An ab initio investigation of the He–H2O complex

Vibrationally excited intermolecular potential energy surfaces and the predicted near infrared overtone (vOH = 2 ← 0) spectra of a H2O-Ne complex

The ab initio intra- and inter-molecular potential energy surfaces (PESs) for the H₂O-Ne system that explicitly incorporate the intramolecular overtone state (v_OH = 2) of H₂O are presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster theory [CCSD(T)-F12] level with an augmented correlation-consistent triple zeta basis set and an additional bond function. The vibrationally averaged three-dimensional intermolecular potentials for |00⁺〉, |02⁺〉, |02^-〉 and |11⁺〉 are obtained analytically by fitting to the multi-dimensional Morse/Long-range potential function form. These fits to 46 980 points have a root-mean-square (RMS) discrepancy of 0.12 cm^-1 for interaction energies less than 1000.0 cm^-1. With the vibrationally averaged PESs for the H₂O-Ne, we employed the combined radial discrete variable representation/angular finite basis representation method and Lanczos algorithm to calculate rovibrational energy levels (J = 0-10, n_s ≤ 2). The predicted infrared transitions and intensities of the para- and ortho-H₂O-Ne complex are in good agreement with the available experimental data for |02^-〉 ← |00⁺〉, |02⁺〉 ← |00⁺〉 transitions. In particular, the RMS discrepancy for |02^-〉∑^e(0₀₀,0) ← |00⁺〉∑^e(1₀₁,0), including P and R branch patterns, is only 0.045 cm^-1, which is comparable with the experimental values. These results will provide reliable theoretical guidance for the future infrared overtone spectroscopy of clusters.

A close coupling study of the bending relaxation of H2O by collision with He

We present a close coupling study of the bending relaxation of H₂O by collision with He, taking explicitly into account the bending-rotation coupling within the rigid-bender close-coupling method. A 4D potential energy surface is developed based on a large grid of ab initio points calculated at the coupled-cluster single double triple level of theory. The bound states energies of the He-H₂O complex are computed and found to be in excellent agreement with previous theoretical calculations. The dynamics results also compare very well with the rigid-rotor results available in the Basecol database and with experimental data for both rotational transitions and bending relaxation. The bending-rotation coupling is also demonstrated to be very efficient in increasing bending relaxation when the rotational excitation of H₂O increases.

Explicitly correlated ab initio potential energy surface and predicted rovibrational spectra for H2O-N2 and D2O-N2 complexes

An ab initio intermolecular potential energy surface (PES) for the van der Waals complex of H₂O-N₂ that explicitly incorporates the intramolecular Q₂ bending normal mode of the H₂O monomer is presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster theory [CCSD(T)-F12] with an augmented correlation-consistent triple zeta basis set and an additional bond function. Analytic five-dimensional intermolecular PESs for ν₂(H₂O) = 0 and 1 are obtained by fitting to the multi-dimensional Morse/long-range potential function form. These fits to 40 890 points have the root-mean-square (rms) discrepancy of 0.88 cm^-1 for interaction energies less than 2000.0 cm^-1. The resulting vibrationally averaged PESs provide good representations of the experimental microwave and infrared data: for microwave transitions of H₂O-N₂, the rms discrepancy is only 0.0003 cm^-1, and for infrared transitions of the A₁ symmetry of the H₂O(ν₂ = 1 ← 0)-N₂, the rms discrepancy is 0.001 cm^-1. The calculated infrared band origin shifts associated with the ν₂ bending vibration of water are 2.210 cm^-1 and 1.323 cm^-1 for H₂O-N₂ and D₂O-N₂, respectively, in good agreement with the experimental values of 2.254 cm^-1 and 1.266 cm^-1. The benchmark tests and comparisons of the predicted spectral properties are carried out between CCSD(T)-F12a and CCSD(T)-F12b approaches.

