Search

The computed electron density of several congested saturated hydrocarbons and halogenated derivatives has been analyzed by the method of interacting quantum atoms (IQA). For all the molecules studied, the calculations show the existence of a bond path between the congested atoms and which, according to the Quantum Theory of Atoms in Molecules, indicates that there is a stabilizing interaction between these atoms. The bond path is found to exist up to interatomic distances well-beyond the sum of the van der Waals radii. The IQA results indicate that steric hindrance is not a repulsive force between the congested atoms but that is the result of an increase in the intra-atomic or self-energy of the congested atoms. This increase in self-energy is caused by the deformation of the atomic basin of the congested atoms. © 2013 Wiley Periodicals, Inc.

Toward cracking the problem of understanding, characterizing, and predicting “solvent-effect” while the world awaits an effective explicit solvent model, we introduce and justify herein a novel set of atomic radii to be used within the most commonly used continuum reaction field, the polarizable continuum model (PCM). The radial values emerge from a quantitative description of the elemental electronic density distribution and are shown to be accurate in such a self-consistent reaction field (SCRF); labeled accordingly as isodensity-based SCRF (IDSCRF) radii. Transition row elements with dynamic oxidation states are addressed through an averaging of the electronic properties from all states in the determination of their effective radii. All results for nonmetal elements have been verified with Guthrie's SAMPLE1 test set and are in quantitative agreement with experimental values from the literature and self-consistent isodensity polarizable continuum model (SCIPCM) calculations. For the compounds with transition metal elements, our IDSCRF results have been verified with SCIPCM results as there are rarely experimental results available. Finally, explicit solvent particles “solvating” Pd- and Ni-containing homogeneous catalysts are also shown to be in close agreement with the IDSCRF radii calculations. © 2012 Wiley Periodicals, Inc.

This article is an account and extension of a series of recent investigations, which using extensive quantum chemical methods provide analytical hyperspherical representations of the potential energy surfaces for the interactions of rare gases with H2S as a rigid molecule, and H2S2, considered as a floppy molecule with respect to torsional mode. For the H2S-rare gas systems, the representation is based on a minimal model, here introduced and discussed. For H2S2, the study of the interaction with Xe, not considered previously, completes the series. The results are discussed with reference to the properties and trends expected for interactions of van der Waals type. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

In this work, first-principle calculations at different levels of theory have been carried out to investigate a series of complexes formed between iodo-perfluorobenzene and several heteroatomic rings. Such model systems are selected to mimic halogen bonding interactions experimentally found within supramolecular architectures. In all cases, the predicted intermolecular distances are significantly shorter than the sums of van der Waals radii of I and N (O or S) atoms and the interactions appear to be highly directional. Halogen bonding energies, calculated at the MP2/lan12dz level, are substantial, ranging from −2.50 to −7.53 kcal/mol. Given the strength as well as the directionality comparable to hydrogen bonds, these synthons should be of general use in many fields of chemistry. Finally, to gain more insights into the nature of such interactions, we have used the natural bond orbital and atoms in molecules analyses on the studied systems, which show a considerable magnitude of charge transfer, significant orbital interaction, and distinct bond critical points between the two interacting atoms. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

The Au(I)–Au(I) closed-shell or aurophilic attraction has been the subject of interest in the experimental and theoretical chemistry fields, due to the intriguing properties associated to it. The presence of phosphorescence in “aurophilic” compounds has been addressed to a wide range of applications, but it has not yet been fully understood. A theoretical study on the electronic and phosphorescent properties of the following series of dinuclear gold complexes has been performed: [Au2(dmpm) (i-mnt)] (1), [Au2(μ-Me-TU) (μ-dppm)] (2), and [Au2(μ-G)(μ-dmpe)] (3). Full geometry optimizations at the second-order Moller–Plesset perturbation theory (MP2) were carried out for each of the species. These calculations made evident that, at the ground-state geometry, the Au(I) cations allocated at the center of the ring show a short Au–Au distance below the sum of the van der Waals radii, at the range of the aurophilic attraction. An intermolecular Au(I)–Au(I) closed-shell attraction for a pair of the systems under study is found. This attraction is comparable to that of the hydrogen bonds. The phosphorescent properties experimentally observed for this series were also characterized through ab initio techniques. The obtained results allow to fit reasonably the excitation energies with the experimental data and to identify a correlation between the strength of the Au(I)–Au(I) interaction and the phosphorescent behavior. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

