Two-dimensional (2D) materials deriving from graphene, such as graphdiyne and 2D polyphenylene honeycomb (2DPPH), have been recently synthesized and exhibit uniformly distributed sub-nanometer pores, a feature that can be exploited for gas filtration applications. Accurate first principles electronic structure calculations are reported showing that graphdiyne pores permit an almost unimpeded helium transport while it is much more difficult through the 2DPPH openings. Quantum dynamical simulations on reliable new force fields are performed in order to assess the graphdiyne capability for helium chemical and isotopic separation. Exceptionally high He/CH 4 selectivities are found in a wide range of temperatures which largely exceed the performance of the best membranes used to date for helium extraction from natural gas.Moreover, due to slight differences in the tunneling probabilities of 3 He and 4 He, we also find promising results for the separation of the fermionic isotope at low temperature.

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.

Intermolecular potentials for the three lowest multiplet states (singlet, triplet and quintet) of the O2(3Sigma)-O2(3Sigma) dimer have been investigated in detail by means of high level ab initio calculations. The methods used include MRCI, ACPF, CASPT2, using different active spaces and basis sets. The results for the quintet state are compared with benchmark CCSD(T) calculations. As expected, the former methods do not account accurately for dispersion interactions, although the CASPT2 method performs better than the CI based ones. On the other hand, it is shown that highly correlated methods are necessary to accurately describe the splittings among the multiplet states. We propose to obtain singlet and triplet interaction potentials by combining CCSD(T) quintet potentials and multiconfigurational singlet-quintet and triplet-quintet splittings, respectively. The calculated splittings are quite stable regarding the method employed, except for the well region of the singlet and triplet states within the rectangular configuration, which corresponds to the absolute minima of these multiplet states. Nevertheless, we have been able to assess adequate upper and lower bounds to the interaction potential for this particular region.

Recent progress in the production of new two-dimensional (2D) nanoporous materials is attracting considerable interest for applications to isotope separation in gases. In this paper we report a computational study of the transmission of (4)He and (3)He through the (subnanometer) pores of graphdiyne, a recently synthesized 2D carbon material. The He-graphdiyne interaction is represented by a force field parametrized upon ab initio calculations, and the (4)He/(3)He selectivity is analyzed by tunneling-corrected transition state theory. We have found that both zero point energy (of the in-pore degrees of freedom) and tunneling effects play an extraordinary role at low temperatures (≈20-30 K). However, both quantum features work in opposite directions in such a way that the selectivity ratio does not reach an acceptable value. Nevertheless, the efficiency of zero point energy is in general larger, so that (4)He tends to diffuse faster than (3)He through the graphdiyne membrane, with a maximum performance at 23 K. Moreover, it is found that the transmission rates are too small in the studied temperature range, precluding practical applications. It is concluded that the role of the in-pore degrees of freedom should be included in computations of the transmission probabilities of molecules through nanoporous materials.

Global potentials for the physisorption of raregas atoms on graphene and graphite, amenable for a variety of dynamics simulations, are reported. An atom−bond pairwise additive form of the potential is used, where the interaction pairs, represented by proper analytical functions, are constituted by the Rg atom (Rg = He, Ne, Ar, Kr) and the C−C bonds of the graphene sheet(s). The parameters of the atom−bond pair potential, derived from the polarizability of the interacting partners, are fine-tuned, exploiting calculations of the prototypical Rg−coronene system using high-level electronic structure methods and large basis sets. The atom−graphene/graphite potential is further expanded in a Fourier series, and it is found that for an accurate representation of the interaction only a small number of corrugation terms need to be added to the laterally averaged potential. Furthermore, this corrugation part of the potential is both identical for Rg−graphene and Rg− graphite; in other words, inner layers of graphite only play a role in the laterally averaged Rg−graphite potential. For all systems, the hollow at the center of the carbon ring is the preferred adsorption site, although diffusion barriers are low. The present results compare well with previous data regarding well depths and equilibrium distances at different adsorption sites and, for graphite, the long-range dispersion coefficient C 3 . In addition, binding energies (eigenvalues of the laterally averaged potentials) are in a fairly good agreement with experimental determinations, providing further support for the reliability of the potentials.

Coronene-doped helium clusters have been studied by means of classical and quantum mechanical (QM) methods using a recently developed He-C24H12 global potential based on the use of optimized atom-bond improved Lennard-Jones functions. Equilibrium energies and geometries at global and local minima for systems with up to 69 He atoms were calculated by means of an evolutive algorithm and a basin-hopping approach and compared with results from path integral Monte Carlo (PIMC) calculations at 2 K. A detailed analysis performed for the smallest sizes shows that the precise localization of the He atoms forming the first solvation layer over the molecular substrate is affected by differences between relative potential minima. The comparison of the PIMC results with the predictions from the classical approaches and with diffusion Monte Carlo results allows to examine the importance of both the QM and thermal effects.

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.

We propose a fast and non-destructive spectroscopic method for single molecular ions that implements quantum logic schemes between an atomic ion and the molecular ion of interest. Our proposal relies on a hybrid coherent manipulation of the two-ion system, using optical or magnetic forces depending on the types of molecular levels to be addressed (Zeeman, rotational, vibrational or electronic degrees of freedom). The method is especially suited for the non-destructive precision spectroscopy of single molecular ions, and sets a starting point for new hybrid quantum computation schemes that combine molecular and atomic ions, covering the measurement and entangling steps.