A new hybrid density functional, APF, is introduced, which avoids the spurious long-range attractive or repulsive interactions that are found in most density functional theory (DFT) models. It therefore provides a sound baseline for the addition of an empirical dispersion correction term, which is developed from a spherical atom model (SAM). The APF-D empirical dispersion model contains nine adjustable parameters that were selected based on a very small training set (15 noble gas dimers and 4 small hydrocarbon dimers), along with two computed atomic properties (ionization potential and effective atomic polarizability) for each element. APF-D accurately describes a large portion of the potential energy surfaces of complexes of noble gas atoms with various diatomic molecules involving a wide range of elements and of dimers of small hydrocarbons, and it reproduces the relative conformational energies of organic molecules. The accuracy for these weak interactions is comparable to that of CCSD(T)/aug-cc-pVTZ c...

A Density Functional with Spherical Atom Dispersion Terms

A procedure for “morphing” an ab initio potential energy surface to obtain agreement with experimental data is presented. The method involves scaling functions for both the energy and the intermolecular distance. In the present work, the scaling functions are parametrized and determined by least-squares fitting to the experimental data. The method is tested on the system Ne–HF, for which high-resolution infrared spectra are available. It is shown to work well even with relatively low-level ab initio calculations. Several basis sets are investigated at the CCSD(T) correlation level, including various aug-cc-pVnZ basis sets and the specially-tailored Ne–HF basis set of ONeil et al. All give good results after morphing, but the changes needed to match experiment are much smaller for the ONeil basis set. The use of MP2 calculations is also investigated: again, the MP2 potential is quite satisfactory after morphing, but requires much more modification than the CCSD(T) potential.

/pdf/morphing-ab-initio-potentials-a-systematic-study-of-ne-hf-9d5r5ny912.pdf

Morphing ab initio potentials: A systematic study of Ne–HF

We review the recent progress achieved in the theoretical description of chemical reactions at low temperatures. In particular, we discuss the crucial role played by the van der Waals interaction potential in quantum-mechanical scattering calculations of atom–diatom collisions in the cold and ultracold regimes, where abstraction reactions proceed by tunnelling. The importance of zero-energy resonances in enhancing the reactivity in the zero-temperature limit is assessed. The impact of Feshbach resonances associated with the decay of metastable states of van der Waals complexes on reactive scattering is addressed through a series of examples. Finally, we discuss the sensitivity of the reaction dynamics to the topology of the van der Waals well. Contents PAGE 1.  Introduction 284 2.  Atom–diatom quantum scattering at low temperatures 286 2.1  Theoretical method286 2.2  Properties of low-temperature scattering287 3.  Reactive scattering at low energies 288 4.  Sensitivity of the reaction dynamics to details ...

Importance of long-range interactions in chemical reactions at cold and ultracold temperatures

Symmetry‐adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–CO complex. The interaction energy is found to be dominated by the first‐order exchange contribution and the dispersion energy. The ab initio potential has a single minimum of em=−24.895 cm−1 for the linear CO–He geometry at Rm=6.85 bohr. The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–CO molecule and the infrared spectrum, which corresponds to the simultaneous excitation of vibration and internal rotation in the CO subunit within the complex. The predicted positions and intensities of lines in the infrared spectrum are in good agreement with the experimental spectrum [C.E. Chuaqui et al., J. Chem. Phys. 101, 39 (1994)]. The theoretical potential was also checked by comparison of computed excess second virial coefficients with the experimental data. The ab initio interaction ...

/pdf/ab-initio-potential-energy-surface-infrared-spectrum-and-3yv5e0ilom.pdf

Ab initio potential energy surface, infrared spectrum, and second virial coefficient of the He–CO complex

The need for better potential-energy models for atom–molecule and molecule–molecule interactions is discussed and the utility of the exchange–coulomb (XC) model is critically examined, by fitting a potential based on it to new high-resolution discrete infrared data for the He–CO Van der Waals molecule. In addition to explaining the observed spectrum as well as does an optimized empirical potential previously determined from the same data, the resulting XC surface is expected to be more realistic in regions not directly sampled by the fitted data.