Zero- and high-pressure mechanisms in the complex forming reactions of OH with methanol and formaldehyde at low temperatures

A recent Ring Polymer Molecular Dynamics study of the reactions of OH with methanol and formaldehyde, at zero pressure and below 100 K, has shown the formation of long lived complexes, with long lifetimes, longer than 100 ns for the lower temperatures studied, 20-100 K (del Mazo-Sevillano et al., 2019). These long lifetimes support the existence of multi collision events with the He buffer-gas atoms under experimental conditions, as suggested by several transition state theory studies of these reactions. In this work we study these secondary collisions, as a dynamical approach to study pressure effects on these reactions. For this purpose, the potential energy surfaces of He with H2CO, OH, H2O and HCO are calculated at highly accurate ab initio level. The stability of some of the complexes is studied using Path Integral Molecular dynamics techniques, determining that OH-H2CO complexes can be formed up to 100 K or higher temperatures, while the weaker He-H2CO complexes dissociate at approximately 50 K. The predicted IR intensity spectra shows new features which could help the identification of the OH-H2CO complex. Finally, the He-H2CO + OH and OH-H2CO + He collisions are studied using quassi-classical trajectories, finding that the cross section to produce HCO + H2O products increases with decreasing collision energy, and that it is ten times higher in the He-H2CO + OH case.

Collision energy dependence of state-to-state differential cross sections for rotationally inelastic scattering of H2O by He

The inelastic scattering of H₂O by He as a function of collision energy in the range 381 cm^-1 to 763 cm^-1 at an energy interval of approximately 100 cm^-1 has been investigated in a crossed beam experiment using velocity map imaging. Change in collision energy was achieved by varying the collision angle between the H₂O and He beam. We measured the state-to-state differential cross section (DCS) of scattered H₂O products for the final rotational states J_KaKc = 1₁₀, 1₁₁, 2₂₁ and 4₁₄. Rotational excitation of H₂O is probed by (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectroscopy. DCS measurements over a wide range of collision energies allowed us to probe the H₂O-He potential energy surface (PES) with greater detail than in previous work. We found that a classical approximation of rotational rainbows can predict the collision energy dependence of the DCS. Close-coupling quantum mechanical calculations were used to produce DCS and partial cross sections. The forward-backward ratio (FBR), is introduced here to compare the experimental and theoretical DCS. Both theory and experiments suggest that an increase in the collision energy is accompanied with more forward scattering.

Rovibrational energies and spectroscopic constants for H2O-Ng complexes

In this work, rovibrational energies and spectroscopic constants for the water -Ng complexes (Ng = He, Ne, Ar, Kr and Xe) were calculated through two different approaches (by solving the Nuclear Schrödinger equation and by applying the Dunham's method) and using two different potential energy curves (PEC). These PEC were determined using potential parameters obtained through molecular beam scattering experiments and accurate theoretical calculation, respectively. It was found that the theoretical rovibrational energies are in a good agreement (only for the lowest numbers of vibrational states) with those obtained through experimental PEC. Another important conclusions was regarding the calculated first two rovibrational energies for the H 2 O-Ar system, that are in a good agreement with the experimental data.

Ground state potential energy surfaces and bound states of M-He dimers (M=Cu,Ag,Au): a theoretical investigation

We present an ab initio investigation on the ground state interaction potentials [potential energy surface (PES)] between helium and the group 11 metal atoms: copper, silver, and gold. To the best of our knowledge, there are no previous theoretical PESs proposed for Cu-He and Au-He, and a single one for Ag-He [Z. J. Jakubek and M. Takami, Chem. Phys. Lett. 265, 653 (1997)], computed about 10 years ago at MP2 level and significantly improved by our study. To reach a high degree of accuracy in the determination of the three M-He potentials (M=Cu,Ag,Au), we performed extensive series of test computations to establish the appropriate basis set, the theoretical method, and the computational scheme for these systems. For each M-He dimer we computed the PES at the CCSD(T) level of theory, starting from the reference unrestricted Hartree-Fock wave function. We described the inner shells with relativistic small core pseudopotentials, and we adopted high quality basis sets for the valence electrons. We also performed CCSDT computations in a limited set of M-He internuclear distances, adopting a medium-sized basis set, such as to define for each dimer a CCSD(T) to CCSDT correction term and to improve further the quality of the CCSD(T) interaction potentials. The Cu-He complex has minimum interaction energy (E(min)) of -28.4 microhartree at the internuclear distance of 4.59 A (R(min)), and the short-range repulsive wall starts at 4.04 A (R(E=0)). Quite interestingly, the PES of Ag-He is more attractive (E(min)=-33.8 microhartree) but presents nearly the same R(min) and R(E=0) values, 4.60 and 4.04 A, respectively. The interaction potential for Au-He is markedly deeper and shifted at shorter distances as compared to the lighter complexes, with E(min)=-69.6 microhartree, R(min)=4.09 A and R(E=0)=3.60 A. As a first insight in the structure of M-He(n) aggregates, we determined the rovibrational structure of the three M-He dimers. The Cu-He and Ag-He potentials support just few rotational excitations, while the Au-He PES admits also a bound vibrational excitation.

