Thermodynamics of hydrogen adsorption in slit-like carbon nanopores at 77 K. Classical versus path-integral Monte Carlo simulations

Hydrogen in slit-like carbon nanopores at 77 K represents a quantum fluid in strong confinement. We have used path-integral grand canonical Monte Carlo and classical grand canonical Monte Carlo simulations for the investigation of the "quantumness" of hydrogen at 77 K adsorbed in slit-like carbon nanopores up to 1 MPa. We find that classical simulations overpredict the hydrogen uptake in carbon nanopores due to neglect of the quantum delocalization. Such disagreement of both simulation methods depends on the slit-like carbon pore size. However, the differences between the final uptakes of hydrogen computed from both classical and quantum simulations are not large due to a similar effective size of quantum/classical hydrogen molecules in carbon nanospaces. For both types of molecular simulations, the volumetric density of stored energy in optimal carbon nanopores exceeds 6.4 MJ dm(-3) (i.e., 45 kg m(-3); Department of Energy target for 2010). In contrast to the hydrogen adsorption isotherms, we found a large reduction of isosteric enthalpy of adsorption computed from the quantum Feynman's path-integral simulations in comparison to the classical values at 77 K and pressures up to 1 MPa. Depression of the quantum isosteric enthalpy of adsorption depends on the slit-like carbon pore size. For the narrow pores (pore width H in [0.59-0.7] nm), the reduction of the quantum isosteric enthalpy of adsorption at zero coverage is around 50% in comparison to the classical one. We observed new phenomena called, by us, the quantum confinement-inducing polymer shrinking. In carbon nanospaces, the quantum cyclic polymers shrink, in comparison to its bulk-phase counterpart, due to a strong confinement effect. At considered storage conditions, this complex phenomenon depends on the size of the slit-like carbon nanopore and the density of hydrogen volumetric energy. For the smallest nanopores and a low density of hydrogen volumetric energy, the reduction of the polymer effective size is the highest, whereas an increase of the pore size and the density of hydrogen volumetric energy causes the polymer swelling up to a value slightly below the one computed from the bulk phase. Quantum confinement-inducing polymer shrinking is of great importance for realizing the potential of quantum molecular sieves.     

A quantum propagator for path-integral simulations of rigid molecules

The expression for the quantum propagator for rigid tops, proposed by Mu?ser and Berne [Phys. Rev. Lett. 77, 2638 (1996)], has been extended to asymmetric tops. Path-integral Monte Carlo simulations are provided that show that the quantum propagator proposed in this work exactly reproduces the rotational energy of free asymmetric tops as evaluated from the partition function. This propagator can subsequently be used in path-integral simulations of condensed phases if a rigid molecular model is used.     

Computational study of the melting-freezing transition in the quantum hard-sphere system for intermediate densities. II. Structural features

The structural features of the quantum hard-sphere system in the region of the fluid-face-centered-cubic-solid transition, for reduced number densities 0.45相似文献   

Calculation of heat capacities of light and heavy water by path-integral molecular dynamics

As an application of atomistic simulation methods to heat capacities, path-integral molecular dynamics has been used to calculate the constant-volume heat capacities of light and heavy water in the gas, liquid, and solid phases. While the classical simulation based on conventional molecular dynamics has estimated the heat capacities too high, the quantum simulation based on path-integral molecular dynamics has given reasonable results based on the simple point-charge/flexible potential model. The calculated heat capacities (divided by the Boltzmann constant) in the quantum simulation are 3.1 in the vapor H2O at 300 K, 6.9 in the liquid H2O at 300 K, and 4.1 in the ice Ih H2O at 250 K, respectively, which are comparable to the experimental data of 3.04, 8.9, and 4.1, respectively. The quantum simulation also reproduces the isotope effect. The heat capacity in the liquid D2O has been calculated to be 10% higher than that of H2O, while it is 13% higher in the experiment. The results demonstrate that the path-integral simulation is a promising approach to quantitatively evaluate the heat capacities for molecular systems, taking account of quantum-mechanical vibrations as well as strongly anharmonic motions.     

On the quantum nature of an excess proton in liquid hydrogen fluoride.

Liquid hydrogen fluoride consists of chains of hydrogen-bonded molecules. The nature of an excess proton in liquid HF, which is of interest not only for its own sake, but also in relation to super-acid chemistry and to its behavior in water, has been studied using computer simulations. The methodology employed is the density-functional-theory-based path-integral Car-Parrinello ab initio molecular dynamics. The excess proton, which formally exists as a H2F+ or a H2F2+ defect in an HF chain, is found to strongly perturb the chain to which it is attached. Moreover, due to large zero-point energy, the charge defect is largely delocalized over several HF molecules.     

