Accurate Intermolecular Potentials Research Articles

Intermolecular potential for methane in zeolite A and Y: Adsorption isotherm and related properties

Based on energies derived from ab initio calculations on methane in zeolite A and Y a more accurate intermolecular potential is proposed for hydrocarbon-zeolite interaction. We then employ the proposed potential POTJI (consisting of sets ABCHA and ABCHY) as well as another potential (termed KDMG) based on the work of Kiselev and Du and Murad and Gubbins in grand canonical Monte Carlo simulations to obtain adsorption isotherms and molecular dynamics simulations to obtain self-diffusion coefficients. For methane in zeolite A we show that the derived potential predicts accurately adsorption isotherm, heats of sorption and diffusion coefficients. For methane in zeolite Y a somewhat less accurate reproduction of adsorption isotherm is obtained. In general, these potentials predict values that are closer to experimentally obtained values than existing potential. Radial distribution function is more structured than for existing KDMG potential. Translational diffusion coefficients are generally lower than KDMG potential while the rotational diffusion coefficients are actually higher than seen for KDMG potential. Further, it is seen that the set of potential parameters derived for zeolites A and Y are similar suggesting that it might be possible to derive one unified set of parameters that would perform reasonably across a large set of zeolite structures. Similar transferability of potentials across the range of hydrocarbons may also be possible. Finally, these results suggest that it may be possible to derive accurate classical potentials for interaction between organic and inorganic systems based on ab initio calculations.

Intermolecular Potential of the O2−O2 Dimer. An ab Initio Study and Comparison with Experiment

Accurate intermolecular potentials for the lowest three multiplet states of O2-O2 dimer have been produced on the basis of ab initio calculations. The quintet potential was taken from previous highly correlated CCSD(T) calculations. In this work, we perform MRCI calculations, with large basis sets including bond functions, of the singlet and triplet states, which are of multireference character. As expected the size inconsistency and lack of higher order excitations limit the accuracy of the MRCI potentials specifically in describing the long range interactions. We show that the Heisenberg Hamiltonian provides an accurate representation of the exchange interactions in this system and this enables us to combine the accurate CCSD(T) potentials with the MRCI spin-exchange parameter to obtain accurate singlet and triplet potentials. The reliability of these potentials is tested by computing integral cross sections and comparing them with the detailed experimental study of the Perugia group, with excellent results. More interestingly, comparison with the experimentally derived potential shows important discrepancies for some angular orientations including that corresponding with the global minima, indicating the need for further work, both theoretical and experimental, to clarify their origin.

Ice Nucleation on a Model Hexagonal Surface

The adsorption of water on a model hexagonal surface has been studied using accurate intermolecular potentials. The structure and binding energies of single molecules, clusters, and adlayers are obtained. The limiting case of weak, nondirectional surface-water interactions presented here is compared with other cases involving water-water and water-surface interactions of a similar magnitude (partial templating) and dominating water-surface interactions (perfect templating) from the literature. None of these models is conducive to the nucleation of ice, each for different reasons. We comment on the requirements for a good ice-nucleating surface.

HM-IE: Quantum Chemical Hybrid Methods for Calculating Interaction Energies

Accurate intermolecular potentials are needed for quantitative molecular simulations, but their calculation from quantum mechanics can be very demanding. We have developed several variations of a procedure, which we collectively refer to as quantum mechanical Hybrid Methods for Interaction Energies (HM-IE), to accurately estimate interaction energies from CCSD(T) calculations with a large basis set (LBS). HM-IE was tested for interaction energies of Ne 2 ,( C 2H2)2, and N2-benzene for many orientations sampling the entire potential energy surface and was found to be in excellent agreement with the CCSD(T)/LBS results while requiring considerably less computational time and resources. Furthermore, for neon, an intermolecular potential fit to interaction energies using HM-IE and a potential fit to CCSD(T)/LBS energies resulted in nearly identical predictions for densities and vapor pressures.

Theoretical studies of the interface between water and Langmuir films of aliphatic alcohols

The interface between water and Langmuir films of long chain aliphatic molecules is investigated using accurate intermolecular potentials. The stabilities of various ice structures which could form at the interface are examined. Antiferroelectric ice is found to be the most stable, but this stability depends crucially on the first layer of water. Ferroelectric structures are found to collapse upon relaxation. Our model was not able to differentiate between the different nucleation properties of C31H63OH and C30H61OH. A better description of the alcohol–water interaction is probably required to account for this difference.

