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The objective of this study is to understand the fracture mechanisms in the lithium manganese oxide (LiMn2O4) electrode at the molecular level by studying mechanical properties of the material at different values of the State of Charge (SOC) using the principles of molecular dynamics (MD). A 2 × 2 × 2 cubic structure of the LiMn2O4 unit cell containing eight lithium ions, eight trivalent manganese ions, eight tetravalent manganese ions, and 32 oxygen ions is studied using a large-scale atomic/molecular massively parallel simulator. As part of the model validation, the lattice parameter and volume changes of LixMn2O4 as a function of SOC (0 < x < 1) have been studied and validated with respect to the experimental data. This validated model has been used for a parametric study involving the SOC value, strain rate (charge and discharge rate), and temperature. The MD simulations suggest that the lattice constant varies from 8.042 Å to 8.235 Å during a full discharging cycle, in agreement with the experimental data. The material at higher SOC shows more ductile behavior compared to low SOC values. Furthermore, yield and ultimate stresses are less at lower SOC values except when SOC values are within 0.125 and 0.375, verifying the phase transformation theory in this range. The strain rate does not affect the fully intercalated material significantly but seems to influence the material properties of the partially charged electrode. Finally, a study of the effect of temperature suggests that diffusion coefficient values for both high and low-temperature zones follow an Arrhenius profile, and the results are successfully explained using the vacancy diffusion mechanism. [ABSTRACT FROM AUTHOR]

Thermodiffusion or the Ludwig-Soret effect is known as the cross effect between the temperature gradient and induced separation of mixture species in multicomponent mixtures. The performance of the boundary driven non-equilibrium molecular dynamics enhanced heat exchange (eHEX) algorithm was validated by evaluating the sign and magnitude of the thermodiffusion process in methane/n-butane/n-dodecane (nC1–nC4–nC12) ternary mixtures. The eHEX algorithm consists of an extended version of the HEX algorithm with an improved energy conservation property. In addition to this, the transferable potentials for phase equilibria-united atom augmented force field was employed in all molecular dynamics (MD) simulations to accurately represent molecular interactions in the fluid. Our newly employed MD algorithm was capable to appropriately reflect the thermophobicity concept and the coupled effect of relative density and mole fraction of the mixture species on the thermodiffusion process. The separation ratio of the ternary mixture for five different compositions (at 333.15 K and 35 MPa) showed good agreement with experimental data and better accuracy in predicting the sign change of the intermediate component (nC4) as its concentration in the mixture increases, when compared to other MD models. [ABSTRACT FROM AUTHOR]

The anion photoelectron (PE) spectra along with supporting results of density functional theory (DFT) calculations on SmO-, SmCeOy-, and Sm2Oy- (y = 1, 2) are reported and compared to previous results on CeO- [M. Ray et al., J. Chem. Phys. 142, 064305 (2015)] and Ce2Oy- (y = 1, 2) [J. O. Kafader et al., J. Chem. Phys. 145, 154306 (2016)]. Similar to the results on CexOy- clusters, the PE spectra of SmO-, SmCeOy-, and Sm2Oy- (y = 1, 2) all exhibit electronic transitions to the neutral ground state at approximately 1 eV e-BE. The Sm centers in SmO and Sm2O2 neutrals can be described with the 4f56s superconfiguration, which is analogous to CeO and Ce2O2 neutrals in which the Ce centers can be described with the 4f 6s superconfiguration (ZCe = ZSm - 4). The Sm center in CeSmO2, in contrast, has a 4f6 occupancy, while the Ce center maintains the 4f 6s superconfiguration. The less oxidized Sm centers in both Sm2O and SmCeO have 4f6 6s occupancies. The 4f6 subshell occupancy results in relatively weak Sm-O bond strengths. If this extra 4f occupancy also occurs in bulk Sm-doped ceria, it may play a role in the enhanced O2- ionic conductivity in Sm-doped ceria. Based on the results of DFT calculations, the heteronuclear Ce-Sm oxides have molecular orbitals that are distinctly localized Sm 4f, Sm 6s, Ce 4f, and Ce 6s orbitals. The relative intensity of two electronic bands in the PE spectrum of Sm2O- exhibits an unusual photon energy-dependence, and the PE spectrum of Sm2O2- exhibits a photon energy-dependent continuum signal between two electronic transitions. Several explanations, including the high magnetic moment of these suboxide species and the presence of low-lying quasi-bound anion states, are considered. [ABSTRACT FROM AUTHOR]

