Intermolecular forces for hydrogen,nitrogen and acetylene

Orientation-dependent intermolecular potentials for H₂, N₂, and C₂H₂ have been determined on the basis of electron charge density contours, octopolar induction in the dispersion force, electrostatic quadrupolar interaction, and the observed second virial coefficients. The recently settled structure of low-temperature solid acetylene has been discussed.     

Intermolecular potential energy surface of Ar-NO

Rotational spectra of an open-shell complex, Ar-NO, in the electronic ground state have been analyzed by employing an analysis using a free-rotor model, where previously observed data by Mills et al. [J. Phys. Chem. 90, 3331 (1986); 90, 4961 (1986)] and additional transitions observed by Fourier-transform microwave spectroscopy in the present study are simultaneously analyzed with a standard deviation of the least-squares fit to be 27.5 kHz. A two-dimensional intermolecular potential energy surface for Ar-NO has been determined from the analysis. The determined potential energy surface is compared with those of Ar-OH and Ar-SH, which are also complexes containing an open-shell species with the 2Pi ground electronic state.     

Intermolecular potential functions and the properties of water

A wide range of gas, liquid and solid state properties are calculated using most of the presently available potential functions for the water pair interaction. It is shown that no one model gives a satisfactory account of all three phases. We propose a new semi-empirical model that has some success as an effective pair potential in all three phases.     

Studies of intersystem crossing dynamics in acetylene

We report a new ab initio study of the acetylene T3 potential energy surface, which clarifies the nature of its energy minimum, and present computed equilibrium geometries and diabatic frequencies. This information enables the computation of harmonic vibrational overlap integrals of T3 vibrational levels with the S1 3nu3 state. The results of this calculation support the interpretation of two local perturbations of S1 3nu3, revealed in ultraviolet laser-induced fluorescence/surface electron ejection by laser excited metastables spectroscopy and Zeeman anticrossing measurements, respectively, as arising from two rotational submanifolds of a single T3 vibrational state. We present plausible assignments for this state as a guide for future experimental work.     

Molecular dynamics calculations for solid and liquid acetylene

Two recently proposed intermolecular potentials are used in computer simulations of bulk acetylene. One of the potentials may prove a useful starting point in the construction of a satisfactory model, but progress is hampered by the lack of suitable experimental data.     

Intermolecular potential and second virial coefficient of the water-hydrogen complex

We construct a rigid-body (five-dimensional) potential-energy surface for the water-hydrogen complex using scaled perturbation theory (SPT). An analytic fit of this surface is obtained, and, using this, two minima are found. The global minimum has C2v symmetry, with the hydrogen molecule acting as a proton donor to the oxygen atom on water. A local minimum with Cs symmetry has the hydrogen molecule acting as a proton acceptor to one of the hydrogen atoms on water, where the OH bond and H2 are in a T-shaped configuration. The SPT global minimum is bound by 1097 microEh (Eh approximately 4.359744 x 10(-18) J). Our best estimate of the binding energy, from a complete basis set extrapolation of coupled-cluster calculations, is 1076.1 microEh. The fitted surface is used to calculate the second cross virial coefficient over a wide temperature range (100-3000 K). Three complementary methods are used to quantify quantum statistical mechanical effects that become significant at low temperatures. We compare our results with experimental data, which are available over a smaller temperature range (230-700 K). Generally good agreement is found, but the experimental data are subject to larger uncertainties.     

Intermolecular potential and second virial coefficient of the water-nitrogen complex

The authors construct a rigid-body (five-dimensional) potential energy surface for the water-nitrogen complex using the systematic intermolecular potential extrapolation routine. The intermolecular potential is then extrapolated to the limit of a complete basis set. An analytic fit of this surface is obtained, and, using this, the global minimum energy is found. The minimum is located in an arrangement in which N2 is near the H atom of H2O, almost collinear with the OH bond. The best estimate of the binding energy is 441 cm-1 (1 cm-1 approximately 1.986 43x10(-23) J). The extrapolated potential is then used to calculate the second cross virial coefficient over a wide temperature range (100-3000 K). These calculated second virial coefficients are generally consistent with experimental data, but for the most part the former have smaller uncertainties.     

Short-time vibrational dynamics of acetylene versus its isotopic variants

The short-time intramolecular dynamics of highly vibrationally excited HCCD and DCCD, as determined by classical trajectories, have qualitative features distinct from HCCH and from one another. The possible differences are also considered from the point of view of the symmetries of the normal modes. The short-time evolution will be reflected in the coarse-grained frequency spectrum and could be detectable via stimulation emission pumping.     