This study is intended as a continuation of previous experimental and theoretical works on the systems H2O-Rg, H2S-Rg, H2O2-Rg, and H2S2-Rg, where Rg = He, Ne, Ar, Kr, Xe. For the H2O-Rg and H2S-Rg systems, molecular and atomic polarizabilities have been calculated and from them, using phenomenological correlation formulas modeling the dispersion-repulsion (van der Waals) forces, the isotropic interaction parameters have been determined and compared with experimental data from this laboratory. For the H2O2-Rg and H2S2-Rg systems, the molecular polarizabilities have been calculated and used in correlation formulas to predict well depths and positions of van der Waals forces and a comparison made with the corresponding potential energy surfaces calculated in previous works. The approach correctly predicts the interaction parameters, except for H2O and H2O2 with the heavier rare gases. The correlation formulas have been then extended to include an attractive induction contribution accounting for the interaction between the permanent molecular dipole moment and the instantaneous induced atomic dipole moment, to improve the predicted parameters for H2O and H2O2-Ar, Kr and Xe. The agreement with experimental and theoretical data is improved but the predicted data still underestimate the interaction. This is probably due to the presence of a significant non van der Waals contribution to the interaction for the heavier gases, as suggested by analogy with the previously studied water-Rg case. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Chemical species are usually defined in terms of the potential energy function. In this work we explore the relationships between this definition and an alternative one, based on the concept of molecular shape. The molecular shape is given a quantitative expression in the framework of the shape group method. The model representation chosen for the molecular surfaces is the continua of fused-sphere van der Waals surfaces. Simple molecular systems are considered, where the “chemical species” are represented by 1-dimensional potentials as torsional rotamers. It is shown that within well-defined ranges of atomic van der Waals radii and nuclear geometries, the identity of the molecular species, as defined by shape, remains invariant under torsional motions. The number and size of such invariance domains can, under certain conditions, follow closely the regions in configuration space where a rotamer is defined according to the minima and curvature of the potential energy function. These findings provide new tools for the study of the interplay between molecular shape and electronic energy, regarding the recognition of active conformations.

An analysis of the electronic charge distributions of porphine and chlorin, their magnesium-substituted analogs, and ethyl chlorophyllide a has been carried out, using point-charge and symmetrically orthogonalized representations of ab initio molecular fragment scf-mo wave functions. The results indicate that both representations provide essentially identical descriptions. Molecular electrostatic potential calculations have also been carried out using the point-charge representation and from the wave function directly. Both approaches are in good agreement in regions beyond the van der Waals radius of the various atoms in the molecule.

The data on charge-transfer complexes in the vapor phase are analyzed and compared with the data for solutions. An attempt is made to distinguish between the effects of true charge-transfer forces and those attributable to simple van der Waals forces. The complex in the iodine-benzene system in the vapor phase appears to be held together almost solely by van der Waals forces, whereas in other cases the charge-transfer forces contribute in varying degree. Certain inferences are made concerning the mutual orientation of molecules in true charge-transfer complexes, and its effects on the equilibrium constants and absorption intensities. Some remarks on the effect of the complexes on the rate of association of iodine atoms have been appended.