We report a detailed study of the local composition and structure of a model, bi-phasic nanoglass with nominal stoichiometry Cu55Nb45. Three dimensional atom probe data suggest a nanoscale-phaseseparated glassy structure having well defined Cu-rich and Nb-rich regions with a characteristic length scale of 3 nm. However, extended x-ray absorption fine structure analysis indicates subtle differences in the local environments of Cu and Nb. While the Cu atoms displayed a strong tendency to cluster and negligible structural order beyond the first coordination shell, the Nb atoms had a larger fraction of unlike neighbors (higher chemical order) and a distinctly better-ordered structural environment (higher topological order). This provides the first experimental indication that metallic glass formation may occur due to frustration arising from the competition between chemical ordering and clustering. These observations are complemented by classical as well as ab initio molecular dynamics simulations. Our study indicates that these nanoscale phase-separated glasses are quite distinct from the single phase nanoglasses (studied by Gleiter and others) in the following three respects: (i) they contain at least two structurally and compositionally distinct, nanodispersed, glassy phases, (ii) these phases are separated by comparatively sharp inter-phase boundaries, and (iii) thermally induced crystallization occurs via a complex, multi-step mechanism. Such materials, therefore, appear to constitute a new class of disordered systems that may be called a composite nanoglass. [ABSTRACT FROM AUTHOR]

We recently proposed a multistage ab initio wavepacket dynamics (MS-AIWD) treatment for the study of delocalized electronic systems as well as electron transport through donor-bridge-acceptor systems such as those found in molecular-wire/electrode networks. In this method, the full donor-bridge-acceptor open system is treated through a rigorous partitioning scheme that utilizes judiciously placed offsetting absorbing and emitting boundary conditions. In this manner, the electronic coupling between the bridge molecule and surrounding electrodes is accounted. Here, we extend MS-AIWD to include the dynamics of open-electronic systems in conjunction with (a) simultaneous treatment of nuclear dynamics and (b) external electromagnetic fields. This generalization is benchmarked through an analysis of wavepackets propagated on a potential modeled on an Al27 - C7 - Al27 nanowire. The wavepacket results are inspected in the momentum representation and the dependence of momentum of the wavepacket as well as its transmission probabilities on the magnitude of external bias are analyzed. [ABSTRACT FROM AUTHOR]

We present a generalization to our previously developed quantum wavepacket ab initio molecular dynamics (QWAIMD) method by using multiple diabatic electronic reduced single particle density matrices, propagated within an extended Lagrangian paradigm. The Slater determinantal wavefunctions associated with the density matrices utilized may be orthogonal or nonorthogonal with respect to each other. This generalization directly results from an analysis of the variance in electronic structure with quantum nuclear degrees of freedom. The diabatic electronic states are treated here as classical parametric variables and propagated simultaneously along with the quantum wavepacket and classical nuclei. Each electronic density matrix is constrained to be N-representable. Consequently two sets of new methods are derived: extended Lagrangian-QWAIMD (xLag-QWAIMD) and diabatic extended Lagrangian-QWAIMD (DxLag-QWAIMD). In both cases, the instantaneous potential energy surface for the quantum nuclear degrees of freedom is constructed from the diabatic states using an on-the-fly nonorthogonal multireference formalism. By introducing generalized grid-based electronic basis functions, we eliminate the basis set dependence on the quantum nucleus. Subsequent reuse of the two-electron integrals during the on-the-fly potential energy surface computation stage yields a substantial reduction in computational costs. Specifically, both xLag-QWAIMD and DxLag-QWAIMD turn out to be about two orders of magnitude faster than our previously developed time-dependent deterministic sampling implementation of QWAIMD. Energy conservation properties, accuracy of the associated potential surfaces, and vibrational properties are analyzed for a family of hydrogen bonded systems. [ABSTRACT FROM AUTHOR]

We propose a multistage quantum wavepacket dynamical treatment for the study of delocalized electronic systems as well as electron transport through donor-bridge-acceptor systems such as those found in molecular-wire/electrode networks. The full donor-bridge-acceptor system is treated through a rigorous partitioning scheme that utilizes judiciously placed offsetting absorbing and emitting boundary conditions. These facilitate a computationally efficient and potentially accurate treatment of the long-range coupling interactions between the bridge and donor/acceptor systems and the associated open system boundary conditions. Time-independent forms of the associated, partitioned equations are also derived. In the time-independent form corresponding to the bridge system, coupling to donor and acceptor, that is long-range interactions, is completely accounted. For the time-dependent study, the quantum dynamics of the electronic flux through the bridge-donor/acceptor interface is constructed using an accurate and efficient representation of the discretized quantum-mechanical free-propagator. A model for an electrode-molecular wire-electrode system is used to test the accuracy of the scheme proposed. Transmission probability is obtained directly from the probability density of the electronic flux in the acceptor region. Conductivity through the molecular wire is computed using a wavepacket flux correlation function. [ABSTRACT FROM AUTHOR]

