Álvaro Valdés

Intermolecular Ab initio potential and spectroscopy of the ground state of HeI2 complex revisited

The structure, energetics, and spectroscopy of ground-state HeI(2) molecule are analyzed from first principles. Ab initio methodology at CCSD(T) level of theory was employed, and large basis sets were used to compute the interaction energies. Scalar relativistic effects were accounted for by relativistic effective core potentials for the iodine atoms. Recent experimental investigations of the HeI(2) rovibronic spectra have estimated the ground-state binding energies of 16.6 +/- 0.6 and 16.3 +/- 0.6 cm(-1) for the T-shaped and linear isomers, respectively. Given the extremely small difference between the two conformers, special attention was paid in the choice of basis sets used and the extrapolation schemes employed, as well as the fitting process for its analytical representation. The complete analytical form is provided, and variational fully quantum mechanical calculations were carried out by using the new parametrized surface, to evaluate vibrationally averaged structures and binding energies for the different conformers. The results obtained are in good accord with recent data available from experimental investigations of the He-I(2) rovibronic spectra.

HeBr2 complex: ground-state potential and vibrational dynamics from ab initio calculations

The three-dimensional interaction potential for HeBr2 was studied using a coupled-cluster (CCSD(T)) method. In our calculations, the Stuttgart group (SDD) effective-core potentials, augmented with diffusion (sp) and polarization (3df) functions (denoted SDD+G(3df)), basis sets are employed for the bromine atoms. For the He atom, the augmented correlation-consistent aug-cc-pV5Z basis set, supplemented with a set of bond functions, is used. The potential energy surface is constructed by fitting the CCSD(T) calculated ab initio data to an analytical expression. The present ground-state potential for HeBr2 shows a double-minimum topology, with wells for both linear and T-shaped configurations. Bound-state calculations are carried out and the lowest vibrational levels are assigned to linear and T-shaped isomers. Dissociation energies and vibrationally averaged structures for both species are determined and found to be in very good agreement with the available experimental data.

Quantum-dynamics study of the H5+ cluster: full dimensional benchmark results on its vibrational states.

A full-dimensional quantum dynamics study is carried out for the highly fluxional H(5)(+) cation on a recent reference potential energy surface by using the multi configuration time-dependent Hartree method. With five equivalent light atoms and shallow barriers between various low-lying stationary points on the surface, the spectroscopic characterization of H(5)(+) represents a huge challenge for accurate quantum dynamics simulations. The present calculation is the first such a study on this cation, which together with its isotope analogies are of primary importance in the interstellar chemistry. The vibrational ground state properties and several vibrationally excited states corresponding to low vibrational frequency motions, not yet directly observable by the experiment, are presented and analyzed.

CCSD(T) potential energy surface and bound rovibrational level calculations for the Ar–ICl(X) complex

The intermolecular potential between Ar atom and ICl fixed at its equilibrium is determined by CCSD(T) calculations. We used effective-core potentials for iodine, augmented correlation consistent basis sets (aug-cc-pVnZ, n=T,Q) for Cl and Ar atoms with an additional set of bond functions. The global potential minimum corresponds to the linear Ar–I–Cl structure with well-depth De=328.1 cm−1, r(Ar⋯I)=3.555 A and ks=3.45 N m−1, in accord with the one determined by the experiment with r(Ar⋯I)=3.576 A and ks=3.20 N m−1. Bound state calculations are carried out for J=0 and 1 rotational states. Accordingly, the lowest level corresponds to linear Ar–I–Cl isomers, while stable isomers are also found for T-shaped and antilinear species.

Vibrational dynamics of the H5+ and its isotopologues from multiconfiguration time-dependent Hartree calculations

Full-dimensional multiconfiguration time-dependent Hartree (MCTDH) computations are reported for the vibrational states of the H(5)(+) and its H(4)D(+), H(3)D(2)(+), H(2)D(3)(+), HD(4)(+), D(5)(+) isotopologues employing two recent analytical potential energy surfaces of Xie et al. [J. Chem. Phys. 122, 224307 (2005)] and Aguado et al. [J. Chem. Phys. 133, 024306 (2010)]. The potential energy operators are constructed using the n-mode representation adapted to a four-combined mode cluster expansion, including up to seven-dimensional grids, chosen adequately to take advantage in representing the MCTDH wavefunction. An error analysis is performed to quantify the convergence of the potential expansion to reproduce the reference surfaces at the energies of interest. An extensive analysis of the vibrational ground state properties of these isotopes and comparison with the reference diffusion Monte Carlo results by Acioli et al. [J. Chem. Phys. 128, 104318 (2008)] are presented. It is found that these systems are highly delocalized, interconverting between equivalent minima through rotation and internal proton transfer motions even at their vibrational ground state. Isotopic substitution affects the zero-point energy and structure, showing preference in the arrangements of the H and D within the mixed clusters, and the most stable conformers of each isotopomer are the ones with the H in the central position. Vibrational excited states are also computed and by comparing the energies and structures predicted from the two surfaces, the effect of the potential topology on them is discussed.

