A variational localized representation calculation of the vibrational levels of the water molecule up to 27 000 cm−1

Noncovalently bound molecular complexes beyond diatom–diatom systems: full-dimensional, fully coupled quantum calculations of rovibrational states

The methodological advances made in recent years have significantly extended the range and dimensionality of noncovalently bound molecular complexes for which full-dimensional quantum calculations of their rovibrational states are feasible.

HCl–H2O dimer: an accurate full-dimensional potential energy surface and fully coupled quantum calculations of intra- and intermolecular vibrational states and frequency shifts

The present work reports a new full-dimensional potential energy surface (PES) of the HCl–H₂O dimer, and the first fully coupled 9D quantum calculations of the intra- and intermolecular vibrational states of the complex, utilizing this PES.

Semiclassical basis sets for the computation of molecular vibrational states

In this paper, we extend a method recently reported [F. Revuelta et al., Phys. Rev. E 87, 042921 (2013)] for the calculation of the eigenstates of classically highly chaotic systems to cases of mixed dynamics, i.e., those presenting regular and irregular motions at the same energy. The efficiency of the method, which is based on the use of a semiclassical basis set of localized wave functions, is demonstrated by applying it to the determination of the vibrational states of a realistic molecular system, namely, the LiCN molecule.

Absorption cross sections of surface-adsorbed H2O in the 295-370 nm region and heterogeneous nucleation of H2O on fused silica surfaces

We have determined absorption cross sections of a monolayer of H2O adsorbed on the fused silica surfaces in the 295-370 nm region at 293 ± 1 K by using Brewster angle cavity ring-down spectroscopy. Absorption cross sections of surface-adsorbed H2O vary between (4.66 ± 0.83) × 10(-20) and (1.73 ± 0.52) × 10(-21) cm(2)/molecule over this wavelength range, where errors quoted represent experimental scatter (1σ). Our experimental study provides direct evidence that surface-adsorbed H2O is an absorber of the near UV solar radiation. We also varied the H2O pressure in the surface study cell over the 0.01-17 Torr range and obtained probe laser absorptions at 295, 340, and 350 nm by multilayer of adsorbed H2O molecules until the heterogeneous nucleation of water occurred on fused silica surfaces. The average absorption cross sections of multilayer adsorbed H2O are (2.17 ± 0.53) × 10(-20), (2.48 ± 0.67) × 10(-21), and (2.34 ± 0.59) × 10(-21) cm(2)/molecule at 295, 340, and 350 nm. The average absorption cross sections of transitional H2O layer are (6.06 ± 2.73) × 10(-20), (6.48 ± 3.85) × 10(-21), and (8.04 ± 4.92) × 10(-21) cm(2)/molecule at 295, 340, and 350 nm. The average thin water film absorption cross sections are (2.39 ± 0.50) × 10(-19), (3.21 ± 0.81) × 10(-20), and (3.37 ± 0.94) × 10(-20) cm(2)/molecule at 295 nm, 340 nm, and 350 nm. Atmospheric implications of the results are discussed.

Spectra of water dimer from a new ab initio potential with flexible monomers

We report the definition and testing of a new ab initio 12-dimensional potential for the water dimer with flexible monomers. Using our recent accurate CCpol-8s rigid water pair potential [W. Cencek, K. Szalewicz, C. Leforestier, R. van Harrevelt, and A. van der Avoird, Phys. Chem. Chem. Phys. 10, 4716 (2008)] as a reference for the undistorted monomers' geometries, a distortion correction has been added, which was taken from a former flexible-monomer ab initio potential. This correction allows us to retrieve the correct binding energy D(e)=21.0 kJ mol(-1), and leads to an equilibrium geometry in close agreement with the one obtained from benchmark calculations. The kinetic energy operator describing the flexible-monomer water dimer has been expressed in terms of Radau coordinates for each monomer and a recent general cluster polyspherical formulation describing their relative motions. Within this formulation, an adiabatic scheme has been invoked in order to decouple fast (intramolecular) modes and slow (intermolecular) ones. Different levels of approximation were tested, which differ in the way in which the residual potential coupling between the intramolecular modes located on different monomers and the dependence of the monomer rotational constants on the dimer geometry are handled. Accurate calculations of the vibration-rotation-tunneling levels of (H(2)O)(2) and (D(2)O)(2) were performed, which show the best agreement with experiments achieved so far for any water potential. Intramolecular excitations of the two monomers were calculated within two limiting cases, to account for the lack of non-adiabatic coupling between intramolecular modes due to the intermolecular motion. In the first model, the excitation was assumed to stay either on the donor or the acceptor molecule, and to hop between the two moieties upon donor-acceptor interchange. In the second model, the excitation remains on the same molecule whatever is the dimer geometry. Marginal frequency differences, less than 2 cm(-1), were obtained for all modes, and the resulting infrared shifts are in good agreement with experiments.