We investigate and analyze the vibrational properties, including hydrogen/deuterium isotope effects, in a fundamental organic hydrogen bonded system using multiple experimental (infrared multiple photon dissociation and argon-tagged action spectroscopy) and computational techniques. We note a qualitative difference between the two experimental results discussed here and employ ab initio molecular dynamics simulations to explain these results. A deeper understanding of the differences between the isotopically labeled systems arises from an analysis of the simulated cluster spectroscopy and leads to a system-bath coupling interpretation. Specifically, when a few active modes, involving the shared hydrogen/deuterium stretch, are identified and labeled as “system,” with all other molecular vibrational modes being identified as “bath” modes, we find critical differences in the coupling between the system modes for the shared proton and shared deuteron cases. These differences affect the energy repartitioning between these modes resulting in a complex spectral evolution as a function of temperature. Furthermore, intensity borrowing across modes that are widely distributed in the frequency domain plays an important role on the simulated spectra. [ABSTRACT FROM AUTHOR]

Experimental investigations on thermodiffusion have been conducted for five different ternary mixtures of methane, n-butane, and n-dodecane at a high temperature and pressure. While the mole fraction of methane was fixed at 0.2 the mole fraction of n-dodecane was varied from 0.7 to 0.2. The experiments were performed in a microgravity environment on board the satellite FOTON-M3. It was found that in all mixtures, n-dodecane separated to the cold side whereas methane segregated to the hot side. n-butane, the species with an intermediate density, showed a change in sign as its mole fraction was increased. At low concentrations it collected on the cold side but moved in the opposite direction with an increase in its mole fraction. The role of the relative density coupled with the species concentrations has been used to explain the thermodiffusion factor in each mixture. Computational investigations showed a similar behavior. However, the theoretical model was not able to capture the sign change of n-butane accurately. The inadequate representation of the significance of the relative densities and the mole fraction of the species has been found as the reason for this. [ABSTRACT FROM AUTHOR]

We have used finite temperature ab initio molecular dynamics simulations in conjunction with computation of critical quantum nuclear effects to probe the differences between single-photon argon tagged action spectral results and infrared multiple-photon dissociation experiments for a proton bound molecular ion system. We find that the principal difference between the results in these experimental techniques is essentially that of cluster temperature. The multiple-photon dissociation experiments conducted using room temperature ions reflect a larger degree of conformational freedom compared to the colder single-photon argon tagged action spectral results. Our ab initio molecular dynamics simulation techniques accurately capture the effects of conformational sampling, adequately reproduce both spectra, and can be utilized to assign the dynamically averaged finite temperature spectra. [ABSTRACT FROM AUTHOR]

Ion-water-cluster properties are investigated both through the multistate empirical valence bond potential and a polarizable model. Equilibrium properties of the ion-water clusters H+(H2O)100, Na+(H2O)100, Na+(H2O)20, and Cl-(H2O)17 in the temperature region 100–450 K are explored using a hybrid parallel basin-hopping and tempering algorithm. The effect of the solid-liquid phase transition in both caloric curves and structural distribution functions is investigated. It is found that sodium and chloride ions largely reside on the surface of water clusters below the cluster melting temperature but are solvated into the interior of the cluster above the melting temperature, while the solvated proton was found to have significant propensity to reside on or near the surface in both the liquid- and solid-state clusters. [ABSTRACT FROM AUTHOR]

We have carried out ab initio molecular-dynamics studies on hydroperoxyl water clusters. Our studies are complemented by optimization, frequency, and excited-state calculations. The three main results we obtained are (a) the dynamically averaged energy gap between the highest-occupied molecular orbital and the lowest-unoccupied molecular orbital monotonically decreases as the number of water molecules is increased in a hydroperoxyl water cluster system, (b) the dynamical averaging of the potential-energy surface at finite temperature broadens the electronic excitation spectrum and changes the infrared spectrum in nontrivial ways, and (c) the structural analysis of our dynamics simulation indicates that the oxygen-oxygen distance in a solvated hydroperoxyl-water cluster is very similar to that found in protonated water clusters (Zundel: H5O2+) inspite of the fact that the latter possesses a positive charge and the hydroperoxyl-water cluster does not. Dynamical charge analysis and the weak acidity of HO2 are used to justify this result. [ABSTRACT FROM AUTHOR]

