Ab Initio Intermolecular Potentials Research Articles

Towards an understanding of the helium–acetylene van der Waals complex

The weakly bound complex He–C2D2 is studied in the ν 3 fundamental band of C2D2 (≈2440 cm−1) using a tuneable infrared diode laser to probe a pulsed supersonic slit jet expansion. This is the first published spectrum for helium–acetylene. Transitions observed in the region of the C2D2 R(0) line are assigned with the help of theoretical results based on an ab initio intermolecular potential, and fitted using a simple Coriolis model. The results indicate that the complex is rather close to the free rotor limit, helping to explain the absence of previous data. Scaled parameters from the model are used to predict a spectrum for He–C2H2.

The influence of temperature, pressure, salinity and capillary force on the formation of methane hydrate

We present here a thermodynamic model for predicting multi-phase equilibrium of methane hydrate liquid and vapor phases under conditions of different temperature, pressure, salinity and pore sizes. The model is based on the 1959 van der Waals–Platteeuw model, angle-dependent ab initio intermolecular potentials, the DMW-92 equation of state and Pitzer theory. Comparison with all available experimental data shows that this model can accurately predict the effects of temperature, pressure, salinity and capillary radius on the formation and dissociation of methane hydrate. Online calculations of the p–T conditions for the formation of methane hydrate at given salinities and pore sizes of sediments are available on: www.geochem-model.org/models.htm.

Molecular dynamics simulations of self-diffusion coefficient and thermal conductivity of methane at low and moderate densities

This article demonstrates a highly accurate molecular dynamics (MD) simulation of thermal conductivity of methane using an ab initio intermolecular potential. The quantum effects of the vibrational contribution to thermal conductivity are more efficiently accounted for in the present MD model by an analytical correction term as compared to by the Monte Carlo method. The average deviations between the calculated thermal conductivity and the experimental data are 0.92% for dilute methane and 1.29% for methane at moderate densities, as compared to approximately 20% or more in existing MD calculations. The results demonstrate the importance of considering vibrational contribution to the thermal conductivity which is mainly through the self-diffusion process.

Contributions of multipolar polarizabilities to the isotropic and anisotropic light scattering induced by molecular interactions in gaseous methane

The binary isotropic and anisotropic collision-induced light scattering spectra of gaseous methane at room temperature are analyzed in terms of a recent ab initio intermolecular potential and interaction-induced pair polarizability trace and anisotropy models, using quantum line-shapes computations. The translational spectra at relatively low frequencies are determined largely by the effects of bound and free transitions. At intermediate frequencies the spectra are sensitive to both the attractive part of the potential and the short-range part of the polarizability trace and anisotropy. The high frequency wings are discussed in terms of the collision-induced rotational Raman effect and estimates for the dipole-quadrupole polarizability A and the dipole-octopole polarizability E are obtained and checked with recent ab initio theoretical values.

A Quantum Chemical and Molecular Dynamics Study of the Coordination of Cm(III) in Water

Molecular dynamics simulations of Cm(III) in water were performed at two different temperatures, namely, T = 300 K and T = 473 K. Fully ab initio intermolecular potentials were employed. At the lower temperature, T = 300 K, nine water molecules coordinate preferentially the Cm(III) ion in the first coordination sphere, while at the higher temperature, T = 473 K, the preferential coordination number is eight instead of nine. The number of water molecules in the second coordination sphere is not uniquely defined, but the most probable number is 16.

Rotational Structure of Small 4He Clusters Seeded with HF, HCl, and HBr Molecules

Diffusion Monte Carlo calculations are performed for ground and excited rotational states of HX(4He)N, complexes with N<or=20 and X=F, Cl, Br. The calculations are done using ab initio He-HX intermolecular potentials whose computation is described. Intermolecular energies and He radial and angular probability density distributions are computed as a function of the number of solvent atoms. Excited states are calculated using fixed-node diffusion Monte Carlo methods, and molecule-solvent angular momentum coupling is studied as a function of cluster size and potential anisotropy. Nodal surfaces of the many-body wave function are computed approximately by making an adiabatic Born-Oppenheimer-like separation of radial and angular degrees of freedom of the cluster. This procedure is extended to include radial dependencies in the adiabatic nodal function. We predict that the observed decrease in the gas-phase rotational constants for HCl and HBr in a 4He nanodroplet will be smaller than that observed for HF, despite HF's having the largest (by far) gas-phase rotational constant of the three molecules. This suggests that the specifics of the solvation dynamics of a molecule in a 4He cluster are the result of a delicate interplay between the magnitude of the gas-phase rotational constant of the molecule and the anisotropic contributions to the atom-molecule potential energy.