Close-coupling study of rotational energy transfer and differential scattering in H2O collisions with He atoms

Quantum close-coupling scattering calculations of rotational energy transfer (RET) of rotationally excited H(2)O due to collisions with He are presented for collision energies between 10(-6) and 1000 cm(-1) with para-H(2)O initially in levels 1(1,1), 2(0,2), 2(1,1), and 2(2,0) and ortho-H(2)O in levels 1(1,0), 2(1,2), and 2(2,1). Quenching cross sections and rate coefficients for state-to-state RET were computed. Both elastic and inelastic differential cross sections are also calculated and compared with relative experimental results giving generally good agreement in all cases, but less so for inelastic results. Significant differences in the computed collisional parameters, obtained on three different potential energy surfaces (PESs), were found particularly in the ultracold regime. In the thermal regime, the rate coefficients calculated on each of the surfaces are generally in better agreement and comparable, but typically larger, than those obtained in a previous calculation. Unfortunately, a lack of absolute differential or integral inelastic experimental data prevents firm determination of a preferred PES.

Application of valence-bond techniques to the study of weakly bound complexes. The potential energy surface of the Ne–CH4system

We present a comprehensive survey of the Molecular Orbital-Valence Bond (MO-VB) method, a theoretical scheme developed within the framework of the Valence Bond theory to deal with weakly bound intermolecular complexes. According to the MO-VB, the wavefunction of the system is expressed as a truncated non-orthogonal Configuration Interaction expansion, which is size extensive and a priori free of basis set superposition error. We report on the recent developments of the method, which extend the range of application of the MO-VB to intermolecular complexes with a quite large number of correlated electrons, showing that VB-based methods are nowadays a valid alternative to Molecular Orbital approaches also in this field. The MO-VB has been applied to study extensively the Ne-CH(4) complex, and compared with the more standard MP4 and CCSD(T) results. We determined two analytical Potential Energy Surfaces (PES) for this system, computed at MO-VB and MP4 level, which represent the first ones coming entirely from ab initio computations. The features of our potentials are discussed, and compared to the single analytical potential which includes the anisotropy available in the literature, determined about twenty years ago by Udo Buck and co-workers using a semiempirical approach [U. Buck, A. Kolhase, D. Secrest, T. Phillips, G. Scoles and F. Grein, Mol. Phys., 1985, 55, 1233]. The differences among the three PES are quite relevant, and are due to play a relevant role in the theoretical simulations of the dynamical properties of the Ne-CH(4) system.

Molecular-beam study of the water-helium system: Features of the isotropic component of the intermolecular interaction and a critical test for the available potential-energy surfaces

We report molecular-beam measurements of the total integral cross sections for the scattering of water molecules by helium atoms. A combined analysis of the new experimental data together with available differential cross section results has allowed an accurate determination of the isotropic component of the interaction potential for this prototypical system. The potential well shows a depth of 0.265 +/- 0.010 kJ/mol at a distance between He and the center of mass of the water molecule of 0.345 +/- 0.02 nm. An effective isotropic long-range attraction constant C(LR) = (6.3+/-0.3) x 10(-4) kJ mol(-1) nm(-6), including both dispersion and induction contributions, has also been determined. The most recent and accurate ab initio potential-energy surfaces have been tested against these new experimental results.

Potential energy surface, bound states, and rotational inelastic cross sections of the He-CH[sub 4] system: A theoretical investigation

We determined two potential energy surfaces (PES) for the He-CH(4) system by means of MP4 and Valence Bond (VB) calculations. The MP4 potential is similar to the one commonly adopted for this system [U. Buck, K. H. Kohl, A. Kolhase, M. Faubel, and U. Staemmler, Mol. Phys. 55, 1255 (1985)], while the VB PES is slightly more attractive. To evaluate the reliability of these potentials, we investigated the scattering properties by performing close coupling calculations, and concluded that: (i) the available experimental data do not permit the ranking among the PES considered; (ii) some theoretical predictions differ considerably from the experimental data, and these discrepancies cannot entirely be ascribed to the inaccuracy of the ab initio calculations; (iii) the scattering properties at low energy might discriminate between the MP4 and VB potentials.