T.G.A. Heijmen

Symmetry-adapted perturbation theory for the calculation of Hartree-Fock interaction energies

A symmetry-adapted perturbation theory is formulated for the calculation of Hartree-Fock interaction energies of closed-shell dimers. The proposed scheme leads to a basis-set-independent interpretation of the Hartree-Fock interaction energy in terms of basic concepts of the theory of intermolecular forces: electrostatics, exchange and induction. Numerical results for different geometries of HE2, Ne2, He-C2H2, He-CO, Ar-HF, (HF)2 and (H2O)2 complexes show that in the region of the van der Waals minimum the proposed perturbation theory reproduces accurately the Hartree-Fock interaction energy. This fast convergence and relatively small computational cost of the proposed perturbation scheme suggest that this method is a practical alternative for the standard supermolecular approach.

A new He–CO interaction energy surface with vibrational coordinate dependence. I. Ab initio potential and infrared spectrum

The intermolecular potential energy surface of the He–CO complex including the CO bond length dependence has been calculated using symmetry-adapted perturbation theory (SAPT). The potential has a minimum of em=−23.734 cm−1 with Rm=6.53 bohr at a skew geometry (ϑm=48.4°) if the molecular bond length is fixed at the equilibrium value of 2.132 bohr. We have applied the potential in the calculation of bound state levels and the infrared spectrum for the 3He–CO and 4He–CO complexes. The computed ab initio transition frequencies are found to agree within 0.1 cm−1 with experiment. In paper II [J. P. Reid, H. M. Quiney, and C. J. S. M. Simpson, J. Chem. Phys. 107, 9929 (1997)], the potential surface is used to calculate vibrational relaxation cross sections and rate constants.

AB INITIO POTENTIAL-ENERGY SURFACE AND ROTATIONALLY INELASTIC INTEGRAL CROSS SECTIONS OF THE AR-CH4 COMPLEX

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential-energy surface of the Ar–CH4 complex. The interaction energy, including high-level intramonomer correlation effects, is found to be dominated by the first-order exchange contribution and the dispersion energy. The ab initio potential has four equivalent minima of em=−144.30 cm−1 at Rm=7.00 bohr, for structures in which the argon atom approaches the face of the CH4 tetrahedron. The computed potential-energy surface has been analytically fitted and used in converged close-coupling calculations to generate state-to-state integral cross sections for rotational excitation of CH4 in collisions with argon. The computed cross sections are generally in good agreement with the experimental data [W. B. Chapman et al., J. Chem. Phys. 105, 3497 (1996)]. Some discrepancies for the smallest cross sections can be explained by the influence of sequential collision channels, with the use of a master equation approach.

Ab initio collision-induced polarizability, polarized and depolarized Raman spectra, and second dielectric virial coefficient of the Helium diatom

Symmetry‐adapted perturbation theory has been applied to compute the interaction‐induced polarizability for the helium diatom. The computed polarizability invariants have been analytically fitted, and used in quantum‐dynamical calculations of the binary collision‐induced Raman spectra. The predicted intensities of the depolarized spectrum are in good agreement with the experimental data [M.H. Proffitt et al., Can. J. Phys. 59, 1459 (1981)]. The computed polarized spectrum shows agreement with the experiment within the large experimental uncertainties. The calculated trace polarizability was also checked by comparison of computed second dielectric virial coefficients with the experimental data. The ab initio dielectric virial coefficients, including first and second quantum corrections, agree well with the experimental data from indirect measurements.

Symmetry-adapted perturbation theory applied to interaction-induced properties of collisional complexes

A symmetry-adapted perturbation theory (SAPT) is formulated for interaction-induced electrical properties of weakly bound complexes. Asymptotic (large R) expressions are reported for the contributions to the collision-induced dipole moments and polarizabilities up to and including second order in the intermolecular potential. These long-range expressions require knowledge only of the multipole moments and (hyper)polarizabilities of the isolated monomers. Numerical results are given for the dipole moment of He-H2 and the polarizability of He2 and the accuracy of the SAPT approach is examined by comparison with full configuration interaction results. The role of various physical contributions to the dipole moment of He-H2 and the polarizability of He2 is investigated. The validity of the long-range approximation and the importance of charge penetration (damping) effects are discussed.