An accurate and simple quantum model for liquid water

The path-integral molecular dynamics and centroid molecular dynamics methods have been applied to investigate the behavior of liquid water at ambient conditions starting from a recently developed simple point charge/flexible (SPC/Fw) model. Several quantum structural, thermodynamic, and dynamical properties have been computed and compared to the corresponding classical values, as well as to the available experimental data. The path-integral molecular dynamics simulations show that the inclusion of quantum effects results in a less structured liquid with a reduced amount of hydrogen bonding in comparison to its classical analog. The nuclear quantization also leads to a smaller dielectric constant and a larger diffusion coefficient relative to the corresponding classical values. Collective and single molecule time correlation functions show a faster decay than their classical counterparts. Good agreement with the experimental measurements in the low-frequency region is obtained for the quantum infrared spectrum, which also shows a higher intensity and a redshift relative to its classical analog. A modification of the original parametrization of the SPC/Fw model is suggested and tested in order to construct an accurate quantum model, called q-SPC/Fw, for liquid water. The quantum results for several thermodynamic and dynamical properties computed with the new model are shown to be in a significantly better agreement with the experimental data. Finally, a force-matching approach was applied to the q-SPC/Fw model to derive an effective quantum force field for liquid water in which the effects due to the nuclear quantization are explicitly distinguished from those due to the underlying molecular interactions. Thermodynamic and dynamical properties computed using standard classical simulations with this effective quantum potential are found in excellent agreement with those obtained from significantly more computationally demanding full centroid molecular dynamics simulations. The present results suggest that the inclusion of nuclear quantum effects into an empirical model for water enhances the ability of such model to faithfully represent experimental data, presumably through an increased ability of the model itself to capture realistic physical effects.     

Signatures of quantum behavior in the microscopic dynamics of liquid hydrogen and deuterium

We discuss the microscopic dynamics and structure of liquid hydrogen and deuterium, as probed by inelastic x-ray scattering measurements. Samples are kept in corresponding thermodynamic conditions, at which classical systems are expected to exhibit the same dynamic and static responses. On the contrary, we observe clear differences revealing the onset of quantum deviations, both in the broadening of inelastic excitations and in the position of the first sharp diffraction peak. These features are discussed, compared to path-integral Monte Carlo simulations, and finally associated with the different de Broglie wavelengths of the two isotopes.     

A combinatorial approach to the electron correlation problem

Starting from a path-integral formulation of quantum statistical mechanics expressed in a space of Slater determinants, we develop a method for the Monte Carlo evaluation of the energy of a correlated electronic system. The path-integral expression for the partition function is written as a contracted sum over graphs. A graph is a set of distinct connected determinants on which paths can be represented. The weight of a graph is given by the sum over exponentially large numbers of paths which visit the vertices of the graph. We show that these weights are analytically computable using combinatorial techniques, and they turn out to be sufficiently well behaved to allow stable Monte Carlo simulations in which graphs are stochastically sampled according to a Metropolis algorithm. In the present formulation, graphs of up to four vertices have been included. In a Hartree-Fock basis, this allows for paths which include up to sixfold excitations relative to the Hartree-Fock determinant. As an illustration, we have studied the dissociation curve of the N(2) molecule in a VDZ basis, which allows comparison with full configuration-interaction calculations.     

Direct assessment of quantum nuclear effects on hydrogen bond strength by constrained-centroid ab initio path integral molecular dynamics

The impact of quantum nuclear effects on hydrogen (H-) bond strength has been inferred in earlier work from bond lengths obtained from path integral molecular dynamics (PIMD) simulations. To obtain a direct quantitative assessment of such effects, we use constrained-centroid PIMD simulations to calculate the free energy changes upon breaking the H-bonds in dimers of HF and water. Comparing ab initio simulations performed using PIMD and classical nucleus molecular dynamics (MD), we find smaller dissociation free energies with the PIMD method. Specifically, at 50 K, the H-bond in (HF)(2) is about 30% weaker when quantum nuclear effects are included, while that in (H(2)O)(2) is about 15% weaker. In a complementary set of simulations, we compare unconstrained PIMD and classical nucleus MD simulations to assess the influence of quantum nuclei on the structures of these systems. We find increased heavy atom distances, indicating weakening of the H-bond consistent with that observed by direct calculation of the free energies of dissociation.     

