Search

We performed first-principles computations to investigate the complex interplay of molecular interaction energies in determining the lattice structure and stability of CO2@sH clathrate hydrates. Density functional theory computations using periodic boundary conditions were employed to characterize energetics and the key structural properties of the sH clathrate crystal under pressure, such as equilibrium lattice volume and bulk modulus. The performance of exchange−correlation functionals together with recently developed dispersion-corrected schemes was evaluated in describing interactions in both short-range and long-range regions of the potential. Structural relaxations of the fully CO2-filled and empty sH unit cells yield crystal structure and lattice energies, while their compressibility parameters were derived by including the pressure dependencies. The present quantum chemistry computations suggest anisotropy in the compressibility of the sH clathrate hydrates, with the crystal being less compressible along the a-axis direction than along the c-axis one, in distinction from nearly isotropic sI and sII structures. The detailed results presented here give insight into the complex nature of the underlying guest−host interactions, checking earlier assumptions, providing critical tests, and improving estimates. Such entries may eventually lead to better predictions of thermodynamic properties and formation conditions, with a direct impact on emerging hydrate-based technologies.

We introduce an efficient quantum fully coupled computational scheme within the multiconfiguration time-dependent Hartree (MCTDH) approach to handle the otherwise extremely costly computations of translational-rotational-vibrational states and energies of light-molecule endofullenes. Quantum calculations on energy levels are reported for a water molecule inside Cfullerene by means of such a systematic approach that includes all nine degrees of freedom of HO@Cand does not consider restrictions above them. The potential energy operator is represented as a sum of natural potentials employing then-mode expansion, along with the exact kinetic energy operator, by introducing a set of Radau internal coordinates for the HO molecule. On the basis of the present rigorous computations, various aspects of the quantized intermolecular dynamics upon confinement of HO@Care discussed, such as the rotational energy level splitting and the significant frequency shifts of the encapsulated water molecule vibrations. The impact of water encapsulation on quantum features is explored, and insights into the nature of the underlying forces are provided, highlighting the importance of a reliable first-principles description of the guest-host interactions.

We implemented a systematic procedure for treating the quantal rotations by including all translational and vibrational degrees of freedom for any triatomic bent molecule in any embedded or confined environment, within the MCTDH framework. Fully coupled quantum treatments were employed to investigate unconventional properties in nanoconfined molecular systems. In this way, we facilitate a complete theoretical analysis of the underlying dynamics that enables us to compute the energy levels and the nuclear spin isomers of a single water molecule trapped in a C fullerene cage. The key point lies in the full 9D description of both nuclear and electronic degrees of freedom, as well as a reliable representation of the guest-host interaction. The presence of occluded impurities or inhomogeneities due to noncovalent interactions in the interfullerene environment could modify aspects of the potential, causing significant coupling between otherwise uncoupled modes. Using specific n-mode model potentials, we obtained splitting patterns that confirm the effects of symmetry breaking observed by experiments in the ground ortho-HO state. Further, our investigation reveals that the first rotationally excited states of the encapsulated ortho- and para-HO have also raised their 3-fold degeneracy. In view of the complexity of the problem, our results highlight the importance of accurate and computational demanding approaches for building up predictive models for such nanoconfined molecules.

