Fausto Cargnoni

Chemical information from the source function

The source function, which enables one to equate the value of the electron density at any point within a molecule to a sum of atomic contributions, has been applied to a number of cases. The source function is a model‐independent, quantitative measure of the relative importance of an atoms or groups contribution to the density at any point in a system, and it represents a potentially interesting tool to provide chemical information. It is shown that the source contribution from H to the electron density ρb at the bond critical point in HX diatomics decreases with increasing Xs electronegativity, and that this decrease is a result of significant changes in the Laplacian distribution within the H‐basin. It is also demonstrated that the source function from Li to ρb in LiX diatomics is a more sensitive index of atomic transferability than it is the lithium atomic energy or population. The observed changes are such as to ensure a constant percentage source contribution from Li to ρb throughout the LiX series, rather than a constant source as one would expect in the limit of perfect atomic transferability. Application of the source function to planar lithium clusters has revealed that the source function clearly discriminates between a nonnuclear electron density maximum and a maximum associated to a nucleus, on the basis of the relative weight of the source contributions from the basin associated to the maximum and from the remaining basins in the cluster. The source function has also allowed for a classification of hydrogen bonds in terms of characteristic source contributions to the density at the H‐bond critical point from the H involved in the H‐bond, the H‐donor D, and the H‐acceptor A. The source contribution from the H appears as the most distinctive marker of the H‐bond strength, being highly negative for isolated H‐bonds, slightly negative for polarized assisted H‐bonds, close to zero for resonance‐assisted H‐bonds, and largely positive for charge‐assisted H‐bonds. The contributions from atoms other than H, D, and A strongly increase with decreasing H‐bond strength, consistently with the parallel increased electrostatic character of the interaction. The correspondence between the classification provided by the Electron Localization Function topologic approach and by the source function has been highlighted. It is concluded that the source function represents a practical tool to disclose the local and nonlocal character of the electron density distributions and to quantify such a locality and nonlocality in terms of a physically sound and appealing chemical partitioning.

An ab initio investigation of the He–H2O complex

Abstract We determined the potential energy surface (PES) of the He–H 2 O complex by means of Coupled-Cluster, Moller–Plesset, and valence bond calculations. The main features of the different PES are discussed and compared to previous literature results. We determined the rotovibrational structure of the complex under the assumption that the water molecule rotates freely. Whatever the computational scheme, the complex is predicted to have a single bound vibrational state and three rotational excitations.

Predicting atomic dopant solvation in helium clusters: The MgHen case

We present a quantum Monte Carlo study of the solvation and spectroscopic properties of the Mg-doped helium clusters MgHe(n) with n=2-50. Three high-level [MP4, CCSD(T), and CCSDT] MgHe interaction potentials have been used to study the sensitivity of the dopant location on the shape of the pair interaction. Despite the similar MgHe well depth, the pair distribution functions obtained in the diffusion Monte Carlo simulations markedly differ for the three pair potentials, therefore indicating different solubility properties for Mg in He(n). Moreover, we found interesting size effects for the behavior of the Mg impurity. As a sensitive probe of the solvation properties, the Mg excitation spectra have been simulated for various cluster sizes and compared with the available experimental results. The interaction between the excited 1P Mg atom and the He moiety has been approximated using the diatomics-in-molecules method and the two excited 1pi and 1sigma MgHe potentials. The shape of the simulated MgHe50 spectra shows a substantial dependency on the location of the Mg impurity, and hence on the MgHe pair interaction employed. To unravel the dependency of the solvation behavior on the shape of the computed potentials, exact density-functional theory has been adapted to the case of doped He(n) and various energy distributions have been computed. The results indicate the shape of the repulsive part of the MgHe potential as an important cause of the different behaviors.

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.