We employ all-electron ab initio time-dependent density functional theory (DFT)-based method to calculate the long range dipole–dipole dispersion coefficient (van der Waals coefficient) C6 of potassium atom clusters (Kn) containing even number of atoms, n ranging from 2 to 20. The dispersion coefficients are obtained via Casimir–Polder relation. The calculations are carried out with the asymptotically correct statistical average of orbital potential and compared with the results obtained using Vosko–Wilk–Nusair representation of exchange-correlation potential within local density approximation. We report the dispersion coefficients between clusters of sodium and potassium atoms as well. The present results have been compared with the available jellium-based model and other DFT results in the literature. We also study the relationship between volume of the cluster and the C6 for K clusters. It is observed that the C6 scales as square of the volume. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

The supermolecular CCSD(T) ab initio calculations of potential energy surface for the electronic ground state of van der Waals complex formed from a mercury atom and a nitrogen molecule are presented. Our calculations indicate the bent orientation (Jacobi coordinates are rNN = 1.103 A, R = 4.38 A, angle θ = 64.4°) of the van der Waals (vdW) system with a well depth De = 89.1 cm−1. The physical origin of the stability of the studied vdW structure was analyzed by the Symmetry Adapted Perturbation Theory. The separation of its interaction energy shows that the dispersion interaction is approximately two-times stronger than the induction one and approximately four-times higher than the electrostatic energy. The theoretical coefficient of diffusion is in good agreement with the experimental value. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Short interfluorine contacts (FF) are ubiquitous in crystal structures. This article presents a theoretical study of five dimeric complexes formed between fluorine-containing molecules and hydrogen fluorine at the MP2 computational levels. The results derived from ab initio calculations show that in all cases, the intermolecular distances are less than the sum of van der Waals radii. Difluorine bond energies, computed at the MP2/aug-cc-pVTZ level of theory, span over a range from −0.71 to −3.77 kJ/mol, thus indicating that interfluorine interactions are relatively very weak but non-negligible. The electrostatic force contributes dominantly to the stability of the systems under investigation, and as such the dispersion interaction makes energetic contributions to binding. As an NBO analysis suggested, the charge-transfer force plays a minor role in the formation of the complexes. Finally, to gain more insights into the nature of interfluorine interactions, topological analysis of the electron density distribution and properties of bond critical points are determined by means of the quantum theory of atoms in molecules. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

High level ab initio calculations at the B3LYP/6-311G(d,p) and MP2(full)/6-311G(d,p) levels employing PCM/UA0 model with different van der Waals' radii for the systems that contain lithium atoms have been carried out, in order to see if the van der Waal's radius for lithium atom employed in the default PCM/UA0 model is proper or not. Comparative analysis indicated that the van der Waals' radius for alkali metals, especially for lithium atom in the default PCM/UA0 model within the Gaussian 03 package, is too small, which causes erroneous redundant imaginary frequencies (RIFs) in the characterization of Li-containing compounds from moderate to big size. A new set of van der Waals' atomic radii based on QTAIM, proposed by Bader, was suggested for a better choice in the characterization of compounds containing alkali metals, for which it can effectively avoid the erroneous RIFs for corresponding geometries of these Li-containing systems. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem 108: 1422-1434, 2008

We report on natural orbital functional calculations of the potential energy curve of the helium dimer. The performance of the recently proposed natural orbital functional (Piris et al. J Chem Phys 2007, 126, 214103) for the treatment of van der Waals interactions is assessed with respect to the increase of the number functions of the augmented correlation consistent polarized valence basis set series. Satisfactory converged towards the experimental equilibrium distance and binding energy is observed with the increase of the number of functions of the basis set. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Interplay of electrostatic and van der Waals forces in the coronene dimer is studied. Our results show that the lowest-energy configuration of the dimer is not necessarily a stack, as it might had been expected a priori. This is a surprising result for the dimer of such a large polycyclic aromatic hydrocarbon (PAH) as coronene (C24H12). The energy of the T-shaped configuration at all highest feasible levels of density functional theory (DFT) (B3LYP, PBE/6-31G(d), D95, cc-pVDZ, cc-pVTZ) is lower than the energies of the three plausible stack configurations. To get a better description of the van der Waals interaction, the DFT results were corrected by a phenomenological van der Waals-type term. This correction gives a slight edge to the parallel-displaced stack configuration. However, the magnitude of the correction is somewhat arbitrary, depending on the set of parameters used. This makes the definitive conclusion impossible at the currently achievable level of theory. A simple model is proposed that is useful for the qualitative understanding of possible geometries of the coronene dimer and larger coronene clusters. The model represents the coronene dimer as two sets of charged rings interacting via Coulomb and Lennard-Jones potentials. The model provides an intuitively clear explanation as to why the T-shaped dimers can be of importance even for some of moderately large PAHs such as coronene, and perhaps for circumcoronene as well. The unexpectedly strong competitiveness of the T-shaped