Energetics and structural properties of selected type and size He@hydrate frameworks, e.g., from regular structured ice channels to clathrate-like cages, are presented from first-principles quantum chemistry methods. The scarcity of information on He@hydrates makes such complexes challenging targets, while their computational study entails an interesting and arduous task. Some of them have been synthesized in the laboratory, which motivates further investigations on their stability. Hence, the main focus is to examine the performance and accuracy of different wave function-based electronic structure methods, such as MP2, CCSD(T), their explicitly correlated (F12) and domain-based local pair-natural orbital (DLPNO) analogs, as well as modern and conventional density functional theory (DFT) approaches, and analytical model potentials available. Different structures are considered, starting from the "simplest system"formed by a noble gas atom (such as He) and one water molecule, followed by the study of the "fundamental units"present in all ice-like and clathrate-like frameworks (such as pentamers and hexamers) and finally the description of interactions in the "building blocks"of three-dimensional (3D) ice channels (e.g., horizontal and perpendicular ice II and Ih) and clathrate-like cages, such as the 512 present in the most common sI, sII, and sH clathrate-hydrate structures. The idea is to provide well-converged DLPNO-CCSD(T) and DFMP2/CBS reference datasets that in turn are used to validate how DFT functionals (in total, 29 approaches from generalized-gradient approximation (GGA), meta-GGA, to hybrid and range-separated functionals, including dispersion correction treatments, were checked) and analytical semiempirical/ab initio-based potentials perform compared with high-level alternatives. Within all tested approaches, those best-performing were identified and classified. Most of the DFT/DFT-D functionals, as well as available analytical pairwise model potentials

We have investigated different approaches to handling parallel-tempering Monte Carlo (PTMC) simulations in the isothermal-isobaric ensemble of molecular cluster/nanoparticle systems for predicting structural phase diagram transitions. We have implemented various methodologies that consist of treating pressure implicitly through its effect on the volume. Thus, the main problem in the simulations under nonzero pressure becomes the volume definition of the finite nonperiodic system, and we considered approaches based on the particles' coordinates. Various volume models, namely container-volume, particle-volume, average-volume, ellipsoids-volume, and convex hull-volume, were employed, and the required corrections for each of them in the Monte Carlo computations were introduced. Finally, we explored the effects of volume/pressure changes for all models on structural phase transitions of a test system, such as the small "icelike" (HO) water cluster. The temperature and pressure dependence of the cluster's heat capacity and energy-volume Pearson correlation coefficient were studied, phase diagrams were constructed using a multiple-histogram method, and attempts were made to identify phase transitions to particular cluster structures. Our results show significant differences between the employed volume models, and we discuss all pressure-induced, such as solid-solid-, solid-liquid-, and liquid-gas-like, phase transformations in the present study.

We present a theoretical study on the potential energy surface and bound states of He-A2+ complexes, where A is one of the alkali Li or Na atoms. The intermolecular interactions were systematically investigated by high-level ab initio electronic structure computations, and the corresponding raw data were then employed to reproduce accurate analytical expressions of the potential surfaces. In turn, we used these potentials to evaluate bound configurations of the trimers from nuclear quantum calculations and to extract information on the effect of orientational anisotropy of the forces and the interplay between repulsive and attractive interaction within the potential surfaces. The spatial features of the bound states are analyzed and discussed in detail. We found that both systems are going under large amplitude stretching and bending motions with high zero-point energies. Despite the large differences in the potential well-depths, the correct treatment of nuclear quantum effects provides insights on the effect of different strength of the ionic interaction on the spectral structure of such cationic alkali van der Waals complexes, related to the mobility of ions and the formation of cold-molecules in He-controlled environments.

Recently, empty hydrate structures sI, sII, sH, and others have been proposed as low-density ice structures by both experimental observations and computer simulations. Some of them have been synthesized in the laboratory, which motivates further investigations on the stability of such guest-free clathrate structures. Using semiempirical and ab initio-based water models, as well as dispersion-corrected density functional theory approaches, we predict their stability, including cooperative many-body effects, in comparison with reference data from converged wave function-based DF-MP2 electronic structure calculations. We show that large basis sets and counterpoise corrections are required to improve convergence in the interaction/binding energies for such systems. Therefore, extrapolation schemes based on triple/quadruple and quadruple/quintuple ¿ quality basis sets are used to reach high accuracy. Eleven different water structures corresponding to dodecahedron, edge sharing, face sharing, and fused cubes, as a part of the WATER27 database, as well as cavities from the sI, sII, and sH clathrate hydrates formed by 20, 24, 28, and 36 water molecules, are employed, and new benchmark energies are reported. Using these benchmark sets of interaction energies, we assess the performance of both analytical models and direct DFT calculations for such clathrate-like systems. In particular, seven popular water models (TIP4P/ice, TIP4P/2005, q-TIP4P/F, TTM2-F, TTM3-F, TTM4-F, and MB-pol) available in the literature, and nine density functional approximations (3 meta-GGAs, 3 hybrids, and 3 range separated functionals) are used to investigate their accuracy. By including dispersion corrections, our results show that errors in the interaction energies are reduced for most of the DFT functionals. Despite the difficulties faced by current water models and DFT functionals to accurately describe the interactions in such water systems, we found some general trends that could serve to exten