Improved modelling of atom–molecule potential-energy surfaces: illustrative application to He–CO

A reliable new three‐dimensional potential energy surface is obtained for the H2–Ar system using an exchange‐coulomb potential model with five parameters determined empirically from a least‐squares fit to experimental data. This surface fully accounts for new high resolution IR data, virial coefficients, and vibrational transition pressure‐shifting coefficients used in the analysis, and yields excellent predictions of elastic and inelastic scattering cross sections and hyperfine transition intensities not included in the analysis. Quantitative comparisons with the best previous empirical potential and a high quality fully ab initio potential are also presented.

/pdf/a-reliable-new-potential-energy-surface-for-h2-ar-3ftaxedmuc.pdf

A reliable new potential energy surface for H2–Ar

The pure rotational transition (J,j,l)=(101)←(000) of the weakly bound CO-He complex has been observed in the 17 GHz region for five isotopomers, namely 12C16O-4He, 13C16O-4He, 12C18O-4He, 13C18O-4He, and 13C17O-4He, using a pulsed-jet cavity Fourier-transform microwave spectrometer. Hyperfine structure due to the quadrupolar 17O (I=5/2) nucleus has been observed and analyzed to yield the quadrupole coupling parameters. A new microwave-terahertz double resonance spectrometer has been used to carry out an experiment on 13C16O-4He in which a submillimeter-wave pump transition (J,j,l)=(110)←(000) and a microwave signal transition (211)←(110) were detected. Infrared spectra of 13C16O-4He and 12C18O-4He, have been obtained in the 2100 cm−1 region of the C-O stretch using a tunable diode laser spectrometer and a long-path (200 m), low-temperature (46 K) equilibrium gas cell. The combined data are sufficient to construct essentially complete experimental energy level schemes for the bound states of the 12C16O-4H...

Isotopic probing of very weak intermolecular forces : microwave and infrared spectra of co-he isotopomers

Pure rotational spectra of three isotopomers of the Van der Waals complex Ar–N2 have been investigated in the frequency range 3.5–20 GHz, using a pulsed molecular beam cavity microwave Fourier-transform spectrometer. Rotational constants and quartic and sextic centrifugal distortion constants have been obtained, along with N hyperfine constants. The spectra of Ar–14N2 and Ar–15N2 indicate equivalence of the nitrogen nuclei, and thus confirm C2v symmetry for the complexes. The measured transition frequencies and the derived constants have been used to test the best available literature potential-energy surfaces for the Ar–N2 interaction. For this purpose rotational transition frequencies and expectation values of other properties were calculated and compared with the corresponding values from the microwave experiments. A refined version of one of the surfaces has been generated by inclusion of the microwave results.

Pure rotational spectrum of, and potential-energy surface for, the Ar?N2 Van der Waals complex

Measurements of the nuclear magnetic resonance (NMR) longitudinal relaxation time T1 have been carried out for D2–He gas mixtures at temperatures of 293 and 220 K for two concentrations of D2 in He and the results extrapolated linearly to infinite dilution at each temperature. Full quantum close‐coupled scattering calculations have also been carried out for an ab initio D2–He interaction potential. The agreement found between measured and calculated relaxation times is excellent at 293 and good at 220 K. Based on the level of agreement found between theory and experiment, it can be concluded that the molecular hydrogen–helium ab initio potential energy surface employed in the present D2–He calculations as well as in earlier H2–He calculations [J. Chem. Phys. 81, 5275 (1984)] represents well the effects of both the spin–rotation and combined dipolar/quadrupolar intramolecular interactions, which dominate the spin relaxation, respectively, of protons and deuterons in the hydrogen isotopes. At the same time,...

Deuteron magnetic resonance probe of the D2–He potential energy surface

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