band, this model provides results in good agreement with abinitio calculations on Lithium clusters.     

Nonadditive interactions and the relative stability of neutral and anionic silver clusters

We discuss the physical nature of nonadditivity in many-particle systems and the methods of calculations of nonadditive contributions to the interaction energy. For neutral clusters, a closed recurrence formula which expresses the energy of m-body interactions through the energies of 2-, 3-, and (m – 1)-body ones is obtained. The general approach for calculation of the nonadditive contribution in the interaction energy of charged systems is developed. The comparative calculation of anionic and neutral silver clusters shows that the geometry of the most stable anionic clusters is established mainly by the additive forces. The stability of neutral silver clusters is determined by the competition of attractive additive forces and repulsive nonadditive ones. © 1995 John Wiley & Sons, Inc.     

Shell effects in planar electron clusters

We investigate the electronic shell structure of planar metal clusters, having in mind clusters on insulating surfaces with an interface energy such that the cluster covers the surface in a monolayer. In this first survey we concentrate on the shell effects of such a planar electron cloud using the Ultimate Jellium Model where the structural effects of the positive background are completely eliminated. An axially symmetric electron cloud shows shell effects which are, however, somewhat smaller than those of fully free threedimensional clusters. The free variation of the shape for planar clusters on surfaces, leading to many triaxial clusters, diminishes the shell effects even further, leading to the existence of hybrid-deformed clusters and a lack of energetically favored “magic” clusters in an intermediate size range N ≈ 10.30. In contrary to the situation for free clusters the small shell energies have a minor effect on the energetics of the groundstate. As a consequence, electronic shell effects are only one ingredient amongst others to determine the kinetics of cluster growth on (insulating) substrates. With a bold rescaling assumption, we can relate axially symmetric planar clusters to the planar electron cloud in a neutral quantum dot, having the consequence that shell effects persist to play a role in these systems.     

The electronic structure of free water clusters probed by Auger electron spectroscopy

(H2O)(N) clusters generated in a supersonic expansion source with N approximately 1000 were core ionized by synchrotron radiation, giving rise to core-level photoelectron and Auger electron spectra (AES), free from charging effects. The AES is interpreted as being intermediate between the molecular and solid water spectra showing broadened bands as well as a significant shoulder at high kinetic energy. Qualitative considerations as well as ab initio calculations explain this shoulder to be due to delocalized final states in which the two valence holes are mostly located at different water molecules. The ab initio calculations show that valence hole configurations with both valence holes at the core-ionized water molecule are admixed to these final states and give rise to their intensity in the AES. Density-functional investigations of model systems for the doubly ionized final states--the water dimer and a 20-molecule water cluster--were performed to analyze the localization of the two valence holes in the electronic ground states. Whereas these holes are preferentially located at the same water molecule in the dimer, they are delocalized in the cluster showing a preference of the holes for surface molecules. The calculated double-ionization potential of the cluster (22.1 eV) is in reasonable agreement with the low-energy limit of the delocalized hole shoulder in the AES.     

Influence of electron correlation effects on the solvation of Cu2+

Hybrid QM/MM MD simulations including electron correlation effects at MP2 level were performed to obtain an accurate picture of the solvation structure and the Jahn-Teller effect of the Cu2+ ion.     

Electron correlation effects in ionic hydrogen clusters

We employ density functional, post‐Hartree–Fock, and quantum Monte Carlo methods to study the electronic structure, geometries, and behavior of positively charged H_m⁺ clusters with m=3,5,…,17. Their structure consists of a tightly bound H₃⁺ core ion surrounded by successive solvation shells of H₂ molecules. For the largest clusters, we propose new geometries. We find that correlated methods yield the stepwise decrease of enthalpies for dissociation of H₂ from the clusters observed in experiments. Our best results are obtained by the diffusion Monte Carlo method, and by including finite temperature entropic effects, we are able to reproduce the experimental dissociation enthalpies with an unprecedented accuracy of less than 0.5 kcal/mol. These benchmark results contrast with erroneous predictions discovered in the density functional approaches. Finally, our analysis of the cluster energy surfaces indicates that under quantum and thermal fluctuations, the outer solvation shells will exhibit pronounced fluctional behavior. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 86–95, 2001     