The multiconfiguration time-dependent Hartree (MCTDH) method using a six-dimensional Hamiltonian that includes all rotational and vibrational degrees of freedom and an ab initio potential energy surface was employed to calculate the rovibronic states of the HeBr2 van der Waals complex. All rotational states of energies within 7 cm-(1) with respect to the energy of the linear ground state were calculated without restriction of the total angular momentum. In total, we obtained 500 and 320 rotationally excited states of the ground vibrational T-shaped and linear isomers of the HeBr2, respectively, and compared them with those predicted by the rigid rotor model. A thermodynamic model was then introduced to determine the relative stability of the two conformers as a function of the temperature. On the basis of the present results, the linear conformers were found to be energetically more stable than the T-shaped ones by 1.14 cm(-1) at T = 0 K, whereas conversion from linear to T-shaped complexes was observed at temperatures above 2.87 K. © 2016 American Chemical Society

Classical thermodynamic properties of a size specific Kr clathrate-like cluster, modeled by an ab initio and a semiempirical Kr-water potential, were computed from parallel-tempering Monte Carlo simulations. The temperature and pressure dependence of the cluster's heat capacity was studied, and phase diagrams were constructed using a multiple histogram method. By associating the heat capacity (maxima) and the Pearson correlation coefficient (minima/maxima) 9 ri values with the phase transitions, attempts were made to identify such changes to particular cluster structures. Various isomers were computed by local optimizations of the interaction enthalpy at a set of randomly selected configurations at each temperature-pressure grid point. Their energy distributions and relative abundances were then employed to assign the observed phase transitions of the cluster. It was found that the structural changes at high pressures are related to Kr clathrate-like cages, such as those of the sI, sII, and sH hydrates, as well as a new one at higher pressures, formed by tetragons and pentagons. Such transitions, at low temperatures and as pressure increases, are related to the topology of the intermolecular interactions, that are becoming accessible by the sampling in the MC simulations, through the employed volume model. © 2016 American Chemical Society

New potential energy functions (i-TTM) describing the interactions between halide ions and water molecules are reported. The i-TTM potentials are derived from fits to electronic structure data and include an explicit treatment of two-body repulsion, electrostatics, and dispersion energy. Many-body effects are represented through classical polarization within an extended Thole-type model. By construction, the i-TTM potentials are compatible with the flexible and fully ab initio MB-pol potential, which has recently been shown to accurately predict the properties of water from the gas to the condensed phase. The accuracy of the i-TTM potentials is assessed through extensive comparisons with CCSD(T)-F12, DF-MP2, and DFT data as well as with results obtained with common polarizable force fields for X–(H2O)n clusters with X– = F–, Cl–, Br–, and I–, and n = 1–8. By construction, the new i-TTM potentials will enable direct simulations of vibrational spectra of halide-water systems from clusters to bulk and interfaces. © 2015 American Chemical Society