Quantum proton transfer in hydrated sulfuric acid clusters: a perspective from semiempirical path integral simulations

We have carried out path-integral molecular dynamics simulations for hydrated sulfuric acid clusters to understand acid-dissociation and hydrogen-bonded structural rearrangement processes in these clusters from a quantum mechanical viewpoint. The simulations were performed using the PM6 semiempirical electronic structure level whose parameters were modified on the basis of the specific reaction parameters strategy so that relative energies of optimized structures, as well as water binding energies reproduce ab initio and density-functional theory calculations. We have found that the acid dissociation processes, first and second deprotonation, effectively occur in a hydrated cluster with a specific cluster size. The mechanisms of the proton-transfer processes were analyzed in detail and it was found that the distance between O in sulfuric acid and O in the proton-accepting water is playing an important role. We also found that the water coordination number of the poton-accepting water is important in the proton-transfer processes.     

Computational study of the structures of gaseous helium-3 at low temperature

A computational study of gaseous helium-3 at T=5.23 K, for number densities rho N<0.0021 A(-3), analyzing the different pair and triplet structures in both the r and the k spaces, is presented. Structures in r space (i.e., instantaneous, total continuous linear response, and centroids) are determined via path-integral Monte Carlo simulations in the canonical ensemble by utilizing the Aziz-Slaman and the SAPT2 interatomic potentials. Additional results obtained with the application of two closures for triplets in r space, the Kirkwood superposition approximation and the Jackson-Feenberg convolution, are also reported. Besides, an analysis of the nonsuitability of quantum hard spheres for describing this system is included. The pair structures in k space are fixed via Ornstein-Zernike schemes appropriate for dealing with quantum diffraction effects in fluids. The effect on the quality of the computed isothermal compressibilities brought about by increasing the sample size in the simulations and by the subsequent application of a grand ensemble correction to the asymptotic behavior of the canonical pair radial correlation functions is also investigated. Furthermore, it is demonstrated analytically that the methods of classical statistical mechanics for dealing with the higher-order direct correlation functions remain fully valid for studying the higher-order correlations of path-integral centroids. By taking advantage of this result, the triplet structure factors for the centroid (also for the instantaneous) correlations are computed by following a number of distinct closures and methods that involve triplet direct correlation functions. The latter computations are intended to explore an alternative scheme to the very expensive fixing of triplet structure factors through direct path integral simulations, an alternative which is expected to yield the main features of these triplet quantities for this gas. Comparison with experiment is made wherever possible, and the results presented allow one to explain the substantial structural features existing in gaseous helium-3.     

Opposing Electronic and Nuclear Quantum Effects on Hydrogen Bonds in H2O and D2O

The effect of extending the O−H bond length(s) in water on the hydrogen-bonding strength has been investigated using static ab initio molecular orbital calculations. The “polar flattening” effect that causes a slight σ-hole to form on hydrogen atoms is strengthened when the bond is stretched, so that the σ-hole becomes more positive and hydrogen bonding stronger. In opposition to this electronic effect, path-integral ab initio molecular-dynamics simulations show that the nuclear quantum effect weakens the hydrogen bond in the water dimer. Thus, static electronic effects strengthen the hydrogen bond in H₂O relative to D₂O, whereas nuclear quantum effects weaken it. These quantum fluctuations are stronger for the water dimer than in bulk water.     

Computation of the equation of state of the quantum hard-sphere fluid utilizing several path-integral strategies

The compressibility factor of the quantum hard-sphere fluid within the region (rho(N) (*)相似文献   

Ab initio molecular dynamics study of a mixture of HF(aq) and HCl(aq)

We have studied a mixture of HF and HCl molecules in water using Car-Parrinello ab initio molecular dynamics (CPMD). We have done simulations with 1 HF and 3 HCl molecules, 3 HF and 4 HCl, 6 HF and 8 HCl (6/8 simulation), and 14 HF molecules. All simulations consist of 32 molecules, and they were 10-96 ps long. The HF dissociation probability was around 30%, and HCl's was more than 90%. The solvation of the HF molecule was much better than the solvation of HCl. The solvation environment of F, both the F- ion and the F in HF, did not depend much on the acids concentration, whereas the Cl coordination numbers were rather sensitive to the concentration. In the 6/8 simulation, all XH-Y (X, Y = F, Cl) type molecules were observed and the FH-F was the most probable. In general, the molecular structures in mixed aqueous acid systems were similar to the pure HF(aq) and HCl(aq) systems.     

