A new method for deriving atomic charges and dipoles for n, -alkanes: investigation of transferability and geometry dependence

Atomic charge models are known to be unsatisfactory for representing the ab initio electrostatic potential (ESP) of n, -alkanes. A new method for deriving atomic charges and dipoles is proposed and applied to n-alkanes ranging from C₄ to C₁₀. Electrostatic parameters found by this method reproduce accurately the ab initio ESP. The issues of transferability and conformational dependence are also addressed by introducing charges and dipoles taken from a truncated distributed multipole analysis, in the same spirit as the restrained electrostatic potential method. A transferable model is proposed for larger alkanes (>C₁₀). We also estimate the error made when using a set of Boltzmann-weighted electrostatic parameters for all conformers. The reduced number of electrostatic sites considered in our model makes it suitable for computer simulation of liquid n-alkanes.     

Tight-binding study of structural and electronic properties of silver clusters

Tight-binding model is developed to study the structural and electronic properties of silver clusters. The ground state structures of Ag clusters up to 21 atoms are optimized by molecular dynamics-based genetic algorithm. The results on small Ag_n clusters (n = 3-9) are comparable to ab initio calculations. The size dependence of electronic properties such as density of states, s-d band separation, HOMO-LUMO gap, and ionization potentials are discussed. Magic number behavior at Ag₂, Ag₈, Ag₁₄, Ag₁₈, Ag₂₀ is obtained, in agreement with the prediction of electronic ellipsoid shell model. We suggest that both the electronic and geometrical effect play significant role in the coinage metal clusters. Received 7 August 2000     

Ab initio calculation of force constants and equilibrium geometries

Force constants of the molecules HF, NH₃, CH₄ and BH₄ ^- have been calculated ab initio by the force method with a 73/3 + 1 gaussian lobe basis set. The results, including a former calculation on H₂O, agree well with experiment: the average relative error is 12 per cent for the diagonal force constants and the average absolute error is 0·06 mdyn/Å for the off-diagonal ones. The trends are also correctly reproduced. It is concluded that ab initio calculations of this accuracy can help to solve a number of spectroscopic problems. Force constants of BH₄ ^- have been determined from a combination of spectroscopic and ab initio information. Geometries have been obtained with little computing work and show good agreement with experiment.     

Parameterization of a potential function for the Ca2+ −Ne and Ca2+ −N2 interactions using high-level ab initio data

Ab initio calculations have been carried out at the CCSD(T)/aug-cc-pVTZ level on di-nitrogen and neon interacting with a calcium (II) cation. The interaction of nitrogen with the Ca²⁺ cation is examined for end-on and side-on approaches. The two adjustable parameters in the TrAZ potential (A ^Ca2+-Ne , b ^Ca2+-Ne ) are fitted to the Ca²⁺-Ne potential energy curve and combination rules are then applied in reverse to get A ^Ca2+-Ca2+ , b ^Ca2+-Ca2+ . Combining these with values of A^NN, 6^NN enables the Ca²⁺-N₂ potential energy curves to be predicted a priori. Agreement with the ab initio data is found to improve dramatically when the anisotropy of the nitrogen polarizability is included in the induced interaction energy. Other combination rules are tried but are not found to have an impact on the agreement between the a priori potential and the ab initio data     

Interaction potentials,spectroscopy and transport properties of RG+–He (RG=Ar–Rn)

High-level ab initio potential energy curves are calculated for the RG⁺–He complexes (RG=Ar–Rn). RCCSD(T) calculations are employed with large basis sets, and taking account of spin–orbit coupling. The calculated spectroscopic parameters are compared with experimentally determined values, with other high-level ab initio results, and with results from potentials that were obtained by fitting to experimental data. The gas-phase mobilities of RG⁺ ions in He are calculated from our potentials and compared, graphically and statistically, with the experimental mobilities as a function of E/n ₀ at several temperatures. We conclude that more precise experimental data are required in order to discriminate between potentials with more certainty. In addition, we discuss previously reported, unexpectedly large drops in experimental mobility values for RG⁺ in He at 4.35 K as E/n ₀ → 0.     