The ab initio atom-centered density-matrix propagation approach and the multistate empirical valence bond method have been employed to study the structure, dynamics, and rovibrational spectrum of a hydrated proton in the “magic” 21 water cluster. In addition to the conclusion that the hydrated proton tends to reside on the surface of the cluster, with the lone pair on the protonated oxygen pointing “outwards,” it is also found that dynamical effects play an important role in determining the vibrational properties of such clusters. This result is used to analyze and complement recent experimental and theoretical studies. [ABSTRACT FROM AUTHOR]

A methodology to efficiently conduct simultaneous dynamics of electrons and nuclei is presented. The approach involves quantum wave packet dynamics using an accurate banded, sparse and Toeplitz representation for the discrete free propagator, in conjunction with ab initio molecular dynamics treatment of the electronic and classical nuclear degree of freedom. The latter may be achieved either by using atom-centered density-matrix propagation or by using Born–Oppenheimer dynamics. The two components of the methodology, namely, quantum dynamics and ab initio molecular dynamics, are harnessed together using a time-dependent self-consistent field-like coupling procedure. The quantum wave packet dynamics is made computationally robust by using adaptive grids to achieve optimized sampling. One notable feature of the approach is that important quantum dynamical effects including zero-point effects, tunneling, as well as over-barrier reflections are treated accurately. The electronic degrees of freedom are simultaneously handled at accurate levels of density functional theory, including hybrid or gradient corrected approximations. Benchmark calculations are provided for proton transfer systems and the dynamics results are compared with exact calculations to determine the accuracy of the approach. [ABSTRACT FROM AUTHOR]

We present a rigorous analysis of the primitive Gaussian basis sets used in the electronic structure theory. This leads to fundamental connections between Gaussian basis functions and the wavelet theory of multiresolution analysis. We also obtain a general description of basis set superposition error which holds for all localized, orthogonal or nonorthogonal, basis functions. The standard counterpoise correction of quantum chemistry is seen to arise as a special case of this treatment. Computational study of the weakly bound water dimer illustrates that basis set superposition error is much less for basis functions beyond the 6-31+G* level of Gaussians when structure, energetics, frequencies, and radial distribution functions are to be calculated. This result will be invaluable in the use of atom-centered Gaussian functions for ab initio molecular dynamics studies using Born-Oppenheimer and atom-centered density matrix propagation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Periodic distributed approximating functionals are proposed and used to obtain a coordinate representation for the adiabatically adjusting principal axis hyperspherical coordinate kinetic energy operator. The approach is tested and accurate results for adiabatic surface functions for the reaction F + H[sub 2] → HF + H are calculated and compared to those of some existing methods. [ABSTRACT FROM AUTHOR]

In a recently developed approach to ab initio molecular dynamics (ADMP), we used an extended Lagrangian to propagate the density matrix in a basis of atom centered Gaussian functions. Results of trajectory calculations obtained by this method are compared with the Born-appenheimer approach (BO), in which the density is converged at each step rather than propagated. For NaC1, the vibrational frequency with ADMP is found to be independent of the fictitious electronic mass and to be equal to the BO trajectory result. For the photodissociation of formaldehyde, H[SUB2]CO→H[SUB2] +CO, and the three body dissociation of glyoxal, C[SUB2]H[SUB2]O2→H[SUB2]+2CO, very good agreement is found between the Born-appenheimer trajectories and the extended Lagrangian approach in terms of the rotational and vibrational energy distributions of the products. A 1.2 ps simulation of the dynamics of chloride ion in a cluster of 25 water molecules was used as a third test case. The Fourier transform of the velocity-velocity autocorrelation function showed the expected features in the vibrational spectrum corresponding to strong hydrogen bonding in the cluster. A redshift of approximately 200 cm[SUP-1] was observed in the hydroxyl stretch due to the presence of the chloride ion. Energy conservation and adiabaticity were maintained very well in all of the test cases. [ABSTRACT FROM AUTHOR]

A generalization is presented here for a newly developed approach to ab initio molecular dynamics, where the density matrix is propagated with Gaussian orbitals. Including a tensorial fictitious mass facilitates the use of larger time steps for the dynamics process. A rigorous analysis of energy conservation is presented and used to control the deviation of the fictitious dynamics trajectory from the corresponding Born–Oppenheimer dynamics trajectory. These generalizations are tested for the case of the Cl[sup -](H[sub 2]O)[sub 25] cluster. It is found that, even with hydrogen atoms present in the system, no thermostats are necessary to control the exchange of energy between the nuclear and the fictitious electronic degrees of freedom. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the derivation of symmetry-adapted Distribution Approximating Functionals to obtain a coordinate representation for the Adiabatically Adjusting Principal Axis Hyperspherical coordinates kinetic energy operator. Testing of resulting expressions by iterative diagonalization of the Hamiltonian matrix using the Arnoldi procedure.