The details of a density functional that includes van der Waals (vdW) interactions are presented. In particular we give some key steps of the transition from a form for fully planar systems to a procedure for realistic layered compounds that have planar symmetry only on large-distance scales, and which have strong covalent bonds within the layers. It is shown that the random-phase approximation of that original functional can be replaced by an approximation that is exact at large separation between vdW interacting fragments and seamless as the fragments merge. An approximation to the latter which renders the functional easily applicable and which preserves useful accuracy in both limits and in between is given. We report additional data from applications to forms of graphite, boron nitride, and molybdenum sulfide not reported in our previous communication.(C) 2004 Wiley Periodicals, Inc.

The two contributions, vibrational and electronic, to the electrical properties polarizability and first hyperpolarizability of monosubstituted acetylene HCCX (with X = F, Cl, Br, OH, SH, BH2, NH2, CH3, and SiH3) have been evaluated using the self-consistent field wave functions within the double-harmonic oscillator approximation. The results show that both contributions to the electrical properties are sensitive to the nature of the substituent: electronegativity, atomic volume, and X-weight. In some cases, X = NH2, SiH3, and SH, the nonlinear optical response is essentially due to, and only to, the vibrational response β of the molecule. A complete study is made upon two tracks, upon the intrinsic properties of X and upon the IR and Raman spectra as well as the sum-over-modes expression. We also calculated the electrical properties at the AUG-cc-pVDZ basis set and at geometric parameters optimized at different levels of theory (B3LYP and MP2). © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

The Hartree–Fock–Clementi–Corongiu method (HF–CC) is revisited, aiming at an unified formulation for post-HF energy computations in atomic and molecular systems. For atomic systems new parameterizations of the HF–CC functional are proposed for the computation of atoms. The previous HF–CC molecular functional (Clementi, E.; Corongiu, G. Theochem 2001, 543, 39), revisited and recalibrated with a new optimization of the parameters, is tested with a sample of 131 molecules, including radicals, H-bond, and van der Waals systems. The atomization energy is decomposed into “HF classic” energy (the sum of the HF nuclear electron, HF kinetic, and HF Coulomb energies), “HF exchange” energy, and correlation energy; the latter is computed with a scaling functional with atomic, covalent, ionic, and van der Waals contributions. For the sample of 131 molecules, the computed HF–CC atomization energies have an average standard deviation of 1.89 kcal/mol. The atomic and molecular components of the correlation energy are decomposed into nuclear electron, kinetic, Coulomb, and exchange contributions; these decompositions characterize the HF–CC model and are used to explain the origin of the chemical bond. Computations on van der Waals systems show the validity of the HF–CC method also for long-range weak interactions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

In a series of papers we have considered the overall effects of geometrical shape on molecular electronic properties. Molecules are mapped into ellipsoids, based on tabulated bond lengths and van der Waals radii. The main advantage of the approach is to make predictions of the influence of shape itself on important physical properties and their trends, particularly for systems which are computationally difficult or time-consuming. The results include insight into the effect of shape on molecular stopping anisotropy, stopping in ultra-thin films, and anisotropic Compton profiles. In this paper we extend the treatment to polarizability anisotropies and to the scaling of the polarizability of long objects in terms of the number of basic building blocks. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