We present variational calculations on systems containing a few boson helium atoms attached to electronically excited atomic and molecular helium anions He*– and He2*– and characterize their structures and energetics. Previously reported high-level ab initio results [Huber, S. E.; Mauracher, A. Mol. Phys. 2014, 112, 794] to describe the interactions between excited (metastable) anions and a neutral He atom have been employed. For the case of the atomic species He*–, the corresponding interaction with He suggests large anharmonicity effects due to the presence of a deep well of ∼17 500 cm–1 at short distances, together with a more external shallow secondary well of ∼4 cm–1, both supporting bound levels. Moreover, when a sum of pairwise interactions is assumed to describe the full PES corresponding to the presence of several neutral He atoms, geometrical constraints already predict the complete solvation of the anionic impurity by six helium atoms, giving rise to a bipyramidal structure. In turn, for the anisotropic weak interaction He–He2*–, where the anionic dimer is considered as a rigid rotor, the obtained structures show the tendency of the helium atoms to pack themselves together and largely far away from the dopant, thereby confirming the heliophobic character of He2*–.

The structure and relative stability of higher-order HeI clusters are investigated by carrying out full-dimensional quantum calculations within the multiconfiguration time-dependent Hartree framework. The full interaction between the I molecule and the He atoms is based on analytical three-body ab initio He-I potentials obtained from high level ab initio calculations plus the He-He interaction. The low-lying minima on the potential surfaces are found to be very close in energy with the He atoms in a ring encircling the dopant for the global minimum structure, while for the local minima one or two of the He atoms prefer the linear arrangements along the I-axis. Such classical description on the basis of the potential energy landscape is corrected by including anharmonic quantum effects, present in highly floppy systems, by carrying out full dimensional quantum calculations. The potential energy operator was constructed by natural potential fits, while a mode combination scheme was employed to optimize the computational cost of the improved relaxation calculations. The obtained results predict the relative stability of the HeI isomers at zero temperature and provide benchmark data on binding energies and structural properties of these van der Waals systems. The (2,1) and (2,2), involving two He atoms in the T-shape and one or two He atoms in the linear configurations, respectively, are found to be the most stable isomers, although extremely close in energy with the (3,0) and (4,0) ones as predicted by classical optimizations. Comparison with experimental data on similar systems at low temperatures is also discussed. This analysis indicates once more the importance of quantum delocalization and the need of accurate quantum-mechanical treatments to characterize such doped helium nanosystems.

© 2015 American Chemical Society. The strong temperature dependence of the I−·(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process is determined using van’t Hoff analysis of key features that quantify the relative populations of Hbonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I−·(H2O)2 system sample the extended potential energy surface with increasing temperature.

© 2015 American Chemical Society. We perform quantum multiconfiguration time-dependent Hartree calculations for the two cages of the sI CO2 clathrate hydrate, and we report, for first time, results on the translational, rotational, and vibrational states. The potential surface is constructed by the semiempirical SPC/E water model and pairwise additive interactions between the CO2 molecule and all water molecules forming each cage, while a spectroscopic-determined potential is used for the CO2 monomer. The exact kinetic energy operator is derived for the linear CO2 in the cage, and the potential energy operator is adapted to combined mode schemes, chosen adequately in order to obtain the desired product representation. The rotational ability of the CO2 is hindered in the small cavities (512), while in the large cage (51262) translational and rotational degrees of freedom are highly coupled. By analyzing the corresponding T-R eigenstates, we established a connection with experimental X-ray measurements on the orientation of the CO2 molecule in the two cages. Further, vibrational excitations of the fundamental symmetric and antisymmetric stretch modes, their overtones, and combination bands are computed from 7D quantum calculations. We found significant frequency shifts, which are comparable to those observed in the double-peak profile of the experimentally recorded infrared spectra, for the CO2 molecule trapped in the small and large cages of the sI clathrate hydrate. Such good agreement, with the data from recent spectroscopic studies of carbon dioxide clathrate hydrate at low temperatures, indicates a single occupancy of the sI structure cages, allowing an assessment of the theoretical approaches employed.