A correlation between structure and reactivity in ion clusters

Observations on metastable peaks resulting from the unimolecular decomposition of ion clusters show that intensity variations as a function of cluster size can reveal the presence of stable cluster configurations. This technique has been used to confirm that (H₂O)₂₁H⁺ and (D₂O)₂₁D⁺ are stable ion clusters, and the method also provides evidence to suggest that Ar₁₉⁺ is a particularly stable species.     

Excess electron localization sites in neutral water clusters

We present approximate pseudopotential quantum-mechanical calculations of the excess electron states of equilibrated neutral water clusters sampled by classical molecular dynamics simulations. The internal energy of the clusters are representative of those present at temperatures of 200 and 300 K. Correlated electronic structure calculations are used to validate the pseudopotential for this purpose. We find that the neutral clusters support localized, bound excess electron ground states in about 50% of the configurations for the smallest cluster size studied (n = 20), and in almost all configurations for larger clusters (n > 66). The state is always exterior to the molecular frame, forming typically a diffuse surface state. Both cluster size and temperature dependence of energetic and structural properties of the clusters and the electron distribution are explored. We show that the stabilization of the electron is strongly correlated with the preexisting instantaneous dipole moment of the neutral clusters, and its ground state energy is reflected in the electronic radius. The findings are consistent with electron attachment via an initial surface state. The hypothetical spectral dynamics following such attachment is also discussed.     

Polarizability effects and dispersion interactions in alkene-Br2 pi-complexes

Weakly bound molecular complexes play an important role in chemistry, physics, and biodisciplines. The preequilibrium pi-complexes of various alkenes with bromine have been examined quantitatively, and a direct relationship between association constants (KF) of these pi-complexes and polarizability of the olefins was found. The stability of the Br2-olefin pi complexes is affected by both the donor ionization potential and the polarizability of the olefin, and an equation able to take into account both effects is proposed.     

Cooperative effects in the activation of molecular oxygen by anionic silver clusters

A novel size dependence in the adsorption reaction of multiple O2 molecules onto anionic silver clusters Agn- (n = 1-5) is revealed by gas-phase reaction studies in an rf-ion trap. Ab initio theoretical modeling based on DFT method provides insight into the reaction mechanism and finds cooperative electronic and structural effects to be responsible for the size selective reactivity of Agn- clusters toward one or more O2. In particular, Agn- clusters with odd n have paired electrons and therefore bind one O2 only weakly, but they are simultaneously activated to adsorb a strongly bound second oxygen molecule. For the clusters Ag3O4- and Ag5O4-, this cooperative effect results in a superoxo-like, doubly bound O2 subunit with potentially high activity in catalytic silver cluster oxidation processes.     

Design and application of a multicoefficient correlation method for dispersion interactions

A new multicoefficient correlation method (MCCM) is presented for the determination of accurate van der Waals interactions. The method utilizes a novel parametrization strategy that simultaneously fits to very high-level binding, Hartree-Fock and correlation energies of homo- and heteronuclear rare gas dimers of He, Ne, and Ar. The decomposition of the energy into Hartree-Fock and correlation components leads to a more transferable model. The method is applied to the krypton dimer system, rare gas-water interactions, and three-body interactions of rare gas trimers He3, Ne3, and Ar3. For the latter, a very high-level method that corrects the rare-gas two-body interactions to the total binding energy is introduced. A comparison with high-level CCSD(T) calculations using large basis sets demonstrates the MCCM method is transferable to a variety of systems not considered in the parametrization. The method allows dispersion interactions of larger systems to be studied reliably at a fraction of the computational cost, and offers a new tool for applications to rare-gas clusters, and the development of dispersion parameters for molecular simulation force fields and new semiempirical quantum models.     