The fully dimensional potential energy surface of the ground electronic state of the LiFH van der Waals complex was constructed by fitting ab initio energies obtained on a grid of ca. 2000 nuclear geometries. The ab initio calculations were performed using the coupled-cluster approach with single, double, and noniterative perturbative triple excitations [the CCSD(T) method]. The large and carefully optimized basis set, consisting of 140 orbitals, was employed. All CCSD(T) energies were corrected for the effects of the basis set superposition error and deformation of the HF monomer in the LiFH complex. The basis set superposition error-corrected CCSD(T) potential energy surface is characterized by a relatively deep, 1991 cm−1, van der Waals well and a late barrier for the Li+HF→LiF+H reaction located at 2017 cm−1 above the Li+HF asymptote. The LiFH complex is bent (the LiFH angle is 109°). The bending LiFH angle characterizing the saddle point is 71°. The fitted potential energy surface was used to calculate the bound and low-lying quasi-bound vibrational states of the LiFH complex. The required ro-vibrational calculations were performed within the framework of the Sutcliffe–Tennyson Hamiltonian for triatomic molecules. The energy positions and widths of the quasi-bound states were obtained using the stabilization method. The ro-vibrational problem was solved both variationally, by diagonalizing the Hamiltonian matrix in a discrete basis set, and by using the perturbative approach based on the adiabatic separation of vibrational motions. All spectroscopic information obtained in this study was rationalized in terms of effective potentials for the van der Waals stretch and bend motions arising from the adiabatic separation of the high- and low-frequency modes. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 916–933, 2000

Van der Waals interaction potentials of the square planar D{sub 4h} and regular tetrahedral T{sub d} configurations of He{sub 4} were examined at the matrix Hartree-Fock, many-body perturbation theory, coupled-cluster theory, and configuration interaction levels of ab initio theory. Potential energy minima, pairwise and nonpairwise additivity, group theoretical parameters, and isotope substitution effects were found to uniquely characterize these interactions.

Counterpoise-corrected supermolecular calculations on the title molecules at MP2, CCSD, and CCSD(T) levels are used to extract a van der Waals radius of 224 pm for Rn, 6 pm more than for Xe. The spin–orbit effects are found to significantly stabilize Rn2 and RnXe. Van der Waals C6 and C8 coefficients, which are in good agreement with both experiment and TDMP2 calculations, were obtained from calculations at large distances. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 131–140, 1998

Continua of molecular surfaces have been proposed in the past as realistic approaches to modeling molecular shape. A continuum of fused-sphere surfaces combines the simplicity of computations involving hard spheres with a more accurate description of the “fuzzy” boundary of a molecule. In this work, we study some simple properties of a continuum of molecular surfaces derived by linearly scaling the van der Waals radii. We introduce some molecular shape descriptors derived from surface area and volume and study their dependence on a scaling parameter that “swells” the molecular surface. As one moves away from the nuclei, any transitions in molecular shape are reflected by changes in the descriptors. These descriptors convey essential shape features, in the sense that they are rather insensitive to molecular size. Even though the geometrical descriptors characterize the continuum globally, their behavior appears to be strongly determined by local molecular shape features. The procedure can also be extended to more realistic electron density surfaces. As a tool, the method can be helpful in assessing molecular similarity, as well as in studying properties of local neighborhoods within large clusters. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 981–992, 1998

Ab initio calculations at the SCF level were carried out to compute the polarization potential map NM of the nucleic acid bases: cytosine, thymine, uracil, adedine, and guanine. For this purpose, the Dunning`s 9s5p basis set contracted to a split-valence, was selected to perform the calculations. The molecular polarization potential (MPP) at each point was evaluated by the difference between the interaction energy of the molecule with a unit point charge and the molecular electrostatic potential (MEP) at that point. MEPS and MPPS for the different molecules were computed with a density of 5 points/{Angstrom}{sup 2} on the van der Waals surface of each molecule, defined using the van der Waals radii. Due to the symmetry of the molecules, only half the points were computed. The total number of points calculated was 558 for cytosine, 621 for thymine, 526 for uracil, 666 for adenine, and 699 for guanine. The results of these calculations are analyzed in terms of their implications on the molecular interactions between pairs of nucleic acid bases. 23 refs., 5 figs., 1 tab.