The energetics and structure of small HeNI2 clusters are analyzed as the size of the system changes, with N up to 38. The full interaction between the I2 molecule and the He atoms is based on analytical ab initio He-I2 potentials plus the He-He interaction, obtained from first-principle calculations. The most stable structures, as a function of the number of solvent He atoms, are obtained by employing an evolutionary algorithm and compared with CCSD(T) and MP2 ab initio computations. Further, the classical description is completed by explicitly including thermal corrections and quantum features, such as zero-point-energy values and spatial delocalization. From quantum PIMC calculations, the binding energies and radial/angular probability density distributions of the thermal equilibrium state for selected-size clusters are computed at a low temperature. The sequential formation of regular shell structures is analyzed and discussed for both classical and quantum treatments. © 2014 American Chemical Society.

Vibrational calculations are performed for the 12-dimensional He 4ICl van der Waals complex using the multiconfiguration time-dependent Hartree (MCTDH) method. The potential energy surface of the cluster is represented as a sum of the triatomic He-ICl ab initio parametrized terms plus the He-He interactions. The topology of the potential presents higher anisotropy compared to the one with a homonuclear dopant, and this is clearly reflected in the structure and energetics of the low-lying conformers of the system. In order to take advantage of the MCTDH method, natural potential fits are employed for the potential energy operator, and also, a mode combination scheme is introduced in order to speed up the computations. Zero-point energy, binding energies, and vibrationally averaged structures of different isomers of the He4ICl cluster are obtained. The present results predict that the (3,1,0) structure, involving three He atoms in the near T-shaped and one He atom in the linear configurations, to be the most stable one in accord with recent experimental findings. Comparisons with previous theoretical and experimental data are presented, and the stability of the high-order conformers is discussed in connection with the multiple minima (global and local) of the underlying potential surface. © 2013 American Chemical Society.

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. © 2013 American Chemical Society.

Full dimensional quantum dynamics calculations of the three lowest isomers of the He 4Br 2 van der Waals molecule in its ground electronic state are reported. The calculations are performed using the multiconfiguration time-dependent Hartree (MCTDH) method and a realistic potential form that includes the sum of three body ab initio coupled-cluster single double triple [CCSD(T)] He-Br 2 interactions plus the He-He and Br-Br interactions. This potential exhibits several multiple minima, with the three lowest ones lying very close in energy, just within 2 cm -1. Such small differences are also found in the calculated binding energies of the three most stable conformers, indicating the floppiness of the system and, thus, the need of accurate potential forms and quantum full dynamics methods to treat this kind of complexes. The 12 dimensional results reported in this work present benchmark data and, thus, can serve to evaluate approximate methods aiming to describe higher order rare gas-dihalogen (N > 4) complexes. A comparison with previous studies using different potential forms and approaches to the energetics for the He 4Br 2 cluster is also presented. © 2012 American Chemical Society.

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. © 2012 American Chemical Society.

Classical and path integral Monte Carlo (CMC, PIMC) >on the fly> calculations are carried out to investigate anharmonic quantum effects on the thermal equilibrium structure of the H5 + cluster. The idea to follow in our computations is based on using a combination of the above-mentioned nuclear classical and quantum statistical methods, and first-principles density functional (DFT) electronic structure calculations. The interaction energies are computed within the DFT framework using the B3(H) hybrid functional, specially designed for hydrogen-only systems. The global minimum of the potential is predicted to be a nonplanar configuration of C 2v symmetry, while the next three low-lying stationary points on the surface correspond to extremely low-energy barriers for the internal proton transfer and to the rotation of the H2 molecules, around the C 2 axis of H5 +, connecting the symmetric C 2v minima in the planar and nonplanar orientations. On the basis of full-dimensional converged PIMC calculations, results on the quantum vibrational zero-point energy (ZPE) and state of H5 + are reported at a low temperature of 10 K, and the influence of the above-mentioned topological features of the surface on its probability distributions is clearly demonstrated. © 2011 American Chemical Society.

The structure, energetics, and spectroscopy of ground-state HeI2 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 HeI2 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 - I2 rovibronic spectra. © 2009 American Chemical Society.