Intermolecular polarizability dynamics of aqueous formamide liquid mixtures studied by molecular dynamics simulations

A molecular dynamics simulation study is presented for the relaxation of the polarizability anisotropy in liquid mixtures of formamide and water, using a dipolar induction scheme that involves the intrinsic polarizability and first hyperpolarizability tensors of the molecules, and the dipole-quadrupole polarizability of water species. The long time diffusive decay of the collective polarizability anisotropy correlations exhibits a substantial slowing down as the formamide mole fraction increases in the mixture. The diffusive times for the polarizability relaxation obtained from the authors' simulations are in good agreement with optical Kerr effect experimental data, and they are found to correlate nearly linearly with the estimated mean lifetimes of the hydrogen bonds within the mixture, suggesting that the relaxation of the hydrogen bond network is responsible to some extent for the collective relaxation of the polarizability anisotropy of the mixture. The short time behavior of the polarizability anisotropy relaxation was investigated by computing the nuclear response function, R(t), which is very rapidly dominated by the formamide contribution as it is added to water, due to the much larger polarizability anisotropy of formamide molecules compared to that of water. Several contributions to the Raman spectrum were also analyzed as a function of composition, and the dynamical origin of the different bands was determined.     

Intermolecular potential energy surface for CS2 dimer

A new four‐dimensional intermolecular potential energy surface for CS₂ dimer is obtained by ab initio calculation of the interaction energies for a range of configurations and center‐of‐mass separation distances for the first time. The calculations were performed using the supermolecular approach at the Møller–Plesset second‐order perturbation (MP2) level of theory with the augmented correlation consistent basis sets (aug‐cc‐pVxZ, x = D, T) and corrected for the basis‐set superposition error using the full counterpoise correction method. A two‐point extrapolation method was used to extrapolate the calculated energy points to the complete basis set limit. The effect of using the higher levels of theory, quadratic configuration interaction containing single, double, and perturbative triple excitations QCISD(T) and coupled cluster singles, doubles and perturbative triples excitations CCSD(T), on the shape of potential energy surface was investigated. It is shown that the MP2 level of theory apparently performs extremely poorly for describing the intermolecular potential energy surface, overestimating the total energy by a factor of nearly 1.73 in comparison with the QCISD(T) and CCSD(T) values. The value of isotropic dipole–dipole dispersion coefficient (C₆) of CS₂ fluid was obtained from the extrapolated MP2 potential energy surface. The MP2 extrapolated energy points were fitted to well‐known analytical potential functions using two different methods to represent the potential energy surface analytically. The most stable configuration of the dimer was determined at R = 6.23 au, α = 90°, β = 90°, and γ = 90°, with a well depth of 3.980 kcal mol^?1 at the MP2 level of theory. Finally, the calculated second virial coefficients were compared with experimental values to test the quality of the presented potential energy surface. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011.     

Intermolecular potential for thermal H2O-He collisions

Theoretical potentials for rotational excitation of H2O by He were constructed via several methods, all of which start with a large basis set SCF interaction. The semiempirical Hartree-Fock with damped dispersion (HFD) model adds a damped long-range attraction with parameters adjusted to fit experimental total differential cross sections. Purely ab initio potentials add correlation energies obtained via perturbation theory (MP2 and MP4) or a variational method (ICF1). Scattering calculations were performed on all surfaces to compare with available beam scattering and pressure broadening data and to assess sensitivity of state-to-state rates to uncertainties in the potential. From comparison with the limited experimental data, the ICF1 surface appears to be marginally better than the MP4 surface. Thermal rates calculated from this surface should be accurate to better than 50%, at least for the larger, more important rates.     

Intermolecular potential calculations for polynuclear aromatic hydrocarbon clusters

Calculations of intermolecular potentials are presented for homo-molecular and hetero-molecular clusters of 24 peri-condensed PAH spanning monomer masses ranging from 78 to 1830 Da. Binding energies of homo-molecular dimers rise rapidly with molecular size and asymptotically approach the experimentally established exfoliation energy for graphite of 5.0 kJ mol(-1) (carbon atom)(-1). Binding energies of hetero-molecular dimers correlate well with the reduced mass of the pair. From calculations of homo-molecular stacks, binding energies were observed to increase with each added molecule and rise asymptotically, approaching a limit which scales linearly with monomer molecular mass. These results are reviewed in the context of molecular growth in flames and in the context of astrophysical observations.     