Ab initio calculation of the He2 A 1Σ u +└X 1Σ g + absorption spectrum

The absorption spectrum of helium gas near 600 Å, assigned to the A ¹Σ _u ⁺└X ¹Σ _g ⁺ transition in He₂, is calculated at 77 K. The excited state potential is taken from a recent ab initio calculation. The theoretical spectrum shows well-defined bands, corresponding to different vibration levels of the A state, with diffuse rotational structure, in agreement with experiment. Fairly good quantitative agreement with Tanaka and Yoshino's measured spectrum is obtained, except for the somewhat too large separation between calculated vibrational bands, probably due to the theoretical potential for the A state rising too steeply at small inter-nuclear separation. Rotational constants derived from the calculated and experimental spectra are in good agreement. It is shown that they are significantly smaller than actual rotational constants of the upper state.     

An interatomic pontential for fullerences from their vibrational spectrum

Summary The vibrational analysis ofsp ²-bonded carbon clusters with different nearest-neighbour interatomic distances (2 in C₆₀, 8 in C₇₀), performed in the framework of the bond-charge model, leads to the determination of an exponential form for the short-range interatomic potential which is inclusive of charge transfer effects. The potential, besides leading to excellent agreement with the existing spectroscopic data andab initio Car-Parrinello calculations, ensures a good transferability of the model to other clusters and possibly an empirical basis for molecular dynamics simulations. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Coupled ab initio potential energy surfaces for the two lowest 2A′ electronic states of the C2H molecule

Realistic two-valued potential energy surfaces for the reaction C(³P) + CH(X²Π) → C₂ + H have been constructed from a set of high level ab initio data describing the first two ²A′ electronic states of the C₂H system. These states have linear equilibrium configurations, known as the X ²Σ⁺ and A²Π states, and are coupled by a conical intersection. They lead to the formation of C₂(X¹Σ⁺ _g) and C₂(a³Π_u) considering an adiabatic dissociation process. The ab initio calculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using the ab initio adiabatic energies and the matrix elements of the dipole moment, a 2 × 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Σ/Π conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with the ab initio ones, and good qualitative agreement is observed.     

Lifetimes of rovibrational levels of the long-range I′ 1Πg state of H2 and D2: experiment and theory

The lifetimes of the lowest lying rovibrational levels of the outer well I′ ¹Π_g state of molecular hydrogen were measured for both H₂ and D₂. The measurements were made by direct observation of the time-dependent decay of the fluorescence. The observed lifetimes depend on isotopomer and increase with vibrational excitation. The predominant decay route for these levels is fluorescence. Previously published ab initio lifetimes calculated for these states, which accounted for non-adiabatic interactions [J. Chem. Phys. 92, 7461 (1990)], are in good agreement with experiment for H₂ but are too long by four or five orders of magnitude for D₂. We present new ab initio results at the adiabatic level for the fluorescence lifetimes. The current results are in reasonable agreement with the experimental lifetimes for both H₂ and D₂. We explain the isotopomer and vibrational dependence of the lifetimes and discuss the neglected interactions and decay pathways.     

Structures and its evolution of Ba n ( n =2 ∼ 14) clusters

We have studied the atomic structure and the electronic properties of Ba_n clusters by the ab initio molecular dynamics method. We find that a structural transition to the bulk-like structure begins at Ba₉ cluster, and the structures of the clusters are transferred to be icosahedral-like around n = 13. The relatively high stability for Ba₄, Ba₁₀ and Ba₁₃ clusters are observed. Received 1st December 2000     

Infrared ion dip and ultraviolet spectroscopy of 4-phenyl imidazole, its tautomer, 5-phenyl imidazole, and its multiply hydrated clusters