Size and structure of molecular clusters in supercritical water

The type and topology of hydrogen-bonded molecular clusters of water are investigated by the molecular dynamics method for five models of water in supercritical conditions. Small clusters (of the order of 10 molecules) are present in all models, even at densities of less than 0.2g/cm³. When the density increases, a phase transition occurs from vapor-like to fluid-like state. Among small clusters, linear structures are predominant.     

Application of a semi-empirical dispersion correction for modeling water clusters

The Grimme-D3 semi-empirical dispersion energy correction has been implemented for the original effective fragment potential for water (EFP1), and for systems that contain water molecules described by both correlated ab initio quantum mechanical (QM) molecules and EFP1. Binding energies obtained with these EFP1-D and QM/EFP1-D methods were tested using 27 benchmark species, including neutral, protonated, deprotonated, and auto-ionized water clusters and nine solute–water binary complexes. The EFP1-D and QM/EFP1-D binding energies are compared with those obtained using fully QM methods: second-order perturbation theory, and coupled cluster theory, CCSD(T), at the complete basis set (CBS) limit. The results show that the EFP1-D and QM/EFP1-D binding energies are in good agreement with CCSD(T)/CBS binding energies with a mean absolute error of 5.9 kcal/mol for water clusters and 0.8 kcal/mol for solute–water binary complexes. © 2018 Wiley Periodicals, Inc.     

Vibrational and correlation effects on properties of water

Large basis set, ab initio potential energy and property surfaces of water have been used with quantum Monte Carlo vibrational analysis in the evaluation of the molecule's rotational constants, zero-point energy, and dipole moment. While there are clearly differences in vibrational state parameters due to including correlation effects, the vibrational averaging effect on rotational constants is very nearly additive with the correlation effect. This has implications for evaluation and estimation of properties of molecules in specific vibrational states. Received: 9 February 1999 / Accepted: 11 February 1999 / Published online: 5 May 1999     

Hofmeister anionic effects on hydration electric fields around water and peptide

Specific ion effects on water dynamics and local solvation structure around a peptide are important in understanding the Hofmeister series of ions and their effects on protein stability in aqueous solution. Water dynamics is essentially governed by local hydrogen-bonding interactions with surrounding water molecules producing hydration electric field on each water molecule. Here, we show that the hydration electric field on the OD bond of HOD molecule in water can be directly estimated by measuring its OD stretch infrared (IR) radiation frequency shift upon increasing ion concentration. For a variety of electrolyte solutions containing Hofmeister anions, we measured the OD stretch IR bands and estimated the hydration electric field on the OD bond to be about a hundred MV∕cm with standard deviation of tens of MV∕cm. As anion concentration increases from 1 to 6 M, the hydration electric field on the OD bond decreases by about 10%, indicating that the local H-bond network is partially broken by dissolved ions. However, the measured hydration electric fields on the OD bond and its fluctuation amplitudes for varying anions are rather independent on whether the anion is a kosmotrope or a chaotrope. To further examine the Hofmeister effects on H-bond solvation structure around a peptide bond, we examined the amide I' and II' mode frequencies of N-methylacetamide in various electrolyte D(2)O solutions. It is found that the two amide vibrational frequencies are not affected by ions, indicating that the H-bond solvation structure in the vicinity of a peptide remains the same irrespective of the concentration and character of ions. The present experimental results suggest that the Hofmeister anionic effects are not caused by direct electrostatic interactions of ions with peptide bond or water molecules in its first solvation shell. Furthermore, even though the H-bond network of water is affected by ions, thus induced change of local hydration electric field on the OD bond of HOD is not in good correlation with the well-known Hofmeister series. We anticipate that the present experimental results provide an important clue about the Hofmeister effect on protein structure and present a discussion on possible alternative mechanisms.