Massimiliano Bartolomei

A bond–bond description of the intermolecular interaction energy: the case of weakly bound N2–H2 and N2–N2 complexes

The atom-bond pairwise additive approach, recently introduced by us to describe the potential energy surface for atom-molecule cases, is extended here for the first time to molecule-molecule systems. The idea is to decompose the van der Waals interaction energy into bond-bond pair contributions. This must be considered an improvement with respect to the familiar atom-atom pairwise additive representation since, still using a simple formulation, it indirectly accounts for three body effects. Such an approach also allows to include, in a straightforward way, the effect of the bond length on the intermolecular interaction energy. Cases of study are the weakly bound complexes involving the H(2) and N(2) molecules, namely N(2)-H(2) and N(2)-N(2), here described as a single bond-bond pair. For both systems ab initio calculations and experimental molecular beam scattering data, as well as second virial coefficients, have been employed to test the accuracy of the chosen representation of the interaction and to improve the obtained potential energy surfaces. The results of this work are important also for the generalization to the cases involving molecular ions and polyatomic molecules.

Penetration Barrier of Water through Graphynes’ Pores: First-Principles Predictions and Force Field Optimization

Graphynes are novel two-dimensional carbon-based materials that have been proposed as molecular filters, especially for water purification technologies. We carry out first-principles electronic structure calculations at the MP2C level of theory to assess the interaction between water and graphyne, graphdiyne, and graphtriyne pores. The computed penetration barriers suggest that water transport is unfeasible through graphyne while being unimpeded for graphtriyne. For graphdiyne, with a pore size almost matching that of water, a low barrier is found that in turn disappears if an active hydrogen bond with an additional water molecule on the opposite side of the opening is considered. Thus, in contrast with previous determinations, our results do not exclude graphdiyne as a promising membrane for water filtration. In fact, present calculations lead to water permeation probabilities that are 2 orders of magnitude larger than estimations based on common force fields. A new pair potential for the water-carbon noncovalent component of the interaction is proposed for molecular dynamics simulations involving graphdiyne and water.

A full dimensional grid empowered simulation of the CO2 + CO2 processes

A recently introduced bond–bond formulation of the intermolecular interaction has been extended to six‐atom systems to the end of assembling a new potential energy surface (PES) and has been incorporated into a grid empowered simulator able to handle the modeling of the CO2 + CO2 processes. The proposed PES is full dimensional and accounts for the dependence of the intermolecular interaction on some basic physical properties of the colliding partners, including modulations induced by the monomer deformation. The used analytical formulation of the interaction involves a limited number of parameters, each having a clear physical meaning. Guess values for these parameters can also be obtained from analytical correlation formulae. Such estimates can then be fine tuned by exploiting experimental and theoretical information. The resulting PES well describes stretched and bent asymptotic CO2 monomers as well as the CO2–CO2 interaction in the most and less stable configurations. On this potential massive quasiclassical elastic and inelastic detailed scattering trajectories have been integrated, by exploiting the innovative computational technologies of the grid. The efficiency of the approach used and the reliability of the estimates of the dynamical properties obtained in this way is such that we can now plan a systematic evaluation of the state specific rate coefficient matrix elements needed for space craft reentry modeling. Here, we present probabilities and cross sections useful to rationalize some typical mechanisms characterizing the vibrational transitions of the CO2 + CO2 system on the flexible monomer proposed PES. On such PES, the key dynamical outcomes are: (a) there is a strong energy interchange between symmetric stretching of the reactants and bending of the products (and viceversa) while asymmetric stretching is strongly adiabatic (b) reactant energy is more efficiently allocated (with respect to the rigid monomers PES) as product vibration when reactant stretching modes are excited while the contrary is true when the reactant bending mode is excited.