Intermolecular potential parameters and combining rules determined from viscosity data

The law of corresponding states has been demonstrated for a number of pure substances and binary mixtures and provides evidence that the transport properties viscosity and diffusion can be determined from a molecular shape function, often taken to be a Lennard–Jones 12‐6 potential, that requires two scaling parameters: a well depth ε_ij and a collision diameter σ_ij, both of which depend on the interacting species i and j. We obtain estimates for ε_ij and σ_ij of interacting species by finding the values that provide the best fit to viscosity data for binary mixtures and compare these to calculated parameters using several “combining rules” that have been suggested for determining parameter values for binary collisions from parameter values that describe collisions of like molecules. Different combining rules give different values for σ_ij and ε_ij, and for some mixtures the differences between these values and the best‐fit parameter values are rather large. There is a curve in (ε_ij, σ_ij) space such that parameter values on the curve generate a calculated viscosity in good agreement with measurements for a pure gas or a binary mixture. The various combining rules produce couples of parameters ε_ij, σ_ij that lie close to the curve and, therefore, generate predicted mixture viscosities in satisfactory agreement with experiment. Although the combining rules were found to underpredict the viscosity in most of the cases, Kong's rule was found to work better than the others, but none of the combining rules consistently yields parameter values near the best‐fit values, suggesting that improved rules could be developed. © 2010 Wiley Periodicals, Inc. *

1 This article is a U.S. Government work and, as such, is in the public domain of the United States of America.

Int J Chem Kinet 42: 713–723, 2010     

Intermolecular dynamics in crystalline iron octaethylporphyrin (FeOEP)

The new technique of nuclear resonance vibrational spectroscopy (NRVS) has increased the range and quality of dynamical data from Fe-containing molecules that when combined with Raman and infrared spectroscopies impose stricter constraints on normal mode simulations, especially at lower frequencies. Going beyond the usual single molecule approximation, a classical normal-mode analysis that includes intermolecular coupling and the full crystalline symmetry is found to produce a better fit with fewer free parameters for the heme compound iron octaethylporphyrin (FeOEP), using NRVS data from polycrystalline material. Off-diagonal force constants were completely unnecessary, indicating that their role in previous single molecule fits was just to emulate intermolecular coupling. Sound velocities deduced from the calculated phonon dispersion curves are compared to NRVS measurements to further constrain the intermolecular force constants. The NRVS data by themselves are insufficient to rigorously determine all unknown force constants for molecules of this size, but the improved crystal model fit indicates the necessity of including intermolecular interactions for normal-mode analyses.     

On the lattice dynamics of solid nitrogen

Lattice statics and dynamics calculations for α-nitrogen have been done using the 6-exp atom-atom interaction to derive intermolecular forces. The three parameters of this interaction are reduced to one by constraining them so that the unit cell and sublimation energy are calculated to agree with experiment. Choice of the one parameter is then made using spectroscopic data. This 'best' potential is then used to calculate the statics and dynamics of β-nitrogen. This is stable only under pressure, and the calculation of the lattice dynamics under pressure gives a satisfactory agreement with experiment.     

Raman spectra and lattice dynamics of cubic gauche nitrogen

The lattice dynamical properties of the cubic gauche phase of nitrogen are computed using density functional perturbation theory. The structure is found to be stable up to at least 250 GPa. Based on the dynamical data we derive the thermodynamical properties. We also determine the Raman spectra with both peak position and intensity and find excellent agreement with the experimental data, with the A mode dominating the spectra at all pressures.     

Quaternion formulation of lattice dynamics of molecular crystals

The use of quaternions to specify rigid-molecule orientation is reviewed. Application of this formalism to lattice dynamics of molecular crystals is developed. Basic formulae for harmonic and anharmonic approximations are summarised.     

Probing pentacene polymorphs by lattice dynamics calculations

We have performed a lattice dynamics calculation to compute the "inherent structures" of minimum potential energy for pentacene, starting from available X-ray data. The calculation shows that two distinct bulk crystalline phases of pentacene exist, with very subtle structural differences but clearly different phonon spectra. The method of crystal growth (from solution or vapor) is not the determining factor for obtaining either structure.