Density functional calculation of intermolecular potentials

Calculations of intermolecular potentials following the density functional theory (DFT) turn out to be very complicated without using some appropriate approximations. Most often the following three approximations have been considered. In one approximation the disturbed charge distributions during collisions are reduced to sums of undisturbed charge distributions from the colliding species. In another approximation, the so-called local density approximation (LDA), one neglects the fact that the intermolecular potentials that depend on charge densities also depend on gradients in the densities. In a third approximation one assumes that the intermolecular potential can be considered as a sum of two terms: a term for the long-range geometry and a term for the short-range geometry. In this Article the three approximations mentioned will be discussed for numerical accuracy for calculations of potentials between inert gas atoms and for calculations of potentials between surfaces and inert gas atoms. In the discussion a few other approximations will be mentioned too.     

Ab initio-based intermolecular carbon-carbon pair potentials for polycyclic aromatic hydrocarbon clusters

We derived the carbon-carbon pair potentials for polycyclic aromatic hydrocarbon (PAH) clusters, which exhibited a strikingly similar geometry to that of the two-layer graphite. The binding energy of PAH clusters ranging in size from the benzene dimer to the pyrene dimer obtained by ab initio calculations at the MP2 level was used to extract the pair potentials in the form of the Lennard-Jones and Exponential-6 functions. Identical binding energy and equilibrium interlayer distance were reproduced by these functions to those calculated by the ab initio method. The pair potentials for PAHs yield the same equilibrium C-C distance as the known pair potentials for graphite and fullerenes, but nearly twice the well depth because of the polarization of the C-H bond.     

A note on the AB initio calculation of intermolecular potentials: the HF dimer

Large scale SCF and CEPA PNO calculations have been performed for the HF dimer. The geometry has been optimized at the SCF level. Stabilization energies and harmonic force constants have been computed and compared with previous results.     

On calculations of intermolecular potentials

Ab initio MO calculations of intermolecular potentials using conventional Gaussian basis set have a tendency to over emphasize binding due to underestimation of exchange repulsion. A modified semi-empirical method has been suggested and results are found to be reliable at medium- and long-range separation.     

A matrix partitioning approach to the calculation of intermolecular potentials. General theory and some examples

A general expression for the interaction energy of two molecules is obtained by using a matrix partitioning method. The wavefunction of the whole system is expanded in terms of antisymmetrized products of free-molecule functions and using a matrix perturbation scheme it is possible to describe the interaction energy in terms of free-molecule quantities (like frequency-dependent polarizabilities and density or spin density matrices) that, in principle, can be evaluated at any level of approximation. By way of example, the interaction potentials are calculated for (N₂)₂ and Ne₂ using wavefunctions of HF and TDHF form. The results are in substantial accord with those available in the literature. Application of these potentials to the calculation of macroscopic properties, however, leads to considerable errors. From the analysis of our results it appears that the dispersion energy is underestimated, probably on account of the neglect of intrasystem correlation energy in the TDHF approximation. The use of more sophisticated methods of evaluation of dynamic polarizabilities will not involve any extension of the approach presented in this work.     

Theoretical study of water-pyridine complexes using intermolecular potentials

Intermolecular potential functions have been used to determine the equilibrium structures of the water-pyridine complexes. The dimer and symmetrical 2:1 water pyridine systems have been studied. Three water models, ST2, TIPS2, and EMPWI have been combined with two different Lennard Jones nonbonded parameters and various charge distributions for the pyridine molecule to describe the systems. For the dimer, results show two distinguishable classes of preferential hydration sites, which are specific sites corresponding to hydrogen-bonded dimer and nonspecific sites located near the hydrophobic regions. Calculations performed on hydrogen-bonded symmetrical complexes show that the planar complex is generally less stable than the complex with water molecules perpendicular to the pyridine plane. For these complexes, the major factor that influences the hydrogen-bonded configurations is the choice of the water model. The importance of atomic charge distributions for the solute over the choice of potential parameters is pointed out. Finally, the effective lone pair representation on the aromatic nitrogen atom is shown to improve the hydrogen bond geometry and the stability of the complexes.     

Reinforcement of polystyrene by covalently bonded oxo-titanium clusters

Oxo-metallic clusters are employed as inorganic nanobuilding blocks in order to obtain new organic–inorganic hybrid materials. Nanobuilding blocks are well-defined preformed entities which allow a better control of the inorganic domains for the elaboration of hybrid compounds. The oxo-alcoxo cluster Ti₁₆O₁₆(OEt)₃₂ presents a shell of labile ethoxy groups which can be selectively exchanged with the preservation of the oxo-core in order to introduce polymerizable ligands at the surface of this nanobrick. Three different new clusters, Ti₁₆O₁₆(OEt)_32−x(OPhCHCH₂)_x, have been synthesised, each cluster bears exactly 4, 8 and 16 styrenic groups. These functional clusters were copolymerized with styrene leading to three dimensional networks where the inorganic nano-fillers are covalently linked to the organic polymer. Thus new hybrid materials can be obtained and these nanobricks are good models to correlate the structure of hybrid materials and their physical properties especially their mechanical and thermal properties. The structure of the materials in function of the organic–inorganic ratio and in function of the cluster functionalities was investigated by SAXS, and the formation of the different levels of aggregation is reported.     