Graphdiyne Pores: “Ad Hoc” Openings for Helium Separation Applications

Graphdiyne is a novel two-dimensional material deriving fr om graphene that has been recently synthesized and featuring uniformly distributed su b-nanometer pores. We report accurate calculations showing that graphdiyne pores permit an a lmost unimpeded helium transport which can be used for its chemical and isotopic separation. E xceptionally high He/CH4 selectivities are found which largely exceed the performance of t he best membranes used to date for extraction from natural gas. Moreover, by exploiting sl ight differences in the tunneling probabilities of3He and4He, we also find promising results for the separation of the fe rmionic isotope at low temperature. To whom correspondence should be addressed †Instituto de Física Fundamental, Consejo Superior de Inves tigaciones Científicas (IFF-CSIC), Serrano 123, 28006 Madrid, Spain ‡Dipartimento di Chimica, Biologia e Biotecnologie, Univer sità di Perugia, Perugia, Italia ¶Department of Chemical System Engineering, School of Engin eeri g, University of Tokyo, Tokyo, Japan

Long‐range interaction for dimers of atmospheric interest: dispersion, induction and electrostatic contributions for O2O2, N2N2 and O2N2

Electric multipole moments, static dipole polarizabilities, and dynamic dipole, quadrupole, and mixed dipole‐octupole polarizabilities of molecular oxygen and nitrogen in their ground electronic states have been obtained by means of high level multiconfigurational ab initio calculations. From these properties, we have obtained electrostatic, dispersion, and induction coefficients for the long‐range interactions of the O2O2, N2N2, and O2N2 dimers. Our data is a comprehensive and consistent set that for N2N2 shows a very good agreement with previous accurate calculations, whereas for quantities involving open‐shell O2 represents a considerable improvement over previous estimations. Moreover, the long‐range interaction is analyzed and compared for the different interacting partners. It is found that the C8 dispersion interaction plays a nonnegligible role and that the induction component is only important for a detailed description of the highest order anisotropy terms in the spherical harmonics expansion of the long‐range potential. It is also found that the total long‐range interaction is quite similar in O2O2 and O2N2, and that differences with N2N2 are mainly because of the important role of the electrostatic interaction in that dimer. Comparison with high level supermolecular calculations indicates that the present long‐range potentials are accurate for intermolecular distances larger than about 15 bohr.

Global ab initio potential energy surfaces for the O2(Σ3g−)+O2(Σ3g−) interaction

Completely ab initio global potential energy surfaces (PESs) for the singlet and triplet spin multiplicities of rigid O(2)((3)Σ(g)(-))+O(2)((3)Σ(g)(-)) are reported for the first time. They have been obtained by combining an accurate restricted coupled cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)] quintet potential [Bartolomei et al., J. Chem. Phys. 128, 214304 (2008)] with complete active space second order perturbation theory (CASPT2) or, alternatively, multireference configuration interaction (MRCI) calculations of the singlet-quintet and triplet-quintet splittings. Spherical harmonic expansions, containing a large number of terms due to the high anisotropy of the interaction, have been built from the ab initio data. The radial coefficients of these expansions are matched at long range distances with analytical functions based on recent ab initio calculations of the electric properties of the monomers [M. Bartolomei, E. Carmona-Novillo, M. I. Hernández, J. Campos-Martínez, and R. Hernández-Lamoneda, J. Comput. Chem. (2010) (in press)]. The singlet and triplet PESs obtained from either RCCSD(T)-CASPT2 or RCCSD(T)-MRCI calculations are quite similar, although quantitative differences appear in specific terms of the expansion. CASPT2 calculations are the ones giving rise to larger splittings and more attractive interactions, particularly in the region of the absolute minima (in the rectangular D(2h) geometry). The new singlet, triplet, and quintet PESs are tested against second virial coefficient B(T) data and, their spherically averaged components, against integral cross sections measured with rotationally hot effusive beams. Both types of multiconfigurational approaches provide quite similar results, which, in turn, are in good agreement with the measurements. It is found that discrepancies with the experiments could be removed if the PESs were slightly more attractive. In this regard, the most attractive RCCSD(T)-CASPT2 PESs perform slightly better than the RCCSD(T)-MRCI counterpart.