A progress report is given of an extension of the density functional formalism to include long-range interactions such as van der Waals or dispersion forces. This is done by proving a general expression for the so-called exchange-correlation energy to contain and to describe such interactions just as well as any other treatment. The proper long-range forms of the interactions are derived explicitly in the cases of two neutral atoms, an atom outside a metal surface, and two parallel metal surfaces. The long-standing problem of treating the attractive and repulsive forces on the same footing in this way gets a solution. For practical calculations, an approximate form, based on an analysis in the weakly inhomogeneous limit and on a limiting form of the three-point function given by Rapcewicz and Ashcroft, is proposed and applied to some prototype cases. © 1995 John Wiley & Sons, Inc.

Quantum chemical fully ab initio conformational calculations were performed for the weakly bound van der Waals CO...H{sub 2} dimer in the framework of the supermolecule approach. The counterpoise-corrected interaction energies, computed through fourth-order MBPT using the basis sets described recently by Sadlej, were in excellent agreement with results obtained with larger basis sets constructured to give accurate values for the electric moments and polarizabilities of CO and H{sub 2}. The relative stability of the configurations studied here was collinear structures > parallel structure > T-shaped structures. The collinear and parallel structures represent the most stable group of configurations. The present calculations show that they have values of D{sub e} between 5.89 and 10.66 meV. The T-shaped structures represent relatively less stable configurations with values of D{sub 3} between 0.84 and 2.92 meV. 52 refs., 5 figs., 1 tab.

LCGTO-LSD and LCGTO-NLSD methods have been tested for the study of water-carbon dioxide weakly bound binary complex. Different local and nonlocal exchange-correlation energy functionals and many grid radial points have been used. Results show that both nonlocal corrections and a large number of radial points in the grid are mandatory for well reproducing then experimental data. 19 refs., 1 fig., 2 tabs.

Steric hindrance has been regarded as a result of nonbonding atomic repulsion and measured in terms of empirically obtained van der Waals radii. The authors sought the cause of steric hindrance using the partial derivatives of the energy-components with respect to a nuclear coordinate. The results show that electrostatic interactions do not produce steric hindrance but the restriction on electron-movement does. Namely, the kinetic energy pressure is the first cause of steric hindrance. © 1994 John Wiley & Sons, Inc.

The original polarizable continuum model (PCM) of solvation based on the boundary element method for the numerical solution of the Poisson equation for a realistic molecular shape is modified through a classical description of solute charge distribution and solute polarization. In this approach, the solute charge distribution is represented by point charges and polarizabilities and the solute–solvent polarization is evaluated. The solvent–solute back polarization is also considered. The effect of electrolyte ionic strength is included via a dielectric function based on the Debye–Huckel model. A more realistic description of molecular cavity boundaries is introduced with the help of variable van der Waals radii. Approaches of evaluation of dispersion–repulsion and cavitation contributions to solvation Gibbs free energy suitable for large biopolymers are also presented. Test calculations illustrating the performance of the model are given together with examples of its application for larger biopolymer structures.

In this work, computations of density functional theory (DFT) have been carried out to investigate the interactions between the PCl unit of hexachlorocyclotriphosphazene and several common electron donors. Such small systems are selected to explore the potential P-bound halogen bonding interactions within polymeric structure and supramolecules. In all cases, the intermolecular distances are shown to be equal to or below sums of van der Waals radii of the atoms involved. Halogen bonding energies, calculated at B971/6-311+G(d,p), span over a relatively lager range, from −0.95 to −95.30 kcal/mol, which intimate that the interactions are comparable to, or even partly prevail over, the conventional hydrogen bonding. For the charge-assisted systems, calculations show that these systems can result in much stronger halogen bonding than the corresponding neutral ones. The results agree well with those of natural population analysis (NPA). Finally, the theory of atoms in molecules (AIM) was applied to provide more insight into the nature of these interactions. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

We derive basic integrals needed for calculation of matrix elements of the Fourier transformed Coulomb operator and the Fourier transformed square of the Coulomb operator between 1s Slater functions. These integrals arise, for example, in the momentum space (k space) susceptibility formulation of van der Waals forces. The advantages of integration in k space (rather than r space) are that in k space there is a drastic reduction in the number of integration variables, and contour integration can be used. The derivation is completely analytic and the auxiliary integrals are all obtained in closed form.

