Calculation of the rotation–vibration states of water up to dissociation (original) (raw)
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First-principles rotation–vibration spectrum of water above dissociation
Chemical Physics Letters, 2011
High-level ab initio electronic structure and variational nuclear motion computations are combined to simulate the spectrum of the water molecule at and above its first dissociation limit. Results of these computations are compared with the related state-selective multi-photon measurements of Grechko et al. [J. Chem. Phys. 138 ]. Both measured and computed spectra show pronounced structures due to quasi-bound (resonance) states. Traditional resonance features associated with trapping of vibrational or rotational energy of the system are identified and assigned. A strong and broad feature observed slightly above dissociation is found to be associated with direct photodissociation into the continuum.
Journal of Quantitative Spectroscopy and Radiative Transfer, 2010
A new, accurate, global, mass-independent, first-principles potential energy surface (PES) is presented for the ground electronic state of the water molecule. The PES is based on 2200 energy points computed at the all-electron aug-cc-pCV6Z IC-MRCI(8,2) level of electronic structure theory and includes the relativistic one-electron massvelocity and Darwin corrections. For H 2 16 O, the PES has a dissociation energy of D 0 = 41 109 cm À 1 and supports 1150 vibrational energy levels up to 41 083 cm À 1 . The deviation between the computed and the experimentally measured energy levels is below 15 cm À 1 for all the states with energies less than 39 000 cm À 1 . Characterization of approximate vibrational quantum numbers is performed using several techniques: energy decomposition, wave function plots, normal mode distribution, expectation values of the squares of internal coordinates, and perturbing the bending part of the PES. Vibrational normal mode labels, though often not physically meaningful, have been assigned to all the states below 26 500 cm À 1 and to many more above it, including some highly excited stretching states all the way to dissociation. Issues to do with calculating vibrational band intensities for the higher-lying states are discussed.
Chemical Physics Letters, 2000
The major factor determining the accuracy of ro-vibrational spectra of the water molecule is the accuracy of the Ž . underlying potential energy surface PES . We discuss improving the ab initio PES by introducing a correction to represent, accurately, the change in potential from equilibrium to linear geometries. We show the improvements which this has on calculated vibrational band origins and rotational energy levels by comparison with experimental data. We predict a barrier to linearity of 11 105 " 5 cm y1 , consistent with, but more accurate than, recent studies. This potential provides the optimum starting point for a proper fit of the PES. q
Calculating the vibration–rotation spectrum of water
Physica Scripta, 2006
The spectrum of water is ubiquitous and therefore the demand for a reliable theoretical model for it remains pressing. The treatment of nuclear motion using methods based on the variational principle has led to a major advance in the development of the model but places great emphasis on the accurate treatment of the underlying potential energy surface. Progress in constructing high-accuracy potentials both ab initio and by fitting to spectroscopic data is discussed.
2013
A recently computed, high-accuracy ab initio Born−Oppenheimer (BO) potential energy surface (PES) for the water molecule is combined with relativistic, adiabatic, quantum electrodynamics, and, crucially, nonadiabatic corrections. Calculations of ro-vibrational levels are presented for several water isotopologues and shown to have unprecedented accuracy. A purely ab initio calculation reproduces some 200 known band origins associated with seven isotopologues of water with a standard deviation (σ) of about 0.35 cm −1. Introducing three semiempirical scaling parameters, two affecting the BO PES and one controlling nonadiabatic effects, reduces σ below 0.1 cm −1. Introducing one further rotational nonadiabatic parameter gives σ better than 0.1 cm −1 for all observed rovibrational energy levels up to J = 25. We conjecture that the energy levels of closed-shell molecules with roughly the same number of electrons as water, such as NH 3 , CH 4 , and H 3 O + , could be calculated to this accuracy using an analogous procedure. This means that near-ab initio calculations are capable of predicting transition frequencies with an accuracy only about a factor of 5 worse than high resolution experiments.
Vibration-rotation levels of water beyond the Born-Oppenheimer approximation
Chemical Physics Letters, 1996
The value of the adiabatic correction to the Born-Oppenheimer electronic energy is calculated as a function of geometry for water using SCF wavefunctions. A mass-dependent adiabatic function is combined with high-accuracy ab initio electronic structure calculations due to Partridge and Schwenke. Vibrational band origins for H20, D20, T20, HDO, HTO and DTO are analysed. Unlike previous calculations on the H + system, it is suggested that non-adiabatic effects are more important 3 than adiabatic ones in determining accurate isotope dependence of the vibrational band origins of water. Use of the adiabatic surface and effective masses of the heavy particles intermediate between the nuclear and atomic masses is found to significantly improve predictions of rotational term values. The adiabatic correction is found to be of particular importance for rotational levels with high K,,.