Adsorption of water on the BaF2(111) surface

Physisorption of water on the (111) surface of barium fluoride is investigated using accurate intermolecular potentials. A revised version of the successful ASP–W2 water potential is developed together with a new potential describing the interaction between water and the barium fluoride surface. Isolated water molecules are found to have a binding energy of −39.8 kJ mol−1. Monolayer and multilayer coverages are also investigated and compared with previous experimental and theoretical work. We find no evidence to support epitaxial growth of ferroelectric ice on this surface and suggest reasons for this.

Computation of the methane–water potential energy hypersurface via ab initio methods

Accurate intermolecular potentials between hydrocarbons and water are essential for the prediction of properties of systems containing these components. Unfortunately, current experimental techniques are unable to measure directly the interaction potential between methane and water molecules. Therefore we have used quantum mechanical calculations, both ab initio and density functional theory (DFT) calculations, in order to determine the H2O–CH4 potential energy surface (PES) accurately for use in modeling gas hydrates. Ab initio methods were found to be more accurate than DFT methods, which do not account for the substantial dispersion interactions that exist between methane and water. Electron correlation was found to be treated accurately by MP2. However, a large basis set, cc-pVQZ was found to be necessary to compute the binding energies to within 0.1 kcal/mol of the basis set limit. In order to sample accurately the PES, the H2O–CH4 binding energy was computed at 18 000 points. For these computations to be feasible, a new method was developed in which all 18 000 points were computed using MP2/6-31++G(2d,2p) and then corrected to near the accuracy of MP2/cc-pVQZ. The PES calculated from the six-dimensional numerical potential agrees very well with far infrared vibration-rotation-tunneling spectroscopic data and experimental second virial coefficient data at the potential minimum and larger separations. The exp-6 potential proves to be the best representation of the H2O–CH4 interaction.

Accurate Intermolecular Potentials Obtained from Molecular Wave Functions: Bridging the Gap between Quantum Chemistry and Molecular Simulations

Accurate Intermolecular Potentials Obtained from Molecular Wave Functions: Bridging the Gap between Quantum Chemistry and Molecular Simulations

The importance of high-order correlation effects for the CO–CO interaction potential

The CO–CO interaction energy is calculated for several geometries, both by the supermolecule MP4 and CCSD(T) methods, and by symmetry-adapted perturbation theory. Relatively large differences between the MP4 and CCSD(T) results are explained by means of a diagrammatic analysis of electron correlation effects, supported by quantitative calculations of the fifth-order contributions to the electrostatic interaction energy. It follows from this analysis that the calculation of an accurate intermolecular potential for CO is a particularly difficult problem: even the CCSD(T) method is not sufficiently reliable since it lacks important fifth-order correlation contributions.

Self-Association of Amphotericin B in Water. Theoretical Energy and Spectroscopy Studies

This present work is devoted to the study of the behavior in water of important antifungal drug, the amphiphilic Amphotericin-B (AmB) molecule. Experimental spectroscopy data, namely, the modification of the AmB UV absorption spectrum in water (hypsochromic frequency shift) with the increasing AmB concentration, indicate the self-association of the molecule in water. In this work, a possible three-dimensional structure of the AmB self-associated species present in water has been determined by using in concert both spectroscopy and energy information and not by fitting the calculated UV absorption spectrum to the experimental one. The calculations have been performed upon AmB dimers and helicoidal oligomers involving 2−8 unit cells. The total energy of any helicoidal oligomer has been evaluated taking into account, besides the intermolecular interactions (computed using an accurate intermolecular interaction potential), (i) the “thermal” effects evaluated utilizing the well-known formulas of statistical thermodynamics and (ii) “solvent” effects calculated in the framework of an adequate continuum model. It has been shown that all energy results obtained for the different helicoidal oligomers may be extended to infinite helicoidal aggregates. In conclusion, (i) The study of the evolution with Nuc (the number of unit cells) of the total energy (involving all here-above cited terms) of both single and double helices has shown that the highest energy gain ( per AmB molecule) is obtained with the double helix. (ii) Such a stabilization mainly proceeds from van der Waals forces together with entropy factors. (iii) This optimized double helix, which is characterized by an helicoidal angle, θ = 30°, and an intercell distance R as 5.7 Å, (thus a pitch of 62.7 Å), leads to an hypsochromic frequency shift (Δν+) calculated as 3950 cm-1, a value which is quite close to the experimental one, 4368 cm-1.