Ab initio characterization of the Ne-I2 van der Waals complex: Intermolecular potentials and vibrational bound states

A theoretical study of the potential energy surface and bound states is performed for the ground state of the NeI(2) van der Waals (vdW) complex. The three-dimensional interaction energies are obtained from ab initio coupled-cluster, coupled-cluster single double (triple)/complete basis set, calculations using large basis sets, of quadruple- through quintuple-zeta quality, in conjunction with relativistic effective core potentials for the heavy iodine atoms. For the analytical representation of the surface two different schemes, based on fitting and interpolation surface generation techniques, are employed. The surface shows a double-minimum topology for linear and T-shaped configurations. Full variational quantum mechanical calculations are carried out using the model surfaces, and the vibrationally averaged structures and energetics for the NeI(2) isomers are determined. The accuracy of the potential energy surfaces is validated by a comparison between the present results and the corresponding experimental data available. In lieu of more experimental measurements, we also report our results/predictions on higher bound vibrational vdW levels, and the influence of the employed surface on them is discussed.

Theoretical investigation of the infrared spectra of the H5(+) and D5(+) cations.

Reduced dimensional quantum dynamics calculations of the infrared spectrum of the H5(+) and D5(+) clusters are reported in both low, 300–2200 cm(–1), and high, 2400–4500 cm(–1), energy regions. The proposed four-dimensional quantum model describes the motion of the proton between the two vibrating hydrogen molecules. The simulations are performed using time-dependent and time-independent approaches within the multiconfiguration time-dependent Hartree method. Propagation of the wavepackets includes an absorbing scheme to deal with vibrational dissociating states, and to assign the different spectral lines, block improved relaxation computations are performed for both bound and predissociative vibrational states of the systems. The reported computations make use of an analytical ab initio-based potential energy, and “on the fly” DFT dipole moment surfaces. The predominant features in the spectra are assigned to the excitations of the shared-proton stretch mode, and above dissociation the symmetric and antisymmetric stretching of the two H2 and the breathing mode of H3(+) are also involved. The computed infrared absorption spectra for both cations are in very good agreement with the recent experimental measurements available from multiple-photon dissociation and mass-selected single-photon photodissociation spectroscopy techniques. Comparison of the present results with previous theoretical calculations on these systems is also presented. Such comparisons between different theoretical approaches and experimental measurements can serve to evaluate the approximations employed, and to guide higher-order computations.

An ab initio study of the E 3Πg state of the iodine molecule.

The E (3)Π(g) state of the iodine molecule is studied by ab initio multireference methods coupled with effective core potentials and large basis sets. Two potential minima are found, a global featuring an ion-pair character, and a local presenting a purely Rydberg nature. Four avoided crossings along the dissociation coordinate attribute an interesting topology to its potential energy curve, and their effect on the vibrational levels of I(2) is discussed.

Full-dimensional multi configuration time dependent Hartree calculations of the ground and vibrationally excited states of He2,3Br2 clusters

Quantum dynamics calculations are reported for the tetra-, and penta-atomic van der Waals He(N)Br(2) complexes using the multiconfiguration time-dependent Hartree (MCTDH) method. The computations are carried out in satellite coordinates, and the kinetic energy operator in this set of coordinates is given. A scheme for the representation of the potential energy surface based on the sum of the three-body HeBr(2) interactions at CSSD(T) level plus the He-He interaction is employed. The potential surfaces show multiple close lying minima, and a quantum description of such highly floppy multiminima systems is presented. Benchmark, full-dimensional converged results on ground vibrational/zero-point energies are reported and compared with recent experimental data available for all these complexes, as well as with previous variational quantum calculations for the smaller HeBr(2) and He(2)Br(2) complexes on the same surface. Some low-lying vibrationally excited eigenstates are also computed by block improved relaxation calculations. The binding energies and the corresponding vibrationally averaged structures are determined for different conformers of these complexes. Their relative stability is discussed, and contributes to evaluate the importance of the multiple-minima topology of the underlying potential surface.

Theoretical characterization of intermolecular vibrational states through the multi-configuration time dependent Hartree approach: The He2,3ICl clusters

Benchmark, full-dimensional calculations on the ground and excited vibrational states for the tetra-, and penta-atomic weakly bound He(2,3)ICl complexes are reported. The representation of the potential energy surfaces includes three-body HeICl potentials parameterized to coupled-cluster singles, doubles, and perturbative triples ab initio data. These terms are important in accurately describing the interactions of such highly floppy systems. The corresponding 6D/9D computations are performed with the multi-configuration time dependent Hartree method, using natural potential fits, and a mode combination scheme to optimize the computational effort in the improved relaxation calculations. For these complexes several low-lying vibrational states are computed, and their binding energies and radial/angular probability density distributions are obtained. We found various isomers which are assigned to different structural models related with combinations of the triatomic isomers, like linear, T-shaped, and antilinear ones. Comparison of these results with recent experimental data is presented, and the quantitative deviations found with respect to the experiment are discussed.