General principles of NMR spectroscopy of oriented molecules are described. The use of the technique to problems of biological importance is discussed with emphasis on theoretical and experimental difficulties involved. Application of the technique to study the structure and the hindered rotation in N-methylimidazole and phenyl silane, to investigate interactions of small molecules with lyotropic liquid crystals, to determine covalent and van der Waals radii, and to estimate non planar distortions in amides are described.

In traditional quantum electrodynamic derivation of a intermolecular van der Waals force, the molecules are assumed to be localized clusters of electrons. In this paper we have assumed the molecules to be localized clusters of bosons and have shown that the expression of the potential remains formally the same. This vindicates, through a microscopic derivation, the established concept that the van der Waals interaction is essentially a macroscopic phenomenon.

Interaction energies ΔE accompanying the formation of van der Waals (VDW) systems may be estimated by means of the formula ΔE = ΔESCF + BSSE + ED(r−6, r−8, r−10), where BSSE is the basis set superposition error and ED is the dispersion energy in the expanded form. Twenty one stationary points were located on the potential energy surfaces of hydrogen-bonded complexes; of those, 18 were local minima and 3 were saddle points. Thermodynamic characteristics calculated on the basis of molecular constants, quantum chemically generated, were obtained for 12 hydrogen-bonded complexes and for competition equilibria between VDW molecules. The role of the VDW forces in some rate processes and in a prototype of enzyme–substrate interactions is outlined. In connection with enzymic catalysis, the intramolecular discrimination in 1,2-difluorohydrazine, and the stability of hydrogen-bonded complexes in a cation field is discussed.

The appearance of the maximum linewidth by the vibrational predissociation of I2He van der Waals (vdW) molecule as a function of the initial vibrational quantum number of I2 is predicted by the dynamic perturbation theory. The kinetic energy perturbation is introduced in addition to perturbation potential. The linewidth agrees quite well with experiment.

General formulas are derived for molecular integrals which arise in the linear-response theory when it is applied to the van der Waals interactions between molecules. These integrals are essentially the matrix elements of the Fourier transformed double Coulomb operator between any Cartesian Gaussians. The computational aspects of these formulas are also discussed.

The Van der Waals molecules Ar · HF, Ar · H2O, and Ar · HCl have been analyzed with potential surfaces obtained by SCF energies corrected for the basis set superposition error and by including dispersion forces. The computed properties for these Van der Waals molecules are in good agreement with recent experimental observations.

Some aspects of the use of simplified formulas for the evaluation the interaction energy between two molecules is discussed. This energy is obtained as the sum of four terms: electrostatic, polarization, dispersion, and short-range (exponentially decreasing) repulsion. The effect of using several approximations is considered (i) for representing the charge distributions of the isolated molecules (which determine the electrostatic and polarization terms), and (ii) for evaluating the short-range repulsion term. The various charge distributions considered are semiempirical atomic net charges (Del Re + Pariser-Parr), atomic charges and dipoles (CNDO) and corresponding “effective” atomic charges, and many-centered multipole distributions obtained from ab initio SCF calculations (charges, dipoles, and quadrupoles located on the atoms and the middles of segments joining pairs of atoms, whether chemically bonded or not). As concerns the short-range repulsion, the various procedures considered are a sum of atom-atom terms, a sum of bond-bond terms, and the use of anisotropic van der Waals radii for heteroatoms (oxygen and nitrogen). In all cases, the dispersion energy is obtained as a sum of atom-atom terms (of the R−6 type). These various procedures are checked in the case of the interactions between nucleic acid bases, for two rather different kinds of configurations, namely, hydrogen bonded and stacked. This comparison reveals a rather complicated picture, namely the results got from the various levels of approximation of the molecular charge distribution lead to different degrees of agreement according to different situations, e.g., for guanine-cytosine interactions, qualitative agreement is found between the various methods as concerns the relative order of stacked and hydrogen bonded situations, while agreement is much less satisfactory for adenine-uracile interactions. One of the main conclusions is that a sufficiently sophisticated representation of the molecular charge distribution is required in order to get reliable results for all possible configurations of a complex. Such comparative studies should bring significant help as concerns the development of the reliable simplified procedures for evaluating intermolecular interaction energies.