Determination of the orientational pair correlation function of a molecular liquid from diffraction data

Abstract The structure of a molecular liquid is defined by means of the orientational pair correlation function (OPCF), g( r ,ω 1 ,ω 2 ) , which determines the likelihood of a molecule being found at position r, with orientation ω2, given a molecule with orientation ω1 at the origin. Diffraction experiments and most computer simulations of liquids however present only the site-site radial distribution functions, which by definition contain less information than the OPCF. By expanding the OPCF as a series of generalised spherical harmonic functions (SHARM), and by showing that this series, even when truncated, contains important orientational information, it is proposed that the coefficients to this series should provide the essential database for the structure of a molecular liquid as derived from a given set of diffraction data. The most reliable method of estimating these coefficients is to run an auxilliary computer simulation which incorporates all known physical constraints on molecular positions as well as reproducing the measured radial distribution functions or diffraction data as accurately as possible. The Empirical Potential Structure Refinement (EPSR) method of performing the simulation is a robust method of obtaining these structural coefficients and the results appear to be relatively insensitive to the details of the assumed reference intermolecular potential. The possibility of using the EPSR technique as a route to establishing an accurate intermolecular potential is also discussed. Some examples of applying the method to liquid water are presented.

A comparison between the NEMO intermolecular water potential and ab initio quantum chemical calculations for the water trimer tetramer

A comparison between recent ab initio quantum chemical calculations and model potential results for an intermolecular water potential is made for different stationary points in the low energy region of the potential energy surface of the cyclic homodromic water trimer and tetramer. Relative energies, structures and vibrational frequencies are compared for the local stationary points of the potential energy surfaces. The intermolecular model potential was derived almost exclusively from water monomer properties, by use of the NEMO method. The comparison shows good agreement between the NEMO water potential and MP2-R12 calculations. The use of accurate intermolecular potentials instead of ab initio methods allows for the treatment of much larger chemical system due to extensive savings in CPU time. A single point MP2-R12 energy calculation for a non-symmetrical water tetramer conformer without symmetry takes about 40 CPU hours compared with less than 0·5 second for the most sophisticated version of the NEMO potential.

Accurate high-pressure and high-temperature effective pair potentials for the systems N2–N and O2–O

Statistical mechanical chemical equilibrium calculations of N2 and O2 show that these molecules dissociate behind strong shock waves. Our determination of accurate intermolecular potentials has required the consideration of the dissociation products N and O. Our previous theoretical efforts to predict the thermodynamic properties of these molecules relied in part on corresponding states theory and shock wave data of argon, without consideration of the dissociation products. Recent high-pressure Hugoniot measurements, however, allowed a more accurate determination of the potentials and the explicit inclusion of the dissociation products. The best fit to the data is obtained with the exponential-6 coefficients, for O2–O2: ε/k=125 K, r*=3.86 Å, α=13.2; for O–O: ε/k=700 K, r*=2.40 Å, α=11.0; for N2–N2: ε/k=293 K, r*=3.91 Å, α=11.5; and for N–N: ε/k=600 K, r*=2.47 Å, α=10.0. The unlike pair interactions are obtained from these like interactions with a modified Lorentz–Berthelot rule. The coefficients in the modified Lorentz–Berthelot equations are k/l/m=1/1/0.93 for O2–O– and k/l/m=1/1/0.90 for N2–N interactions.

Anisotropic atom-atom potentials

Abstract The forces between polyatomic molecules are traditionally represented by the isotropic atom-atom potential model. However, the implicit assumption that the atoms interact as if they were spherical is a poor approximation for some elements. This paper outlines some of the progress being made in developing anisotropic atom-atom potentials, which can represent the effects of lone pair and m-electron density on intermolecular interactions. It is difficult to determine the form of an atom's anisotropy empirically, and so it has to be derived from the molecular charge distribution, using recently developed theories of intermolecular forces. This can be done for each major contribution to the intermolecular potential for small polyatomics, resulting in more accurate intermolecular potentials. For organic molecules, at the moment, only the electrostatic contribution can be routinely described in this way, through a distributed-multipole analysis. However, computational studies using such an accurate electrostatic model, in conjunction with simple approximations for the other contributions, have been useful for understanding the structures of van der Waals complexes, biochemical interactions and molecular crystal structures. The development of new computer codes is gradually allowing anisotropic atom-atom potentials to be used routinely for an increasing range of types of simulation. Nevertheless, it will often be desirable, and adequate, to approximate an accurate potential by a simpler isotropic site-site form, with additional sites representing the anisotropic features. Assuming the isotropic atom-atom potential, without careful consideration of the distribution of charge in the molecule, can lead to problems in deriving quantitatively adequate potentials for many molecules and can even lead to conceptual problems.