An accurate model to predict the thermodynamic stability of methane hydrate and methane solubility in marine environments

An accurate thermodynamic model is proposed to predict the thermodynamic stability of methane hydrate in marine environments and methane concentration needed to form hydrate in the absence of gas phase. Taking into account the effect of capillary force and salinity on chemical potential of CH 4 and H 2O, this study extends the Van der Waals–Platteeuw model and our approach to calculate the Langmuir constant from angle-dependent ab initio intermolecular potentials to marine environments. The Gibbs–Thomson equation with correct parameters for hydrate–water interface is used to account for the capillary effect of porous sediments. The Pitzer model is used to calculate the activity coefficients of H 2O and CH 4 in methane–seawater system. Comparison with the experimental data shows that this model can predict the equilibrium P– T condition of CH 4 hydrate in porous media and predict CH 4 solubility at hydrate–water equilibrium with high accuracy. The prediction of this model shows that CH 4 solubility in liquid phase at hydrate–liquid equilibrium will be reduced by increasing salinity, but it will be increased by reducing pore sizes of porous sediments. Online calculations of the P– T condition for the formation of methane hydrate and the methane solubility at a given salinity and pore sizes of sediments is made available on: www.geochem-model.org/models.htm.

Hydration Structure and Dynamic Properties of the Square Planar Pt(II) Aquaion Compared to the Pd(II) Case

This work examines, by means of classical Molecular Dynamics simulations, the hydration of a square planar hydrate, [Pt(H2O)4]2+, focussing the attention on the structure and dynamics adopted by water molecules in the regions above and underneath the molecular plane. Results obtained are compared with those previously derived for the case of the [Pd(H2O)4]2+ where the concept of meso-shell was introduced to define this axial region [Martinez et al. (J Phys Chem B 108:15851, 2004)]. Specific ab initio intermolecular potentials describing the interaction between the ion and the solvent have been developed following the statistical implementation of the hydrated ion concept for the case of a planar aquaion. A meso-shell is characterized by a peak in the Pt–O RDF centered at 2.95 A which integrates to two water molecules; the mean residence time for these molecules is in the range 1–7 ps. The vibrational frequency associated to the dynamic variable defined from the distance meso-shell water molecule-cation is used to quantify the linkage degree of the water molecule in this shell. The meso-shell in Pt(II) is more labile than in the Pd(II) case, whereas the first and second hydration shells of both cations are highly similar. The observed differences in meso-shell are discussed in relation with the mechanistic interpretation of the solvent exchange at the first hydration shell.

Prediction of CH 4 and CO 2 hydrate phase equilibrium and cage occupancy from ab initio intermolecular potentials

Presented is an improved model for the prediction of phase equilibria and cage occupancy of CH 4 and CO 2 hydrate in aqueous systems. Different from most hydrate models that employ Kihara potential or Lennard-Jones potential with parameters derived from experimental phase equilibrium data of hydrates, we use atomic site-site potentials to account for the angle-dependent molecular interactions with parameters directly from ab initio calculation results. Because of this treatment, our model can predict the phase equilibria of CH 4 hydrate and CO 2 hydrate in binary systems over a wide temperature-pressure range (from 243–318 K, and from 10–3000 bar for CH 4 hydrate; from 253–293 K, and from 5–2000 bar for CO 2 hydrate) with accuracy close to experiment. The average deviation of this model from experimental data is less than 3% in pressures for a given temperature. This accuracy is similar to previous models for pressures below 500 bar, but is more accurate than previous models at higher pressures. This model is also capable of predicting the cage occupancy and hydration number for CH 4 hydrate and CO 2 hydrate without fitting any experimental data. The success of this study validates the predictability of ab initio intermolecular potentials for thermodynamic properties.

Understanding the Hydration Structure of Square-Planar Aquaions: The [Pd(H2O)4]2+ Case

Ion solvation of monatomic cations has usually been rationalized on the basis of concentric shell models because of the electrostatic field generated by the metal cation. This work examines, by means of molecular dynamics simulations, the solvation phenomenon for a square-planar hydrate, [Pd(H2O)4]2+, addressing the question of the structure adopted by water solvent in the regions above and below the molecular plane. Specific ab initio intermolecular potentials describing the interaction between the ion and the solvent have been developed, extending the statistical implementation of the hydrated ion concept. Results show how water molecules in these regions present structural and dynamic properties markedly different from those of the first and the second shells. Whereas average distances are close to those of the first hydration shell, their orientation is similar to that of the second shell, and their mean residence times are even shorter than those of the second hydration shell. This region, called the meso shell, could help in understanding peculiar properties of transition-metal cation square-planar complexes, such as their particular facility to be incorporated in confined regions such as those present in complex biomolecules or interlayered solid structures.

Theoretical study of the He-HF+ complex. II. Rovibronic states from coupled diabatic potential energy surfaces.

