Ab Initio Potential Data Research Articles

New theoretical study of potassium perturbed by He and a comparison to laboratory spectra

The visible and near-infrared spectra of late L- and T-type dwarf stars are dominated in large part by the resonance lines of neutral Na and K. It is the collision broadening of these atomic lines by H2 and He in the stellar atmosphere that determines the continuum from below 0.5 µm to above 0.9 µm in the spectrum. Their line profiles can be detected as far as 3000 cm−1 from the line center in T dwarfs and consequently an accurate and detailed determination of the complete profile, including the extreme far wing, is required to model the contribution of these strong alkali resonance lines to brown dwarf spectra. We report on our new calculations of unified line profiles of K perturbed by He using ab initio potential data for the conditions prevailing in cool substellar brown dwarfs and hot dense planetary atmospheres with temperatures from Teff = 500 to 3000 K. For such objects with atmospheres of H2 and He, conventional laboratory absorption spectroscopy can be used to examine the line wings and test the line shape theories and molecular potentials. We find that an analytical Lorentzian profile is useful for a few cm−1 from the line center, but not in the line wings, where the radiative transfer is a consequence of the K–He radiative collisions that are sensitive to the interaction potentials. Tables of the K–He absorption coefficients of the resonance lines allow accurate model atmospheres and synthetic spectra. For this purpose, we present new opacities from comprehensive line shape theory incorporating accurate ab initio potentials. Use of these new tables for the modeling of emergent spectra will be an improvement over previous line shape approximations based on incomplete or inaccurate potentials. We also present Lorentzian impact parameters obtained in the semi-classical and quantum theory for the K 4s − 4p resonance line centered at 0.77 µm specifically for the line core regime.

PES and transport properties of the He⋯HBr complex from kinetic theory and molecular dynamics simulations.

The ab initio intermolecular potential of the He⋯HBr van der Waals (vdW) complex was calculated at the CCSD(T)/a5zBF level of theory and expanded in terms of the orthogonal Legendre polynomials. The PES then was implemented to calculate the interaction viscosity (η12) and diffusion (D12) coefficients through classical Mason-Monchick approximation (MMA), quantum mechanical close-coupling (CC), and molecular dynamics (MD) simulations. Energy-dependent Senftleben-Beenakker (SB) cross-sections were calculated using rotationally averaged cross-sections, and Boltzmann averaging was used to reveal the temperature dependence of the SB cross-sections over the temperature range of T = 50-1000 K. The calculated transport properties from MMA are in close agreement with the CC results, especially for temperatures lower than T = 900 K. The ab initio potential data then were used to derive LJ (12,6) and Vashishta MD force fields, and the equilibrium MD simulation methods were implemented to extract η12 and D12 coefficients using Einstein formulas. It was found that the Vashishta 3-body interaction potential model shows better accuracy than the LJ (12,6) model in MD simulations of D12. For η12, however, both MD potential models are successful, and an average absolute deviation of lower than 1% was obtained when compared to the quantum mechanical CC method.

A Refined Intermolecular Interaction Potential for Methane: Spectral Analysis and Molecular Dynamics Simulations

AbstractThe previously constructed methane interaction potential energy surface calculated at the second‐order Møller‐Plesset (MP2) perturbation theory has been significantly improved in two aspects. First, all ab initio potential energy data are calculated by the supermolecule counterpoise corrected coupled cluster with single and double and perturbative triple excitations [CCSD(T)] method with Dunning’s correlation‐consistent aug‐cc‐pVXZ, X=D, T, Q, 5, basis sets and extrapolated to the complete basis set (CBS) limits with a convergence precision of 0.01 kcal/mol. Second, instead of the simple 4‐site model proposed in the previous study, a 5‐site model has been used to represent the ab initio potential data. The simulated infrared spectrum using the potential energy surface seems to be broadly in line with the spectral features observed in experiments. Molecular dynamics simulations using the ab initio force field show quantitative agreements with experiments. The properties examined in this paper include the atom‐to‐atom radial distribution functions in liquid and supercritical phases and the self‐diffusion coefficients over a wide range of thermodynamic conditions. It is shown that the refined ab initio force field can be applied to study fluid properties in different phases.