The rotational and vibrational dynamics of argon–methane. I. A theoretical study

Reported here is a theoretical calculation of the spectrum of the argon–methane complex based upon a previously reported ab initio potential energy surface [T. G. A. Heijmen et al., J. Chem. Phys. 107, 902 (1997)]. The irregular rotational structure observed in the spectrum is traced to a combination of the fact that the excited vibrational state of the methane has vibrational angular momentum and the methane rotates nearly freely within the complex. The theoretical spectrum is compared qualitatively with the experimental results obtained using the optothermal method, a more complete discussion of which is given in a companion paper, hereafter referred to as paper II.

Infrared spectroscopy and ab initio potential energy surface for Ne-C2H2 and Ne-C2HD complexes

The rotationally resolved spectra of Ne–C2H2 and Ne–C2HD were measured in the region of the asymmetric C–H stretch (ν3) band of the acetylene monomer. The transitions in the Ne–C2H2 spectrum are substantially broadened by vibrational predissociation, while those of Ne–C2HD are quite narrow. This difference is attributed to the fact that in the former dissociation proceeds through a “doorway” state, related to a Fermi resonance involving the bending vibrations of C2H2. In C2HD this Fermi resonance is absent. The potential energy surface (PES) for the Ne–acetylene complex has been computed using symmetry-adapted perturbation theory. This PES has been fit to an analytic form and applied in calculations of the rovibrational energy levels of Ne–C2H2 and Ne–C2HD. From these levels and calculated transition intensities we generated the near-infrared spectra of these complexes in the region of the ν3 band. These complexes may be considered as nearly free internal rotors. For Ne–C2H2 the results obtained from the ...

Rotational state-to-state rate constants and pressure broadening coefficients for He–C2H2 collisions: Theory and experiment

Converged close-coupling and coupled-states calculations were used to obtain state-to-state rate constants and pressure broadening coefficients for the collisional rotational (de-)excitation of C2H2 by He. The ab initio potential used in these calculations was previously computed by symmetry-adapted perturbation theory. The computed pressure broadening coefficients and total rate constants agree well with the available experimental data. In the experimental part of the paper stimulated Raman-pumping has been used to prepare acetylene in selected rotational states (ji=2 to 18 and ji=1 to 19 of the C≡C stretching mode). The population decay in the prepared state and the transfer to other rotational states was monitored by laser induced fluorescence. The experimental data can be described by an infinite-order-sudden power law (IOS-P) or directly compared with the ab initio derived rate constants. The influence of multiple collisions possible at the relatively large pressure-delay-products employed has been t...

Symmetry-adapted perturbation theory of nonadditive three-body interactions in van der Waals molecules. II. Application to the Ar2–HF interaction

Symmetry-adapted perturbation theory (SAPT) of three-body forces is applied to characterize the nonadditive interactions in the Ar2–HF trimer. The origins of the anisotropy of the nonadditive Ar2–HF potential are discussed, and the results are compared with the existing ab initio data. The multipole-expanded expressions for the induction, induction–dispersion, and dispersion nonadditivities in terms of the multipole moments and (hyper)polarizabilities are derived for the special case of atom–atom–diatom complexes, and the validity of the multipole approximation is investigated by comparison of the expanded and nonexpanded energies computed at the same level of the theory and in the same basis sets. Finally, recent (semi)empirical models of nonadditive interactions in Ar2–HF based on the exchange quadrupole electrostatic interaction are analyzed in terms of contributions as defined by SAPT. It is shown that the present level of the SAPT theory correctly accounts for the terms included in the exchange quadr...

The rotational and vibrational dynamics of argon-methane. II. Experiment and comparison with theory

Presented here is a detailed comparison between the experimental near infrared spectrum of argon–methane and the results of a theoretical calculation based upon the methods described in a companion paper, hereafter referred to as paper I [T. G. A. Heijmen et al., J. Chem. Phys. 110, 5639 (1999), preceding paper]. Many of the bands in the spectrum are easily assigned directly from this comparison. The spectrum is shown to be highly sensitive to the anisotropy of the argon–methane potential surface and the agreement with the ab initio spectrum, although not quantitative, is very good. The predissociation linewidths observed in the experimental spectra are found to be strongly dependent upon the symmetry of the excited state. Symmetry considerations place restrictions on the final rotational states that can be accessed, possibly explaining the differences in the lifetimes.