Mass-resolved resonant two photon ionisation (R2PI) and infrared ion dip spectra have been recorded for 4-phenylimidazole (4PI) and its singly and multiply hydrated clusters 4PI(H₂O)_{n = 0 - 4}, under supersonic expansion conditions. In the case of 4PI(H₂O)_0,1, it has also been possible to record infrared spectra in both the ground (S₀) and excited (S₁) states. Combining the experimental data with the results of ab initio calculations has led to the structural assignment of each cluster. In each case, the water molecules bind primarily to the NH site of the imidazole ring. Clusters with n≥ 2 incorporate linear water chains, in which the proton donating terminus bridges either to the π-electron system (n = 2) or to the >N: atom site (n = 3, 4) on the imidazole ring. Despite the creation of a “water wire”, connecting the donor and acceptor sites of imidazole, there is no evidence of proton transfer in either the ground or excited state. Received 20 December 2001 Published online 13 September 2002     

Ab initio pair potential and phase equilibria predictions for the refrigerant methyl fluoride

Ab initio pair interaction energies for methyl fluoride (CH₃F) were calculated using symmetry adapted perturbation theory with the aug-cc-pVDZ basis set plus bond functions for a large number of configurations of a pair of methyl fluoride molecules in order to obtain a good representation of the two-body potential energy surface. These interaction energies were used to develop a site-site intermolecular potential that accurately reproduced the calculated energies and had the correct asymptotic behaviour at long range. This pair potential was used in Gibbs ensemble Monte Carlo simulations to predict the phase behaviour of methyl fluoride. The predicted equilibrium properties with the ab initio pair potential are in good agreement with the experimentally measured phase boundary, and give an estimated critical point in excellent agreement with measured values. Multi-body effects were then added to the pair potential using a polarizable model. The small changes to the phase behaviour that resulted established that pairwise interactions account for most of the molecular interactions in CH₃F.     

Effect of melting on ionization potential of sodium clusters

The effect of melting transition on the ionization potential has been studied for sodium clusters with 40, 55, 142, and 147 atoms, using ab initio and classical molecular dynamics. Classical and ab initio simulations were performed to determine the ionization potential of Na₁₄₂ and Na₁₄₇ for solid, partly melted, and liquid structures. The results reveal no correlation between the vertical ionization potential and the degree of surface disorder, melting, or the total energy of the cluster obtained with the ab initio method. However, in the case of 40 and 55 atom clusters, the ionization potential seems to decrease when the cluster melts. Received 1st November 2002 Published online 24 April 2003 RID="a" ID="a"e-mail: ar@phys.jyu.fi     

Kerr effect of molecular oxygen at λ=1064 nm

We report a new measurement of the Kerr effect of molecular oxygen at λ= 1064 nm. The experimental value reported for the anisotropy of the index of refraction Δ n_u^l, (3.15±0.85)×10^-25 m² V^-2 atm^-1, is in good agreement with the value of 3.4×10^-25 m² V^-2 atm^-1 obtained via an ab initio calculation. We show that the dependence of the effect on the pressure is not linear because of the presence of a collision-induced absorption band around 1060 nm due to the transition from the X³ Σ^-_g ground state to the ¹Δ_g state. We also give the value of the quadratic anisotropy Δn_u^q (-1.03±0.68)×10^-25 m² V^-2 atm^-2. We finally compare our ab initio theoretical and experimental results with previous existing data.     

Drainage and water clusters in Gillette foam

Raman measurements on Gillette foam have been carried out to analyze different phases of water in the system. We have shown that in addition to free water molecules, which drain out with aging of foam, water clusters of only a few water molecules are also present in foam. We have analyzed the rate of drainage with the existing theory available in the literature. The nature of water clusters in Gillette foam has also been obtained from ab initio self‐consistent field calculations for [H₂O]_n clusters. Copyright © 2008 John Wiley & Sons, Ltd.     