The bound rovibronic levels of the He-HF+ complex were calculated for total angular momentum J=1/2, 3/2, 5/2, 7/2, and 9/2 with the use of ab initio diabatic intermolecular potentials presented in Paper I and the inclusion of spin-orbit coupling. The character of the rovibronic states was interpreted by a series of calculations with the intermolecular distance R fixed at values ranging from 1.5 to 8.5 A and by analysis of the wave functions. In this analysis we used approximate angular momentum quantum numbers defined with respect to a dimer body-fixed (BF) frame with its z axis parallel to the intermolecular vector R and with respect to a molecule-fixed (MF) frame with its z axis parallel to the HF+ bond. The linear equilibrium geometry makes the He-HF+ complex a Renner-Teller system. We found both sets of quantum numbers, BF and MF, useful to understand the characteristics of the Renner-Teller effect in this system. In addition to the properties of a "normal" semirigid molecule Renner-Teller system it shows typical features caused by large-amplitude internal (bending) motion. We also present spectroscopic data: stretch and bend frequencies, spin-orbit splittings, parity splittings, and rotational constants.

Monte Carlo simulations of thermodynamic properties of argon, krypton and xenon in liquid and gas state using new ab initio pair potentials

The internal energies and compressibility factors of argon, krypton and xenon have been simulated using recent state-of-the-art ab initio pair intermolecular potentials and the best semi-empirical pair potentials, and the Axilrod-Teller-Muto three-body term. The results are compared with experimental data for both sub-critical and super-critical temperatures and for densities ranging up to a 2.5 multiple of the critical density. Both the ab initio and semi-empirical results for argon are in very good agreement with the experimental ones. For krypton and xenon, the ab initio results are worse than the semi-empirical results but they are still acceptable.

Ab initio pair potential and phase equilibria predictions for hydrogen chloride

An ab initio intermolecular pair potential for hydrogen chloride has been computed using symmetry-adapted perturbation theory and an extended basis set at a level of theory equivalent to fourth-order Møller–Plesset perturbation theory. Three different site–site pair potential functions were used to fit the ab initio energies, and these were then used to calculate the second virial coefficient and in Gibbs ensemble Monte Carlo simulations to determine the vapor–liquid equilibria. The accurate predictions of the phase behavior compared to experimental data with only pairwise interactions suggest that these are the predominant contribution, and that nonpairwise additivity and quantum effects are not important for HCl. Our results are also compared with those of another ab initio-based pair potential previously reported in the literature.

Molecular simulation of the vapour–liquid phase coexistence of neon and argon using ab initio potentials

Gibbs ensemble simulations using ab initio intermolecular potentials are reported for the vapour–liquid phase coexistence of neon and argon. For neon two different quantum chemical ab initio potentials of well-known quality are used to investigate the effect of the quality of pair interactions. In addition calculations are also reported for neon using a potential that includes three-body interactions. For argon, simulations are compared with results obtained from NPH-ensemble molecular dynamics simulations. It is found that the results of a perfect pair potential must occur outside the experimental temperature–density phase envelope. Therefore, if a perfect pair potential is used, many-body interactions and quantum effects must be considered to obtain good agreement with experiment.

Experimental and theoretical CO2–Ar pressure-broadening cross sections and their temperature dependence

Line broadening coefficients of the CO2–Ar fundamental ν3 band are obtained experimentally and theoretically for temperatures from 120 to 765 K. The experimental results are obtained by Fourier-transform infrared spectroscopy. Theoretical values are provided ia quantum-mechanical (close coupling and coupled states) approaches as well as semiclassical (improved Smith–Giraud–Cooper and Robert–Bonamy) methods. The most up-to-date CO2–Ar ab initio intermolecular potential (J. M. Hutson, A. Ernesti, M. M. Law, C. F. Roche and R. J. Wheatley, J. Chem. Phys., 1996, 105, 9130) is employed for all calculations. For all the temperatures probed, theoretical values are found to be in a rather good agreement with experiment. In addition, the Raman Q(j) line broadening coefficients and the application of the random phase approximation are presented.

Ab Initio Crystal Structure Predictions for Flexible Hydrogen-Bonded Molecules

Crystal structure predictions for glycol and glycerol are reported. A series of increasingly accurate energy calculations is applied, and the final predictions are based solely on ab initio derived energies. A recently developed transferable ab initio potential is used for intermolecular interactions, augmented with ab initio derived conformational energies. The experimental structure of glycol was predicted with a low energy, 1.1 kJ/mol above the global minimum. For glycerol the experimental structure corresponded to the global minimum. This latter result provides a proposal for the positions of the hydrogen atoms in the crystal structure of glycerol. A three-dimensional hydrogen-bonded network is formed which consists only of intermolecular hydrogen bonds. Together with previous work, the ab initio intermolecular potential has now been applied to predict the crystal structures of six different compounds. The energy difference between the observed crystal structure and the global energy minimum varied fr...