Molecular Dynamics Simulations of Liquid Water Structure and Diffusivity

An ab initio force field has been constructed to study the structure and diffusivity for liquid water using molecular dynamics simulations. The potential energy data were calculated by using both the second order Moller-Plesset (MP2) perturbation theory and the coupled cluster method with single, double, and perturbative triple excitations [CCSD(T)], together with the correlation consistent basis set up to aug-cc-pV5Z. For configuration sampling, we considered 10 dimer orientations, each with about 121 grid points. With an additional ”polarization” term, an analytical function similar to that for the well-established TIP4P model was used to represent the ab initio potential data. The simulation results were compared with those using several empirical force fields (SPC/E, TIP4P etc.) or the recent CC-pol series of ab initio force fields and also with available experimental measurements. We obtained good quantitative agreement for the atom-wise radial distribution functions and the self-diffusion coefficients over a wide range of thermodynamic conditions. This new ab initio force field can thus serve as a suitable starting point for predicting liquid water properties without priori empirical knowledge.

Liquid chloroform structure from computer simulation with a fullab initiointermolecular interaction potential

We have calculated the intermolecular interaction energies of the chloroform dimer in 12 orientations using the second-order Møller-Plesset perturbation theory. Single point energies of important geometries were calibrated by the coupled cluster with single and double and perturbative triple excitation method. Dunning's correlation consistent basis sets up to aug-cc-pVQZ have been employed in extrapolating the interaction energies to the complete basis set limit values. With the ab initio potential data we constructed a 5-site force field model for molecular dynamics simulations. We compared the simulation results with recent experiments and obtained quantitative agreements for the detailed atomwise radial distribution functions. Our results were also consistent with previous results using empirical force fields with polarization effects. Moreover, the calculated diffusion coefficients reproduced the experimental data over a wide range of thermodynamic conditions. To the best of our knowledge, this is the first ab initio force field which is capable of competing with existing empirical force fields for liquid chloroform.

Accurate ab initio Calculations of Methane Dimer Interaction Energies and Molecular Dynamics Simulation of Fluid Methane

AbstractIntermolecular interaction potentials of the methane dimers have been calculated for 12 symmetric conformations using the Hartree-Fock (HF) self-consistent theory, the second-order M�ller-Plesset (MP2) perturbation theory, and the coupled-cluster with single and double and perturbative triple excitations (CCSD(T)) theory. The HF calculations yield unbound potentials largely due to the exchange-repulsion interaction. In MP2 and CCSD(T) calculations, the basis set effects on the repulsion exponent, the equilibrium bond length, the binding energy, and the asymptotic behavior of the calculated intermolecular potentials have been thoroughly studied. We have employed basis sets from the Slater-type orbitals fitted with Gaussian functions, Pople�s medium size basis sets to Dunning�s correlation consistent basis sets. With increasing basis size, the repulsion exponent and the equilibrium bond length converge at the 6-31G** basis set and the 6-311++G(2d, 2p) basis set, respectively, while a large basis set (aug-cc-pVTZ) is required to converge the binding energy at a chemical accuracy (˜0.01 kcal/mol). We used the BSSE corrected results that systematically converge to the destined potential curve with increasing basis size. The binding energy calculated and the equilibrium bond length using the CCSD(T) method are close to the results at the basis set limit. For molecular dynamics simulation, a 4-site potential model with sites located at the hydrogen atoms was used to fit the ab initio potential data. This model stems from a hydrogen-hydrogen repulsion mechanism to explain the stability of the dimer structure. MD simulations using the ab initio PES show good agreement on both the atom-wise radial distribution functions and the self-diffusion coefficients over a wide range of experimental conditions.

Molecular dynamics simulations of fluid methane properties using ab initio intermolecular interaction potentials

Intermolecular interaction energy data for the methane dimer have been calculated at a spectroscopic accuracy and employed to construct an ab initio potential energy surface (PES) for molecular dynamics (MD) simulations of fluid methane properties. The full potential curves of the methane dimer at 12 symmetric conformations were calculated by the supermolecule counterpoise-corrected second-order Møller-Plesset (MP2) perturbation theory. Single-point coupled cluster with single and double and perturbative triple excitations [CCSD(T)] calculations were also carried out to calibrate the MP2 potentials. We employed Pople's medium size basis sets [up to 6-311++G(3df, 3pd)] and Dunning's correlation consistent basis sets (cc-pVXZ and aug-cc-pVXZ, X = D, T, Q). For each conformer, the intermolecular carbon-carbon separation was sampled in a step 0.1 A for a range of 3-9 A, resulting in a total of 732 configuration points calculated. The MP2 binding curves display significant anisotropy with respect to the relative orientations of the dimer. The potential curves at the complete basis set (CBS) limit were estimated using well-established analytical extrapolation schemes. A 4-site potential model with sites located at the hydrogen atoms was used to fit the ab initio potential data. This model stems from a hydrogen-hydrogen repulsion mechanism to explain the stability of the dimer structure. MD simulations using the ab initio PES show quantitative agreements on both the atom-wise radial distribution functions and the self-diffusion coefficients over a wide range of experimental conditions.