Influence of covalence and anion symmetry on the structure of small metal hydroxide clusters: Sodium versus silver hydroxide

An ab initio study of the Na_n(OH)_n, Na_n(OH)_n-1 ⁺, Ag_n(OH)_n, and Ag_n(OH)_n-1 ⁺ clusters with n up to four is presented. The results of this study show that, in accordance with experimental observations, the sodium hydroxide clusters are almost purely ionic, while the Ag-O bond exhibits a significant covalent character. The perturbation caused by the non-spherical OH^- group relatively to an atomic anion, as well as the influence on structures and energies of the covalent character of the metal-oxygen bond are determined. The appearance of metal-metal bonds in the silver hydroxide clusters is also discussed. Finally, the theoretical results obtained on the Na-OH clusters are compared to experimental results available on the dissociation of the Na_n(OH)_n-1 ⁺ clusters. Received 9 August 1999 and Received in final form 1st December 1999     

Theoretical studies of atmospheric triatomic molecules: Ab initio characterization of Rydberg series in CO2

The ab initio Koopmans' Theorem is used to determine energy levels and orbital characteristics for the lowest members of s, p, d, f, g, h Rydberg series in CO₂. The energy levels are in good agreement with results of SCF calculations and experimental energy levels. It appears that unambiguous spectral assignments can be based upon the comparison of ab initio and observed energy levels in the case of penetrating Rydberg orbits. The energy levels associated with the higher angular momentum orbits exhibit degeneracies and separations which more closely approximate those of the hydrogen atom.     

Theoretical investigation of the Renner-Teller effect in Δ electronic states of tetra-atomic molecules. 1. Variational calculation of vibronic structure in the 11Δg state of B2H2

A variational approach for the ab initio handling of the Renner-Teller effect in Δ electronic states of tetra-atomic molecules is presented. The model Hamiltonian involves four nuclear degrees of freedom which correlate for a linear nuclear arrangement with two doubly degenerate bending modes. The bond lengths are assumed to be kept fixed at their equilibrium values and the effect of end-over-end rotations is neglected. The kinetic energy operator and the general form of the potential surfaces employed allow in principle for a treatment of large amplitude bending vibrations. However, because of restrictions implied, such as neglect of coupling between bending and stretching vibrations and interactions with other electronic states, the approach is aimed primarily at molecules bending with relatively small amplitudes around their linear equilibrium geometries. Two algorithms are developed, one for symmetric acetylene-like (A-B-B-A) molecules, the other for asymmetric (A-B-C-D) species. The approach is applied to calculate the vibronic spectrum of the lowest lying excited state, ¹Δ_g, of B₂H₂, employing ab initio computed potential energy surfaces.     

Melting curve of copper measured to 16 GPa using a multi-anvil press

The melting temperature, T _m, of copper has been determined from ambient pressure to 16 GPa using multi-anvil techniques. The melting curve obtained (T _m=1355(5)+44.5(31)P?0.61(21)P ², with T _m in Kelvin and P in GPa) is in good agreement with both the previous experimental studies and with recent ab initio calculations.     

Is the Ar—Br2(X1Σ+ g ) van der Waals complex linear rather than T-shaped? A study in terms of ab initio based potential energy surfaces

The ground state Ar—Br₂ potential energy surface is predicted from ab initio calculations and from an atom—atom model using empirical ArBr potentials and the (evaluated ab initio) perturbation of the interaction between Ar and Br within Br₂. At all levels of modelling, the surface has a double-minimum topology, with wells for both the linear (L-) and T-shaped geometries. This differs from the single-minimum topology predicted by the commonly used pairwise additive Lennard-Jones potential. For both ab initio and atom—atom model surfaces, the L well is found to be significantly deeper than the T well; this relative behaviour is unchanged by zero-point vibrations. Spectroscopic parameters are predicted for the present surfaces. The final surfaces result from a scaling to reproduce the estimated bond energy of the system. Possible reflections of the surface topology in experimental observables are discussed.