Triplet correlations in the quantum hard-sphere fluid

A study of three-particle correlations in the quantum hard-sphere fluid far from exchange is presented. The three types of triplet correlations in a monatomic quantum fluid (instantaneous, linear response, and centroids) are analyzed by utilizing (a) the density derivatives of the corresponding quantum pair radial correlation functions, (b) closures for triplet functions, and (c) path-integral Monte Carlo (PIMC) simulations that have concentrated on the fixing of equilateral and isosceles correlations. For the sake of comparison, the classical hard-sphere fluid is also studied with tools (a) and (b) and Monte Carlo (MC) simulations. The relative usefulness of density derivatives combined with closures is discussed in light of the PIMC and MC results. The exact PIMC correlations between quantum triplets show features that resemble those known to occur at the pair level, such as the close proximity between the instantaneous and the three-particle linear response, the much more pronounced features in centroid triplet structures, and the same global patterns with changes in density and temperature such as the outward shifts of the structures with decreasing temperature and density.     

Ab initio statistical mechanics of surface adsorption and desorption. I. H2O on MgO (001) at low coverage

We present a general computational scheme based on molecular dynamics (MD) simulation for calculating the chemical potential of adsorbed molecules in thermal equilibrium on the surface of a material. The scheme is based on the calculation of the mean force in MD simulations in which the height of a chosen molecule above the surface is constrained and subsequent integration of the mean force to obtain the potential of mean force and hence the chemical potential. The scheme is valid at any coverage and temperature, so that in principle it allows the calculation of the chemical potential as a function of coverage and temperature. It avoids all statistical mechanical approximations, except for the use of classical statistical mechanics for the nuclei, and assumes nothing in advance about the adsorption sites. From the chemical potential, the absolute desorption rate of the molecules can be computed, provided that the equilibration rate on the surface is faster than the desorption rate. We apply the theory by ab initio MD simulation to the case of H2O on MgO (001) in the low-coverage limit, using the Perdew-Burke-Ernzerhof (PBE) form of exchange correlation. The calculations yield an ab initio value of the Polanyi-Wigner frequency prefactor, which is more than two orders of magnitude greater than the value of 10(13) s(-1) often assumed in the past. Provisional comparison with experiment suggests that the PBE adsorption energy may be too low, but the extension of the calculations to higher coverages is needed before firm conclusions can be drawn. The possibility of including quantum nuclear effects by using path-integral simulations is noted.     

Isotope effects in ice Ih: a path-integral simulation

Ice Ih has been studied by path-integral molecular dynamics simulations, using the effective q-TIP4P/F potential model for flexible water. This has allowed us to analyze finite-temperature quantum effects in this solid phase from 25 to 300 K at ambient pressure. Among these effects we find a negative thermal expansion of ice at low temperatures, which does not appear in classical molecular dynamics simulations. The compressibility derived from volume fluctuations gives results in line with experimental data. We have analyzed isotope effects in ice Ih by considering normal, heavy, and tritiated water. In particular, we studied the effect of changing the isotopic mass of hydrogen on the kinetic energy and atomic delocalization in the crystal as well as on structural properties such as interatomic distances and molar volume. For D(2)O ice Ih at 100 K we obtained a decrease in molar volume and intramolecular O-H distance of 0.6% and 0.4%, respectively, as compared to H(2)O ice.     

Quantum effects on adsorption and diffusion of hydrogen and deuterium in microporous materials

Monte Carlo and molecular dynamics simulations and neutron scattering experiments are used to study the adsorption and diffusion of hydrogen and deuterium in zeolite Rho in the temperature range of 30-150 K. In the molecular simulations, quantum effects are incorporated via the Feynman-Hibbs variational approach. We suggest a new set of potential parameters for hydrogen, which can be used when Feynman-Hibbs variational approach is used for quantum corrections. The dynamic properties obtained from molecular dynamics simulations are in excellent agreement with the experimental results and show significant quantum effects on the transport at very low temperature. The molecular dynamics simulation results show that the quantum effect is very sensitive to pore dimensions and under suitable conditions can lead to a reverse kinetic molecular sieving with deuterium diffusing faster than hydrogen.     

On the calculation of time correlation functions by potential scaling

We present and analyze a general method to calculate time correlation functions from molecular dynamics on scaled potentials for complex systems for which simulation is affected by broken ergodicity. Depending on the value of the scaling factor, correlations can be calculated for times that can be orders of magnitude longer than those accessible to direct simulations. We show that the exact value of the time correlation functions of the original system (i.e., with unscaled potential) can be obtained, in principle, using an action-reweighting scheme based on a stochastic path-integral formalism. Two tests (involving a bistable potential model and a dipeptide bond-vector orientational relaxation) are exemplified to showcase the strengths, as well as the limitations of the approach, and a procedure for the estimation of the time-dependent standard deviation error is outlined.