Vibrational Levels of Methanol Calculated by the Reaction Path Version of MULTIMODE, Using an ab initio, Full-Dimensional Potential

An accurate potential energy surface has been determined for methanol from ab initio potential data at the CCSD(T) level of theory with an aug-cc-pVTZ basis. The resulting potential function is valid over all twelve vibrational degrees of freedom for all near-equilibrium and torsional configurations. A torsional reaction path has been derived for this potential, from which the low-lying vibrational levels of methanol have been calculated by the reaction path version of MULTIMODE. Comparisons with experiment and other calculations are made.

The vibrations of glyoxal, studied by ‘Multimode’, with a large amplitude motion, using an ab initio potential surface

The procedure ‘Multimode’ was introduced by Carter and Bowman [1998, J. chem. Phys., 108, 43971, to calculate the vibrations of polyatomic molecules using normal coordinates. Recently Carter and Handy have introduced an extension to ‘Multimode’ to include one large amplitude torsional vibration for molecules for which an analytical potential is available. This procedure is now extended to molecules for which ab initio potential data may be calculated. Glyoxal (CHOCHO) is studied using the density functional approach to generate the potential surface data. The full J = 0 vibrational spectra is presented.

Joint treatment of ab initio and experimental data in molecular force field calculations with Tikhonov’s method of regularization

We describe a novel method for employing calculated ab initio potential data together with Tikhonov’s variational procedure to extract fundamental molecular force field parameters from experimental spectral data, the formal ‘‘inverse problem’’ of vibrational spectroscopy. In this approach, the ab initio quantities serve to ‘‘regularize’’ the initially ill-posed problem (in the sense of Tikhonov), leading to variationally stable and unique force field parameters that optimally mimic overall patterns of the (approximate) ab initio quantities, but exactly reproduce the available experimental data within specified experimental precision. In this manner, ab initio and experimental data can be jointly combined to produce more stable and reliable force fields (improvable to any degree through higher level ab initio treatment, additional experimental data, etc.) than could be attained by theoretical or experimental methods alone. The proposed procedure allows use of any system of generalized coordinates, including redundant systems of internal or symmetry coordinates, simplifying the transfer and comparison of force constants in series of related molecules. The procedure is illustrated with numerical application to CHF2Cl and its isotopomers at MP2/3-21G*, MP2/6-31G* levels of theory, demonstrating the stability and consistency of force fields obtained from different levels of theoretical input.

Semi-classical multi-dimensional study of the inelastic and reactive interaction of D2(v, j) with a non-rigid Cu(111) surface

Classical trajectory and semi-classical wavepacket calculations on the dynamics of deuterium colliding with a Cu(111) surface have been performed. It is shown that both electron–hole pair and phonon excitation may be important dynamical aspects to include in the treatment. The calculations were carried out using an analytical potential surface fit to ab initio potential data. The lowest barrier height to dissociation on this surface is ca. 0.9 eV. This results in very small dissociation probabilities at energies below 1 eV. It is shown that a potential-energy surface with a barrier of ca. 0.5 eV is more reasonable if agreement with recent experimental data is to be obtained.

Experimental and theoretical studies of simple attractive Penning ionization systemsag

Quantum mechanical calculations based on local complex potential theory and ab initio potential data are presented for the Penning ionization systems He*(2 3S) +H, Li, Na, and He*(2 1S) +Na. The dependence of the electron energy spectrum on the complex entrance channel potential and on the collision energy is shown within the “ΔJ≡J+ = -J*=0” approximation (i.e. conservation of the heavy particle angular momentum upon electron emission). Further calculations demonstrate the more detailed structure of the electron spectra if a selected number of heavy particle partial waves is considered only. High resolution, angular dependent electron spectrometric studies of the systems He* + Li, Na show the need to take angular momentum transfer (ΔJ≠0) into account. The new experimental results are discussed in connection with earlier work, and quantal model calculations with the inclusion of rotational energy transfer illustrate how the angular dependence of the electron spectra can be qualitatively understood in terms of a recently given semiclassical model.