Picosecond solvation dynamics of alkali cations in superfluid 4He nanodroplets

Infrared spectra are reported for carbon dioxide and nitrous oxide solvated in superfluid helium droplets, corresponding to the vibrational excitation of the (0201)/(1001) Fermi diad. Although the rotational constants of these two molecules are similar in the gas phase, they are observed to be quite different in liquid helium, namely, 0.154 cm−1 for CO2 and 0.0717 cm−1 for N2O. In addition, solvation in helium results in shifts in the vibrational origin that are in the opposite directions, −0.42 cm−1, for CO2 and +1.2 cm−1 for N2O. The spectra also show strong droplet size dependence, indicative of the interactions between the molecule and the liquid.

Communication: Nucleation of quantized vortex rings in 4He nanodroplets

Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using time-dependent density functional simulations that the solvation of a Ba(+) ion created by photoionization of neutral Ba at the surface of a (4)He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba(+) ion. The process is expected to be quite general and very efficient under standard experimental conditions.

Solubility of metal atoms in helium droplets: exploring the effect of the well depth using the coinage metals Cu and Ag.

We report a theoretical investigation of the solution properties of Cu and Ag atoms dissolved in He clusters. Employing our recent ab initio ground state pair potential for Me-He (Me = Ag, Cu), we simulated the species Me@He (n) (n = 2-100) by means of diffusion Monte Carlo (DMC) obtaining exact information on their energetics and the structural properties. In particular, we investigated the sensitivity of structural details on the well depth of the two interaction potentials. Whereas Ag structures the first He solvation layer similarly, to some extent, to a positive ion such as Na(+), Cu appears to require the onset of a second solvation shell for a similar dense structure to be formed despite an interaction well of 28.4 μhartree. An additional signature of the different solution behavior between Ag and Cu appears also in the dependence of the energy required to evaporate a single He atom on the size of the MeHe(n) clusters. The absorption spectrum for the (2)P ← (2)S excitation of the metals was also simulated employing the semi-classical Lax approximation to further characterize Me@He(n) (n = 2-100) using novel accurate interaction potentials between He and the lowest (2)P state of Ag and Cu in conjunction with the Diatomic-in-Molecules approach. The results indicated that Ag exciplexes should not form via a direct vertical excitation into an attractive region of the excited manifolds and that there is an interesting dependence of the shape of the Cu excitation bands on the local structure of the first solvation shell.

Dynamics of photoexcited Ba+ cations in 4He nanodroplets

We present a joint experimental and theoretical study on the desolvation of Ba(+) cations in (4)He nanodroplets excited via the 6p ← 6s transition. The experiments reveal an efficient desolvation process yielding mainly bare Ba(+) cations and Ba(+)Hen exciplexes with n = 1 and 2. The speed distributions of the ions are well described by Maxwell-Boltzmann distributions with temperatures ranging from 60 to 178 K depending on the excitation frequency and Ba(+) Hen exciplex size. These results have been analyzed by calculations based on a time-dependent density functional description for the helium droplet combined with classical dynamics for the Ba(+). In agreement with experiment, the calculations reveal the dynamical formation of exciplexes following excitation of the Ba(+) cation. In contrast to experimental observation, the calculations do not reveal desolvation of excited Ba(+) cations or exciplexes, even when relaxation pathways to lower lying states are included.

Potential energy surface, bound states, and rotational inelastic cross sections of the He-CH4 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.

Geometrical reconstructions and electronic relaxations of silicon surfaces. I. An electron density topological study of H-covered and clean Si(111)(1×1) surfaces

The relaxations of the first three interlayer distances in the H-covered Si(111)(1×1) surface were calculated using a fully periodic Hartree–Fock approach and a finely tuned slab model. All computed relaxations fall well within the error bounds of the experiment, provided the relevant geometrical parameters and the basis set of the first layer Si atoms (Si1) are both optimized. The quantum theory of atoms in molecules is applied on the wave functions of Si bulk and of H-covered or clean Si(111)(1×1) slabs so as to shed light on how the electronic perturbation caused by H adsorption and surface formation propagates and dampens through the first Si atoms layers. In the H-covered surface, the large charge transfer from Si1 to H induces a noticeable asymmetry in and strengthening of the surface Si1–Si2 back bonds, whereas in the clean slab the same bonds are found to be weakened compared to the bulk in agreement with the well-known tendency of this system to evolve in favor of other reconstructions. The negat...