Theoretical studies of structural stability at high pressure

Theoretical methods are indispensible for the study of matter at high pressure. In the last decade the development of accurate intermolecular potentials and the methodologies in classical molecular dynamics (MD) simulations have greatly facililated the applications of these methods to the study of structural phase transformamtions of solids at high pressures. More recently, it has been possible to incorporate quantum mechanical effects into MD calculations. This method eliminates a great deal of empiricism. These first principles calculations have not only reproduced the experimental results for phase transformations but also provided detailed mechanisms and in some cases predicted new structures that may be found at high pressures. The success of MD calculations is illustrated through a review of our studies of pressure-induced amorphization and phase transitions in SiO2 and TiO2, and the structural memory effect in several materials. Current applications using quantum molecular dynamics on ice are discussed.

Towards an accurate intermolecular potential for water

A new ab initio potential for the water dimer is presented. It is obtained from monomer properties and perturbation theory calculations (IMPT) on the dimer. The long range electrostatic interaction is described by distributed multipoles: charge, dipole and quadrupole on oxygen and charge and dipole on hydrogen. The induction is modelled using polarizabilities on oxygen up to quadrupole. The short range repulsive energy is modelled by an exponential site-site function incorporating site anisotropy. Two models have been used for the dispersion energy: accurate dispersion coefficients for the water dimer with Tang-Toennies damping functions, and a site-site model. Neither is entirely satisfactory. The geometry of the global minimum on the potential surface is in very good agreement with the experimental result. The virial coefficient agrees moderately well with the experimental determination; the discrepancies suggest that the potential is not deep enough.

The calculation of intermolecular potentials

The use of intermolecular perturbation theory to rationalize intermolecular forces and to provide a framework for concepts of intermolecular interactions has been routine for many years. It has long been assumed, however, that the route to accurate calculations of intermolecular potentials lies through ab initio supermolecule calculations, and it is only in relatively recent times that this view has been challenged. It is now clear that accurate intermolecular potentials can be constructed for regions where the molecules do not overlap by assembling the components described by long-range intermolecular perturbation theory —the electrostatic, induction and dispersion terms— though perturbation theory itself does not necessarily provide the best way to calculate them. At short range, the repulsion must be added, and the long-range terms modified to account for the effects of overlap. Here too, perturbation theory provides a useful to accurate numbers.

An overlap model for estimating the anisotropy of repulsion

There is an urgent need for accurate anisotropic site-site intermolecular pair potentials for use in realistic simulations of the condensed phases and spectra of van der Waals molecules. However, all attempts to find an analytical form for the repulsion energy have relied on empirical models with many fitted parameters, whose determination requires large quantities of accurate experimental or ab initio data. In this paper, we develop and assess a procedure for estimating the repulsion anisotropy from the monomer wavefunctions which is straightforward, computationally inexpensive and capable of high accuracy. The method is based on the observation that the intermolecular repulsion energy is closely related to the overlap between the unperturbed charge densities of the interacting molecules. The overlap can be treated analytically, leading to an anisotropic site-site functional form. Model pair repulsion potentials with two sites are obtained for (F2)2 and (Cl2)2 by ignoring the bonding charge density. For ...

Impact broadening of the dt micro formation resonances.

The density dependence of dt\ensuremath{\mu} molecular formation has been treated as a line-broadening process in the impact approximation. In the present paper, elastic cross sections and their coherent difference are calculated for ${\mathrm{D}}_{2}$+${\mathrm{D}}_{2}$ and (dt\ensuremath{\mu})dee+${\mathrm{D}}_{2}$ collisions. Together with the rotationally inelastic cross sections previously calculated [Phys. Rev. A 37, 329 (1988)] using the same accurate intermolecular potential, this provides all cross sections needed for evaluation of the width in the impact approximation. The impact width is dominated by elastic scattering and, at the temperature 23 K and liquid-hydrogen density, has the value 14 meV, which is more than three times the previously calculated value.

An improved potential for Ar–Kr

An accurate intermolecular potential of the HFD-C form is proposed which appears to be the best representation of the Ar–Kr interaction. The potential, which possesses the correct long range behavior, predicts a wide variety of macroscopic and microscopic data. It is consistent with accurate second virial, viscosity, and diffusion coefficients. Differential scattering and low as well as high energy total cross section data are accurately predicted.