Andreas Ernesti

The intermolecular potential energy surface for CO2–Ar: Fitting to high‐resolution spectroscopy of Van der Waals complexes and second virial coefficients

Two potential energy surfaces for CO2–Ar are obtained by least‐squares fitting to the high‐resolution spectra of Van der Waals complexes and the second virial coefficients of Ar+CO2 gas mixtures. The potentials incorporate a repulsive wall based on monomer ab initio calculations and the assumption that the repulsion potential is proportional to the overlap of the monomer charge densities. The dispersion energy is represented in a two‐site model, with dispersion centers located along the C–O bonds of CO2. The resulting potentials give a good representation of all the experimental data with only three or four adjustable parameters. They are quite different from previous empirical CO2–Ar potentials, which all have either a poor representation of the attractive well or a poor representation of the repulsive wall.

An evaluation of existing potential energy surfaces for CO2–Ar: Pressure broadening and high‐resolution spectroscopy of van der Waals complexes

Nine different potential energy surfaces for CO2–Ar taken from the literature are tested for their ability to reproduce the spectra of the Ar–CO2 van der Waals complex and the pressure broadening of CO2 infrared lines by Ar. None of the potentials give a satisfactory account of all the experimental results. All the potentials are found to give significant discrepancies with at least some of the spectroscopic properties of the van der Waals complex. Coupled‐states (CS) and infinite‐order sudden (IOS) calculations of the pressure broadening cross sections are compared for a few of the potential energy surfaces. The IOS approximation is found to be seriously inaccurate for some potential surfaces, especially for high‐j lines, so that CS calculations are essential when comparing with experimental line‐shape data. CS calculations of line‐broadening cross sections are therefore carried out on all nine different potential energy surfaces. For the pressure broadening coefficients, there are substantial uncertaint...

Non-additive intermolecular forces from the spectroscopy of Van der Waals trimers: A comparison of Ar2–HF and Ar2–HCl, including H/D isotope effects

Variational calculations of vibrational energies, rotational constants and angular expectation values are carried out for the trimers Ar2–HCl, Ar2–DCl, Ar2–HF and Ar2–DF. Calculations are performed on pairwise additive potential energy surfaces and on surfaces including a variety of non-additive contributions. Attention is focused on the bending levels corresponding to hindered internal rotation of the HX molecule in the complex, several of which have been observed by high-resolution spectroscopy. The results confirm that it is crucial to include dispersion, induction and short-range effects when modelling the non-additive forces in molecular systems. It is found that the model of non-additive forces previously proposed by Ernesti and Hutson [Phys. Rev. A 51, 239 (1995)] works well for the bending bands of Ar2–HCl, Ar2–DCl and Ar2–DF as well as Ar2–HF. In addition, a new distributed model of the non-additive dispersion energy is proposed, in which the triple-dipole energy is partitioned between two anisot...

On the rotational constants of floppy molecules

Abstract Methods for calculating the rotational constants of floppy triatomic molecules and atom-linear molecule complexes are discussed. It is shown that previously published equations relating the rotational constants to expectation values of moments of inertia correspond to an approximate separation of rotation and vibration. Improved equations, which take account of the Eckart conditions, are presented. The results of the old and new methods are compared with rotational constants obtained by fitting to J -dependent eigenvalues obtained from exact close-coupling calculations on ArCO 2 . The improved equations are found to give substantially more accurate results.

Line shape, transport and relaxation properties from intermolecular potential energy surfaces: The test case of CO2–Ar

Two new potential surfaces for CO2–Ar, obtained principally from the spectra of van der Waals complexes, are tested against measurements of pressure broadening and of transport and relaxation properties, none of which was used in the determination of the surfaces. Pressure broadening of both infrared and Raman lines is considered. The coupled states (CS) approximation is used for all line shape calculations. Thermally averaged infrared and Raman cross sections at 523, 296, 160 and 77 K (infrared) and 295 K (Raman) show good agreement with the experimental data available. Generalized transport and relaxation cross sections are obtained via full classical trajectory and classical CS calculations. Properties tested include diffusion, viscosity and nuclear spin relaxation. They provide a different test of the surfaces and agree well with experiment.

CALCULATIONS OF THE SPECTRA OF RARE GAS DIMERS AND TRIMERS : IMPLICATIONS FOR ADDITIVE AND NONADDITIVE INTERMOLECULAR FORCES IN NE2-AR, NE2-KR, NE2-XE , AR2-NE, AR3, AR2-KR AND AR2-XE

Calculations of ground‐state energies and rotational constants are carried out for a variety of van der Waals dimers and trimers formed from Ne, Ar, Kr and Xe. It is found that the existing pair potentials for Ne–Ar, Ne–Kr and Ne–Xe do not adequately reproduce the measured rotational constants of the van der Waals dimers. Modified pair potentials, with equilibrium distances that differ from the originals by less than 1% but give much better rotational constants, are then proposed. Calculations of rotational constants for Ne2–Ar, Ne2–Kr and Ne2–Xe are carried out using pairwise‐additive potentials constructed from both the original and the modified pair potentials. The modified pair potentials give much better agreement with experiment for the trimers as well as the dimers. The effect of an Axilrod–Teller triple‐dipole term on the rotational constants is considered, and found to be significant, especially for the A rotational constant. However, the best available Ne–Ne potential is not accurate enough to a...

On the choice of inertial axes for interpreting spectroscopic properties of van der Waals complexes

Properties such as the dipole moments and nuclear quadrupole coupling constants of van der Waals complexes are important in the determination of intermolecular potential energy surfaces from high‐resolution spectra. The properties are often interpreted in terms of angular expectation values. It is shown that, when calculating such properties, it is important to use an inertial axis system that satisfies the Eckart conditions. Projections onto other axes, such as the intermolecular vector or the instantaneous principal axes, can lead to substantial errors when the individual monomers have large moments of inertia.

Non-additive intermolecular forces from the spectroscopy of Van der Waals trimers: the effect of monomer vibrational excitation in Ar2–HF and Ar2–HCl

Calculations on the vibration–rotation energy levels of the Van der Waals trimers Ar2–HF and Ar2–HCl are carried out, in order to investigate the role of three-body (non-pairwise-additive) forces. The present calculations focus on the lowest Van der Waals vibrational state formed from HX molecules in different intramolecular vibrational states, v. The calculations use both pairwise-additive potentials, derived from the accurately known Ar–Ar, Ar–HF and Ar–HCl potentials, and various different contributions to the three-body forces. All five intermolecular degrees of freedom are included. The red shift observed in the Ar2–HF fundamental band is fairly well predicted (within 0.5 cm–1) by the pairwise-additive potential, but the agreement with experiment is improved when non-additive forces are included. The rotational constants are found to be very sensitive to non-additive terms, and especially to the ‘exchange quadrupole’ term. Non-additive forces of the type usually used in studies of atomic forces, such as triple dipole forces, are found to be inadequate to reproduce the observed spectra.

Properties of H+ 2 relevant to the He—H+ 2 intermolecular potential: asymptotically increasing multipole moments, polarizabilities and dispersion coefficients

The quadrupole moment and polarizabilities of H+ 2 and the C 6 dispersion coefficients for He-H+ 2 are fitted to functional forms with the correct limiting behaviour for both large and small values of the H—H bond length r. Both the quadrupole moment and the dispersion coefficients increase quadratically with r at long range, while the polarizabilities increase exponentially. The reasons for the behaviour are analysed.