Ab initio potential energy surface and infrared spectra of H2-CO and D2-CO van der Waals complexes

Third-order interactions in symmetry-adapted perturbation theory

We present an extension of many-body symmetry-adapted perturbation theory (SAPT) by including all third-order polarization and exchange contributions obtained with the neglect of intramonomer correlation effects. The third-order polarization energy, which naturally decomposes into the induction, dispersion, and mixed, induction-dispersion components, is significantly quenched at short range by electron exchange effects. We propose a decomposition of the total third-order exchange energy into the exchange-induction, exchange-dispersion, and exchange-induction-dispersion contributions which provide the quenching for the corresponding individual polarization contributions. All components of the third-order energy have been expressed in terms of molecular integrals and orbital energies. The obtained formulas, valid for both dimer- and monomer-centered basis sets, have been implemented within the general closed-shell many-electron SAPT program. Test calculations for several small dimers have been performed and their results are presented. For dispersion-bound dimers, the inclusion of the third-order effects eliminates the need for a hybrid SAPT approach, involving supermolecular Hartree-Fock calculations. For dimers consisting of strongly polar monomers, the hybrid approach remains more accurate. It is shown that, due to the extent of the quenching, the third-order polarization effects should be included only together with their exchange counterparts. Furthermore, the latter have to be calculated exactly, rather than estimated by scaling the second-order values.

Quenching of rotationally excited CO by collisions with H2

Quantum close-coupling and coupled-states approximation scattering calculations of rotational energy transfer in CO due to collisions with H2 are presented for collision energies between 10(-6) and 15,000 cm(-1) using the H2-CO interaction potentials of Jankowski and Szalewicz [J. Chem. Phys. 123, 104301 (2005); 108, 3554 (1998)]. State-to-state cross sections and rate coefficients are reported for the quenching of CO initially in rotational levels j2 = 1-3 by collisions with both para- and ortho-H2. Comparison with the available theoretical and experimental results shows good agreement, but some discrepancies with previous calculations using the earlier potential remain. Interestingly, elastic and inelastic cross sections for the quenching of CO (j2 = 1) by para-H2 reveal significant differences at low collision energies. The differences in the well depths of the van der Waals interactions of the two potential surfaces lead to different resonance structures in the cross sections. In particular, the presence of a near-zero-energy resonance for the earlier potential which has a deeper van der Waals well yields elastic and inelastic cross sections that are about a factor of 5 larger than that for the newer potential at collision energies lower than 10(-3) cm(-1).

A new ab initio interaction energy surface and high-resolution spectra of the H2–CO van der Waals complex

A new four-dimensional intermolecular potential-energy surface for the H(2)-CO complex is presented. The ab initio points have been computed on a five-dimensional grid including the dependence on the H-H separation (the C-O separation was fixed). The surface has then been obtained by averaging over the intramolecular vibration of H(2). The coupled-cluster supermolecular method with single, double, and noniterative triple excitations has been used to calculate the interaction energy. The correlation part of the interaction energy has been obtained from extrapolations based on calculations in a series of basis sets. An analytical fit of the ab initio potential-energy surface has the global minimum of -93.049 cm(-1) at the intermolecular separation of 7.92 bohr for the linear geometry with the C atom pointing toward the H(2) molecule. For the other linear geometry, with the O atom pointing toward H(2), the local minimum of -72.741 cm(-1) has been found for the intermolecular separation of 7.17 bohr. The potential has been used to calculate the rovibrational energy levels of the para-H(2)-CO complex. The results agree very well with those observed by McKellar [A. R. W. McKellar J. Chem. Phys. 108, 1811 (1998)]: the discrepancies are smaller than 0.1 cm(-1). The calculated dissociation energy is equal to 19.527 cm(-1) and significantly smaller than the value of 22 cm(-1) estimated from the experiment. Predictions of rovibrational energy levels for ortho-H(2)-CO have also been done and can serve as a guidance to assign recorded experimental spectra. The interaction second virial coefficient has been calculated and compared with the experimental data.

Infrared spectra of CO2–H2 complexes

Infrared spectra of weakly bound CO(2)-H(2) complexes have been studied in the region of the CO(2) v(3) asymmetric stretch, using a tunable diode laser probe and a pulsed supersonic jet expansion. For CO(2)-paraH(2), results were obtained for three isotopic species, (12)C(16)O(2), (13)C(16)O(2), and (12)C(18)O(2). These spectra were analyzed using an asymmetric rotor Hamiltonian, with results that resembled those obtained previously for OCS- and N(2)O-paraH(2), except that half the rotational levels were missing due to the symmetry of CO(2) and the spin statistics of the (16)O or (18)O nuclei. However, for CO(2)-orthoH(2), more complicated spectra were observed which could not be assigned, in contrast with OCS- and N(2)O-H(2) where the paraH(2) and orthoH(2) spectra were similar, though distinct. The CO(2)-paraH(2) complex has a T-shaped structure with and intermolecular distance of about 3.5 Angstroms, and the CO(2) v(3) vibration exhibits a small redshift (-0.20 cm(-1)) in the complex.

Small para-hydrogen clusters doped with carbon monoxide: Quantum Monte Carlo simulations and observed infrared spectra

The structures and rotational dynamics of clusters of a single carbon monoxide molecule solvated in para-hydrogen, (paraH(2))(N)-CO, have been simulated for sizes up to N=17 using the reptation Monte Carlo technique. The calculations indicate the presence of two series of R(0) rotational transitions with J=1<--0 for cold clusters, similar to those predicted and observed in the case of He(N)-CO. Infrared spectra of these clusters have been observed in the region of the C-O stretch ( approximately 2143 cm(-1)) in a pulsed supersonic jet expansion using a tunable diode laser probe. With the help of the calculations, the observed R(0) rotational transitions have been assigned up to N=9 for the b-type series and N=14 for the a-type series. Theory and experiment agree rather well, except that theory tends to overestimate the b-type energies. The (paraH(2))(12)-CO cluster is calculated to be particularly stable and (relatively) rigid, corresponding to completion of the first solvation shell, and it is observed to have the strongest a-type transition.

Approximate generation of full-dimensionalab initiovan der Waals surfaces for high-resolution spectroscopy

A method for the generation of highly accurate, nearly-exact, full-dimensional interaction energy surfaces for weakly interacting subsystems is proposed. The method is based on the local expansion of the exact interaction energy surface in the Taylor series with respect to intramolecular coordinates. It is shown that without any significant loss of accuracy this expansion can be limited to a few low-order terms. This leads to significant savings in computations of the full-dimensional interaction energy surfaces. Also a method for the direct calculation of the interaction energy surface of reduced dimensionality, corresponding to averaging over the intramolecular vibrations, without explicit knowledge of the full-dimensional surface, is presented. The main ideas and computational features of the proposed scheme are comprehensively tested for the Ar-HF system.