Rita Prosmiti

Ab initio ground state potential energy surfaces for Rg–Br2(Rg=He, Ne, Ar) complexes

High-level ab initio molecular electronic structure calculations are performed for Rg–Br2 (Rg=He, Ne, Ar) complexes at CCSD(T) (coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations) level of theory. Specific augmented correlation consistent basis sets are used for each noble atom (Rg), supplemented with an additional set of bond functions. Effective-core potentials (ECPs), augmented with diffusion (sp) and polarization (3df ) functions, have been employed for the bromine atoms. For all complexes, the CCSD(T) potential energy surfaces (PESs) show double-minimum topology, with wells at both linear and T-shaped configurations; the linear minimum is found to be deeper than the T-shaped one. Vibrational corrections are taken into account for all the complexes and their effects in the stability of the linear and T-shaped conformers are examined. For each complex and each configuration (linear and T-shaped), Re equilibrium intermolecular distances, ...

The van der Waals potential energy surfaces and structures of He–ICl and Ne–ICl clusters

The potential energy surfaces of the ground electronic state of rare gas interhalogen van der Waals molecules, Rg–ICl (Rg=He, Ne), are calculated at CCSD(T) (coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations) level of theory. Calculations have been performed with specific augmented correlation consistent basis sets for the noble atom (Rg), supplemented with an additional set of bond functions. For iodine atom a correlation consistent triple zeta valence basis set in conjunction with large-core Stuttgart–Dresden–Bonn relativistic pseudopotential has been employed. The CCSD(T) results predict the existence of three minima on the Rg–ICl potential energy surfaces at collinear (Rg–ICl), antilinear (Rg–ClI), and near T-shaped configurations, with the collinear structure to be the lowest one. Bound states calculated from the intermolecular potential surfaces show that zero-order vibrational corrections do not alter the stability of the three stru...

An accurate, global, ab initio potential energy surface for the H+ 3 molecule

A new global, ground-state, Born-Oppenheimer surface is presented for the H+ 3 system. The energy switching approach has been used to combine different functional forms for three different regimes: a spectroscopic expansion at low energy, a Sorbie-Murrell function at high energy and known long-range terms combined with accurate diatomic potentials at large separations. At low energies we have used the ultra high accuracy ab initio data of Cencek et al. (1998, J. chem. Phys., 108, 2831). At intermediate energy we have calculated 134 new ab initio energies using a high accuracy, explicitly correlated procedure. The ab initio data of Schinke et al. (1980, J. chem. Phys., 72, 3909) has been used to constrain the high energy region. Two fits are presented which differ somewhat in their behaviour at energies over 45 000 cm−1 above the H+ 3 minimum. Below this energy, the fits reproduce each set of ab initio data close to their intrinsic accuracy. The ground state surface should provide a suitable starting point for renewed studies of the near-threshold photodissociation spectrum originally reported by Carrington et al. (1982, Molec. Phys., 45, 753).

A global potential energy surface for the H3+ molecule

Abstract A global two-valued ground-state potential for H 3 + is constructed. The energy switching approach is used to combine different functional forms for three different energy regimes. The Born-Oppenheimer surface of Dinelli et al. [J. Chem. Phys. 103 (1995) 10433] is used for energies up to 20000 cm −1 , for higher energies the many-body expansion of Sorbie and Murrell is fitted to ab initio calculations [Schinke et al,. J. Chem. Phys. 72 (1980) 3909], at large separations long-range terms are combined with accurate diatomic potentials. This produces an accurate global potential which represents all aspects of ground-state H 3 + including the avoided crossing of the two surfaces and dissociation limits. This surface is suitable for studying spectroscopy, high-lying bound states and reaction dynamics.

He79Br2 B,v=8←X,v″=0 excitation spectrum: Ab initio prediction and spectroscopic manifestation of a linear isomer

The B←X rovibronic excitation spectrum of the HeBr2 van der Waals complex is calculated using an ab initio potential energy surface for the ground electronic state. The coupled-cluster single double triple calculations predict double-minimum topology (linear and T-shaped wells) for the X-state potential with a low isomerization barrier. The two lowest vibrational levels, assigned to T-shaped and linear isomers using the localization patterns of the corresponding wave functions, are almost degenerated and lie slightly above the isomerization barrier. This indicates that T-shaped and linear isomers can coexist even at low temperatures and give rise to two separated bands in the excitation spectrum. The main band of the B←X excitation spectrum is assigned to transitions from the T-shaped isomer, whereas the very good agreement between the observed and calculated spectrum, using the ab initio X-state potential, demonstrates that the unassigned secondary band corresponds to excitation of the linear isomer of t...

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.

Determining the Bulk Viscosity of Rigid Water Models

We use equilibrium molecular dynamics methods to compute the shear and bulk viscosities of the pairwise additive and rigid SPC/E, TIP4P, and TIP4P/2005 water models. For the latter model it was found in a recent study (J. Chem. Phys. 2009, 131, 246101) an excellent agreement with experiment in the prediction of the shear viscosity over a range of different thermodynamic conditions. Here, we examine, for a wide range of temperatures, whether this remarkable accuracy of the TIP4P/2005 model remains in the prediction of the bulk viscosity. Moreover, we examine whether equilibrium molecular dynamics methods provide reasonable accuracy in the calculation of the bulk viscosity, as it was previously found for the shear viscosity (J. Chem. Phys. 2010, 132, 096101). We concluded that, by performing the appropriate data analysis, accurate estimates of the bulk viscosity can be obtained, while, compared to the other simple rigid/pairwise additive water models, the predictions of the TIP4P/2005 model for the bulk viscosity are significantly closer to the experiment.

Vibrational predissociation of NeBr2 (X, v=1) using an ab initio potential energy surface

Quantum mechanical calculations on the vibrational predissociation dynamics of NeBr2 are performed using an ab initio (coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations) potential energy surface. Energy positions, lifetimes, and final rotational state distributions are determined for vibrational predissociation from the two lowest linear (n=0) and T-shaped (n=1) van der Waals levels of NeBr2 (X, v=1). Comparison with the experimental assumption as regards the energy transfer to rotation provides information about the type of isomer involved in the experimental vibrational predissociation process, suggesting that it was the linear one.

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.