The inter molecular potential of NH+ 4-Ar II. Calculations and experimental measurements for the rotational structure of the v 3 band

The mid-infrared spectrum of the v 3,(t 2) transition of the NH+ 4-Ar complex has been recorded at rotational resolution using photofragmentation spectroscopy. The spectrum is divided into perpendicular and parallel subbands corresponding to transitions between different hindered internal rotor states. The P and R branches of the strongest perpendicular subbands are rotationally resolved providing rotational and centrifugal distortion constants. The widths of individual rotational lines are limited by the laser bandwidth of 0.02 cm−1, giving a lower limit of 250 ps for the lifetime of the excited states. Effective intermolecular separations for each internal rotor state are determined from its rotational constant, after correction for the contribution due to Coriolis coupling between the internal and total rotational angular momenta. The absolute energies, rotational and distortion constants for the first few intermolecular bending and stretching levels of the ground intramolecular vibrational state are determined in a numerical solution to the rotation-intermolecular vibration Hamiltonian, employing a three-dimensional ab initio intermolecular potential. The results are compared with the experimental constants in order to assess the accuracy of the calculated potential. The relative energy levels from this calculation are also compared with those from a two-dimensional representation of the potential energy surface (‘fixed-R’ model) in order to judge directly the influence of the radial dependence of the potential.

Transferable ab Initio Intermolecular Potentials. 1. Derivation from Methanol Dimer and Trimer Calculations

An intermolecular potential was derived from ab initio calculations on methanol dimers and trimers. Ninety-four methanol dimer geometries and seventeen trimer geometries have been studied using an interaction-optimized basis set. Because we aimed at transferability, all terms of the model were fitted separately, in order to obtain a potential in which these energy terms have a well-defined physical meaning. Electrostatic interactions were described by atomic multipole moments obtained by fitting to the monomer electrostatic potential. The polarization energy was modeled using atomic dipole polarizabilities. Scaled empirical polarizabilities reproduced the energy nonadditivity in methanol trimers very accurately. The dispersion energy was described by a damped r-6 atom−atom potential, which was fitted to separately calculated dispersion energies. Exchange energy and remaining short-ranged terms were modeled by an exponential repulsion term, including some anisotropic features. This repulsion model was fitt...

Nuclear Dynamics of Benzene···(Ar)n Clusters

In this paper we report on constant energy molecular dynamics (MD) simulations of nuclear dynamics of benzene−Ar and benzene−Arn (n = 2−7) clusters, utilizing the ab initio intermolecular potential of benzene−Ar. The dissociation dynamics of benzene−Ar resulting from a nonselective excitation of the intermolecular modes exhibits an excess energy dependence which can be accounted for in terms of the Rice−Ramsperger−Kassel−Marcus (RRKM) statistical theory. The time-resolved isomerization dynamics of benzeneAr2 and rigid nonrigid isomerization of benzene−Arn (n = 2−7) was investigated. The intracluster vibrational energy redistribution (ICVR) in benzene−Ar from an intramolecular vibration to the intermolecular modes is inefficient on the nanosecond time scale for all of the (one quantum or two quanta) vibrational excitation of each mode. The mode selectivity of the ICVR and its linear dependence on the excess vibrational energy were established. The vibrational predissociation (VP) times for single-mode excitations of benzene−Ar are also slow on the nanosecond time scale (>22 ns), in accord with experimental data. The retardation of ICVR and VP of benzene−Ar reflects the bottleneck effect for intracluster vibrational energy flow, due to the considerable frequency mismatch between the high-frequency vibrational modes of benzene (ω = 402−2955 cm-1) and the low-frequency van der Waals vibrational modes (ω = 26 and 40 cm-1).

Variation of intermolecular interaction and local lattice distortion of parahydrogen crystals upon vibrational excitation

When a hydrogen molecule in a parahydrogen crystal is excited to a high vibrational overtone state, its electronic properties vary significantly since the energy of excitation is a sizable fraction of the energies of the excited electronic states. Thus the vibrational excitation leads to a significant variation of the intermolecular potential and resultant local distortion of the crystal lattice. Such an effect is sensed by the variation of the splitting of $M$ sublevels of the $J=1$ ${\mathrm{H}}_{2}$ impurity that causes the overtone transition. In this paper we attempt to theoretically explain the observed large variations of the pair and crystal-field splitting parameters $\ensuremath{\Delta}B$ and ${\ensuremath{\varepsilon}}_{2c}$ upon the $v=3\ensuremath{\leftarrow}0$ vibrational excitation. Using the ab initio anisotropic intermolecular pair potential and simplified models on the vibrational dependence of the potential and crystal distortion, observed values are well explained.