The Journal of Chemical Physics, 2007
Quantum dynamical calculations are reported for the zero point energy, several low-lying vibrational states, and the infrared spectrum of the H 5 O 2 + cation. The calculations are performed by the multiconfiguration time-dependent Hartree ͑MCTDH͒ method. A new vector parametrization based on a mixed Jacobi-valence description of the system is presented. With this parametrization the potential energy surface coupling is reduced with respect to a full Jacobi description, providing a better convergence of the n-mode representation of the potential. However, new coupling terms appear in the kinetic energy operator. These terms are derived and discussed. A mode-combination scheme based on six combined coordinates is used, and the representation of the 15-dimensional potential in terms of a six-combined mode cluster expansion including up to some 7-dimensional grids is discussed. A statistical analysis of the accuracy of the n-mode representation of the potential at all orders is performed. Benchmark, fully converged results are reported for the zero point energy, which lie within the statistical uncertainty of the reference diffusion Monte Carlo result for this system. Some low-lying vibrationally excited eigenstates are computed by block improved relaxation, illustrating the applicability of the approach to large systems. Benchmark calculations of the linear infrared spectrum are provided, and convergence with increasing size of the time-dependent basis and as a function of the order of the n-mode representation is studied. The calculations presented here make use of recent developments in the parallel version of the MCTDH code, which are briefly discussed. We also show that the infrared spectrum can be computed, to a very good approximation, within D 2d symmetry, instead of the G 16 symmetry used before, in which the complete rotation of one water molecule with respect to the other is allowed, thus simplifying the dynamical problem.
J Phys Chem a, 2000
The aim of the present paper is to evaluate the influence of the solute-solvent interactions on the infrared spectra of water diluted in liquid CCl 4 and in supercritical xenon, considered as the standard 'inert' solvent. This investigation is based upon FTIR spectra analyzed at the light of both analytical treatments and molecular dynamics simulations. For water in supercritical xenon, the rotational relaxation processes mainly determine the shape of the IR profiles associated with the ν 1 and ν 3 stretching modes. The water molecule rotates almost "freely" due to the isotropic character of the van der Waals interactions applied on the solute. Both the J-model for asymmetric molecular rotor and the molecular dynamics simulations properly account for the band shapes associated with the ν 3 and ν 1 vibrational modes of water. Thus, the rotational dynamics of water is primarily governed by "collisional" interactions with the neighboring solvent molecules. For water dissolved in liquid CCl 4 , a structural analysis based upon the simulated radial distribution functions provides evidence for the existence of a short-ranged C‚‚‚H-O arrangement between the solute and its neighboring solvent molecules. It is also found that the reorientational dynamics of water are more perturbed than those in SC xenon fluid, due to the weakly anisotropic character of the water-CCl 4 interactions. In particular, the reorientational motions of the z symmetry axis of water appear to be more specifically affected. We emphasize that a correct treatment of the rotational dynamics of water in liquid CCl 4 is provided only by simulation methods that, in contrast to the analytical J model, include the details of the intermolecular solute-solvent potential. Although the transition dipole moment of the ν 3 mode of water is only weakly affected by the interactions, the oscillator strength of the ν 1 internal mode is found to be enhanced compared to its gas-phase value, a result related to the increase of the transition dipole moment due to the water-solvent interactions. Finally, we argue that the spectral properties can be interpreted without invoking a specific H-bond contribution in the intermolecular potential.
Relativistic correction to the potential energy surface and vibration-rotation levels of water
Chemical Physics Letters, 1998
The relativistic correction to the electronic energy of the water molecule is calculated as a function of geometry using Ž . CCSD T wavefunctions and first-order perturbation theory applied to the one-electron mass-velocity and Darwin terms. Based on the calculated 324 energy points, a fitted relativistic correction surface is constructed. This surface is used with a high-accuracy ab initio non-relativistic Born-Oppenheimer potential energy surface to calculate the vibrational band origins and rotational term values for H 16 O. These calculations suggest that the relativistic correction, has a stronger influence on 2 the vibration-rotation levels of water than the Born-Oppenheimer diagonal correction. The effect is particularly marked for vibrational levels with bending excitation or rotational states with high K . q 1998 Elsevier Science B.V. All rights a reserved.