A study to improve the van der Waals component of the interaction in water clusters

A portable model potential, representing the intermolecular interaction of water as a combination of a few effective components given in terms of the polarizability and dipole moment values of the molecular partners, is here proposed as a building block of the force field of water clusters in molecular dynamics simulations. In this spirit, here, we discuss the key properties of the model potential and its application to water dimers, trimers and tetramers with the purpose of extrapolating the results to very large clusters mimicking the liquid phase. The suitability of the model potential for dynamics investigations is checked by comparing on one hand the value of the second virial coefficient calculated for the gaseous dimer with experimental data measured over a wide range of temperature (273–3000 K) and, on the other hand, the calculated radial distribution functions and density with those obtained from experiments performed using liquid water.

Accurate ab initio intermolecular potential energy surface for the quintet state of the O2(Σg−3)–O2(Σg−3) dimer

A new potential energy surface (PES) for the quintet state of rigid O(2)((3)Sigma(g)(-)) + O(2)((3)Sigma(g)(-)) has been obtained using restricted coupled-cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)]. A large number of relative orientations of the monomers (65) and intermolecular distances (17) have been considered. A spherical harmonic expansion of the interaction potential has been built from the ab initio data. It involves 29 terms, as a consequence of the large anisotropy of the interaction. The spherically averaged term agrees quite well with the one obtained from analysis of total integral cross sections. The absolute minimum of the PES corresponds to the crossed (D(2d)) structure (X shape) with an intermolecular distance of 6.224 bohrs and a well depth of 16.27 meV. Interestingly, the PES presents another (local) minimum close in energy (15.66 meV) at 6.50 bohrs and within a planar skewed geometry (S shape). We find that the origin of this second structure is due to the orientational dependence of the spin-exchange interactions which break the spin degeneracy and leads to three distinct intermolecular PESs with singlet, triplet, and quintet multiplicities. The lowest vibrational bound states of the O(2)-O(2) dimer have been obtained and it is found that they reflect the above mentioned topological features of the PES: The first allowed bound state for the (16)O isotope has an X structure but the next state is just 0.12 meV higher in energy and exhibits an S shape.

Molecular oxygen tetramer (O2)4: intermolecular interactions and implications for the epsilon solid phase

Recent data have determined that the structure of the high pressurephase of solid oxygen consists of clusters composed of four O2 molecules. This finding has opened the question about the nature of the intermolecular interactions within the molecular oxygen tetramer. We use multi- configurational ab initio calculations to obtain an adequate characterization of the ground singlet state of (O2)4 which is compatible with the non magnetic character of thephase. In contrast to previous suggestions implying chemical bonding, we show that (O2)4 is a van der Waals like cluster where exchange interactions preferentially stabilize the singlet state. However, as the clus- ter shrinks, there is an extra stabilization due to many-body interactions that yields a significant softening of the repulsive wall. We show that this short range behavior is a key issue for the understanding of the structure of �-oxygen.

Quantum-mechanical study of the collision dynamics of O2(3Sigma(g)-) + O2(3Sigma(g)-) on a new ab initio potential energy surface.

The quantum mechanical theory for the scattering of two identical rigid rotors is reviewed and applied to the collision of O2(3Sigma(g)-) molecules using a new accurate ab initio potential energy surface (PES) for the quintet state of the composite system. The PES is based on calculations using restricted coupled-cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)] [Bartolomei; et al. J. Chem. Phys. 2008, 128, 214304.]. This PES is extended here for large intermolecular distances using the ab initio long-range coefficients of Hettema et al. [J. Chem. Phys. 1994, 100, 1297.]. Elastic and rotationally inelastic integral cross sections have been obtained by means of close coupling calculations in the subthermal energy range (center-of-mass velocities below 500 m/s). Results are compared with those obtained using a PES derived from molecular beam experiments [Aquilanti; et al. J. Am. Chem. Soc. 1999, 121, 10794.]. General agreement is found between both PESs, although the experimentally derived PES appears as somewhat more anisotropic at least for the studied energy range. There is, however, a significant difference in the absolute value of the elastic cross sections that is due to differences in the long-range dispersion interaction. The performance of the ab initio PES for higher velocities (relevant to experiments) is also explored by retaining just the isotropic component of the interaction. A satisfactory agreement is found for the shape of the glory pattern but shifted toward lower absolute values of the cross sections.