Infrared shifts of the water dimer from the fully flexible ab initio HBB2 potential

We report calculations of the infrared shifts for the water dimer, as obtained from the recent ab initio fully flexible HBB2 potential of Bowman and co-workers. The rovibrational calculations, which formally are 12-dimensional plus overall rotation, were performed within the [6+6]d adiabatic separation which decouples the 'fast' intramolecular modes from the 'slow' intermolecular ones. Apart from this decoupling, each set of modes is treated in a fully variational approach. The intramolecular motion was described in terms of Radau coordinates, using the f-embedding formulation of Wei & Carrington, and neglecting the rovibrational Coriolis coupling terms. Within this adiabatic approximation, the intermolecular motion is handled in a similar way as for rigid monomers, except for the rotational constants B's, averaged over intramolecular modes that depend now on the intermolecular geometry. Comparison with experimental data shows an excellent overall agreement.

New method for calculating bound states: the A(1) states of Li(3) on the spin-aligned Li(3)(1 (4)A(')) potential energy surface

In this paper, we present a calculation for the bound states of A(1) symmetry on the spin-aligned Li(3)(1 (4)A(')) potential energy surface. We apply a mixture of discrete variable representation and distributed approximating functional methods to discretize the Hamiltonian. We also introduce a new method that significantly reduces the computational effort needed to determine the lowest eigenvalues and eigenvectors (bound state energies and wave functions of the full Hamiltonian). In our study, we have found the lowest 150 energy bound states converged to less than 0.005% error, and most of the excited energy bound states converged to less than 2.0% error. Furthermore, we have estimated the total number of the A(1) bound states of Li(3) on the spin-aligned Li(3)(1 (4)A(')) potential surface to be 601.

Comparison of Perturbative and Variational Treatments of Molecular Vibrations: Application to the Vibrational Spectrum of HFCO up to 8000 cm-1†

We calculated highly excited states of the HFCO molecule, comparing results from two methods. In the first method, Van Vleck perturbation theory is used to transform away all off-diagonal couplings except those between nearly degenerate states. This perturbative transformation leads to a matrix representation where eigenvalues are obtained with relatively small matrices. In the second method, variational eigenvalues are obtained by combining the Jacobi-Wilson approach with the block-Davidson scheme. The key ingredient here is a prediagonalized-perturbative scheme applied to a subspace of a curvilinear normal-mode basis set. Comparisons of the two methods provide a critical test of the less time-consuming perturbation theory. Two different coordinate sets are used to test the sensitivity of the results to coordinate choice. Perturbation theory also requires a polynomial fit to the potential. The implications of this restriction are investigated.

An ab initio potential energy surface and vibrational states of MgH2(1 1A′)

A three-dimensional global potential energy surface for the ground electronic state of MgH(2) is constructed from more than 3000 ab initio points calculated using the internally contracted multireference configuration interaction method with the Davidson correction at the complete basis set limit. Low-lying vibrational energy levels of MgH(2) and MgD(2) are calculated using the Lanczos algorithm, and found to be in good agreement with known experimental band origins. The majority of the vibrational energy levels up to 8000 cm(-1) are assigned with normal mode quantum numbers. However, our results indicate a gradual transition from a normal mode regime for the stretching vibrations at low energies to a local mode regime near 7400 cm(-1), as evidenced by a decreasing energy gap between the (n(1),0,0) and (n(1)-1,0,1) vibrational states and bifurcation of the corresponding wave functions.