CNDO calculation of intermolecular V-T and V-V potentials. The CO2-N2 system

The intramolecular electron transfer between two naphthyl moieties (N) of trans-1,4-bis-(α-naphthyl-methyl)-cyclohexane in HMPA was studied over a range of temperatures

. Although the inspection of the model indicates a separation by 9 Å between the naphthyl groups the frequency of transfer is 0.9 × 10⁷ s^?1 at 15°C.     

Studies in hydrogen bonding: Association within small clusters of water,methanol, and ammonia molecules using Jorgensen's intermolecular pair potentials

The geometries of the dimer, trimer, and tetramer hydrogen-bonded clusters of water, methanol, and ammonia molecules have been derived using previously published intermolecular pair potentials containing constants optimized from ab initio calculations. The lowest energy forms for the dimers of all three types of molecules have an open structure, while the trimers and tetramers have cyclic structures. The results are compared with those previously described using another empirical potential, EPEN .     

PCILO calculation of intermolecular energies: The water dimer

The PCILO (perturbative configuration interaction using localized orbitals) method for approximating the electronic structure of molecules has been used with some success for calculating intramolecular interactions in large molecules where intramolecular hydrogen bonding is involved. In this note we show that the PCILO method may be used to calculate the energy of interaction between two water molecules in selected configurations.     

Development of intermolecular potentials for predicting transport properties of hydrocarbons

We explore the ability of a modified form of an n-6 potential to represent transport property data of n-paraffins over a range of temperatures, pressures, and chain lengths. General features of the relationship between the potential form and resulting properties are investigated for methane, where it is shown that the tunable softness of the potential function allows for adjustment of transport properties, through modulation of the magnitude of the density fluctuations, while leaving thermodynamic properties such as liquid density and vapor-liquid coexistence properties essentially unchanged. We tune potential parameters of a united atom model for n-paraffins to viscosity and density data of low molecular weight (C4-C10) species and demonstrate the robust predictive capability of viscosity-temperature-pressure relationships for species up to n-C36.     

The method of introducing correction factors into parameters of atom-atom potentials of intermolecular interaction used for calculation of thermodynamic characteristics of adsorption

Different methods of introducing correction factors into parameters of atom-atom potentials of intermolecular interaction used for calculation of thermodynamic characteristics of adsorption by the semiempirical molecular statistical theory were compared. A method of isostructural fragments based on the application of the atom-atom potential corrected for the molecular fragment was suggested for introducing the correction factors. The advantages of this method were demonstrated for chlorobenzenes, chlorodioxines, and chlorobiphenyls. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 681–687, April, 2000.     

Theoretical study of dihydrogen bonded clusters of water with tetrahydroborate

Ab initio calculations were used to analyze interactions of BH₄ ^? with 1?C4 molecules of H₂O at the MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p) computational levels. The negative cooperativity for dihydrogen bond clusters containing H₂O···H₂O hydrogen bonds is more remarkable. The negative cooperativity is increased with increasing the size and also the number of hydrogen bonds in the cluster. The B?CH stretching frequencies show blue shifts with respect to cluster formation. Also greater blue shift of stretching frequencies where predicted for B?CH bonds which did not contribute in dihydrogen bonding with water molecules. The structures obtained have been analyzed with the Atoms in Molecules (AIM) methodology.     

First-principle molecular dynamics calculation of selenium clusters

The equiliblium structures of small selenium clusters are obtained via first-principle molecular dynamics calculations based on the linearized-augmented-plane-wave (LAPW) method. Resulting equiliblium structures show a good agreement with experimental data and other firstprinciple calculations.     

A simple but reliable method for the prediction of intermolecular potentials

A simple but reliable method for the prediction of intermolecular potentials is presented and tested for the case of the lighter noble gases and some of their mixtures.     

Band structure of solids from clusters SCF potentials

The possibilities and limits of the molecular orbital theory to deal with the problem of determining electronic structure of solids have been explored. A cluster model based on the charge neutrality in the solid has been used in test calculations on some III–V semiconductors and have given quite satisfactory results. Recommendations are given to widen the field of applications of this procedure. © 1995 John Wiley & Sons, Inc.