The TΔS0 term has been estimated for various types of van der Waals molecules and its importance for the stability predictions has been demonstrated. With hydrogen-bonded complexes the absolute values of ΔH0 and TΔS0 are comparable at 289°K. With true van der Waals molecules the TΔS0 term even represents a dominant contribution to ΔG0.

The ground state energy of the He2 system is calculated according to the techniques described in Paper I around the van der Waals equilibrium, without assuming the constancy of the intraatomic correlation energies. The second-order results do not present an attractive region; the S2 decrease of the intraatomic correlation correlation corrections is larger than the attractive interatomic correlation corrections. The further orders reverse progressively the situation and finally give a qualitatively correct potential curve after the fifth order. Reaching almost exact solutions in the considered basis, one can demonstrate that the intermolecular calculation of the system A + B in the union of the basis for A and B involves an energy decrease which simply represents the effect of the extension of the basis set; and that a proper intermolecular calculation must compare the A + B energy with the A and B energies calculated in conveniently extended basis sets including the vacant MO's of the partner.

For several van der Waals molecules, the results of ab initio intermolecular perturbation theory are compared with the simple Buckingham-Fowler model. A selection of dimers is studied, with emphasis on cases where the experimental structure is not reliably predicted by the hard-sphere electrostatic model. Cases are reported where the electrostatic term dominates, in agreement with the model: HF···HF, H3N···HF, N2O···HF, and H2CO··· HF; cases where the electrostatic term dominates but the molecular shapes are not well described by hard spheres: CO2···H2O and CIF···HF; and cases where there is a delicate balance between the electrostatic and other terms:H2O···HF, H2O···HCCH, CO2···HF, CO2···N, and CH3CN···HCN. It is concluded that, in general, the electrostatic interaction is central to the strength and orientation dependence of the forces between molecules and that the model provides a useful approximation to this interaction, while perturbation theory gives a more complete picture.

The infinite summation of intermolecular ring diagrams is extended to any pair of interacting molecules with isotropic polarizabilities. The calculation no longer requires the use of a minimal basis set. The polarizabilities α,α′ may be factorized at all orders, and an expression is derived, which gives modified Van der Waals energies. For heavy atoms the series converges if αα′/R6 is smaller than 1, R being the interatomic distance. This inequality is easily satisfied for the Van der Waals distances, and in practice the correction due to the high orders of the perturbation expansion remains weak. The role of the EPV diagrams and the connection with a configuration interaction treatment are discussed.

A study is made of a variational principle which provides upper and lower bounds for the polarizability under a perturbation with imaginary frequency. As an example bounds for the imaginary frequency polarizability of atomic hydrogen and ensuing limits for the constant of van-der-Waals interaction of two hydrogen atoms are calculated.

The exchange polarization energy for the H2+ ion is expressed as an infinite sum of its angular components. At large internuclear distances R, a particular l-component, referred to as induced2l-pole exchange energy, behaves asymptotically like For larger R this suggests a rather slow l−4 rate of convergence of the series. However, as contrasted to the analogous expansion for the induction energy the convergence speeds up considerably with decreasing internuclear separation. It is also shown that at sufficiently large R a given induced 2l-pole exchange energy can be computed with a high accuracy using a polarization function obtained with neglect of charge overlap effects. Near the bottom of the van der Waals minimum over 99% of the exchange polarization energy can be produced using this simplified form of the polarization function.