We consider further how scattering information (the S-matrix) can be obtained, as a continuous function of energy, by studying wave packet dynamics on a finite grid of restricted size. Solutions are expanded using recursively generated basis functions for calculating Green’s functions and the spectral density operator. These basis functions allow one to construct a general solution to both the standard homogeneous Schrödinger’s equation and the time-independent wave packet, inhomogeneous Schrödinger equation, in the non-interacting region (away from the boundaries and the interaction region) from which the scattering solution obeying the desired boundary conditions can be constructed. In addition, we derive new expressions for a ‘‘remainder or error term,’’ which can hopefully be used to optimize the choice of grid points at which the scattering information is evaluated. Problems with reflections at finite boundaries are dealt with using a Hamiltonian which is damped in the boundary region as was done by Mandelshtam and Taylor [J. Chem. Phys. 103, 2903 (1995)]. This enables smaller Hamiltonian matrices to be used. The analysis and numerical methods are illustrated by application to collinear H+H2 reactive scattering. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A temperature dependent analysis of the vibrational spectrum of the protonated 21-water cluster is provided using the ab initio molecular dynamics method called atom-centered density matrix propagation. It is found that even at low temperatures, finite temperature dynamical averaging greatly influences the spectral results. While the current publication deals with an analysis of the vibrational spectrum at many internal temperatures, the general conclusions are consistent with those in an earlier publication [Iyengar et al., J. Chem. Phys.123, 084309 (2005)]. Furthermore, a critical spectral region (below 1500 cm-1) is found for this system, where the key vibrations of the protonated species can be detected. [ABSTRACT FROM AUTHOR]

The curvy-extended-Lagrangian molecular-dynamics (ELMD) approach [J. M. Herbert and M. Head-Gordon, J. Chem. Phys. 121, 11542 (2004)] is similar to atom-centered density-matrix propagation (ADMP) [H. B. Schlegel, J. M. Millam, S. S. Iyengar, G. A. Voth, A. D. Daniels, G. E. Scuseria, and M. J. Frisch, J. Chem. Phys. 114, 9758 (2001); S. S. Iyengar, H.B. Schlegel, J.M. Millam, G.A. Voth, G.E. Scuseria, and M.J. Frisch, ibid.115, 10291 (2001); H.B. Schlegel, S.S. Iyengar, X. Li, J.M. Millam, G.A. Voth, G.E. Scuseria, and M.J. Frisch, ibid. 117, 8694 (2002); S.S. Iyengar, H.B. Schlegel, G.A. Voth, J.M. Millam, G.E. Scuseria, and M.J. Frisch, Israel J. Chem. 42, 191 (2002)] and based on Car-Parrinello [Phys. Rev. Lett. 55, 2471 (1985)] extended-Lagrangian [H.C. Andersen, J. Chem. Phys. 72, 2384 (1980)] molecular dynamics. Similarities between curvy-ELMD and ADMP arise from using unconverged electronic single-particle density matrices within Gaussian basis functions as dynamical variables. Curvy-ELMD differs from ADMP in not requiring idempotency to be explicitly enforced. In this Comment, we address several misleading remarks in Refs. 1 [J.M. Herbert and M. Head-Gordon, J. Chem. Phys. 121, 11542 (2004)] and 8 [J.M. Herbert and M. Head-Gordon, J. Chem. Phys. (submitted)]. [ABSTRACT FROM AUTHOR]

A correction to the article "Thermodiffusion in multicomponent hydrocarbon mixtures: Experimental investigations and computational analysis" that was published in the March 2, 2010 issue is presented.

We propose and implement an alternative approach to the original Car-Parrinello method where the density matrix elements (instead of the molecular orbitals) are propagated together with the nuclear degrees of freedom. Our new approach has the advantage of leading to an O(N) computational scheme in the large system limit. Our implementation is based on atom-centered Gaussian orbitals, which are especially suited to deal effectively with general molecular systems. The methodology is illustrated by applications to the three-body dissociation of triazine and to the dynamics of a cluster of a chloride ion with 25 water molecules. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Cooperativity is essential for the proper functioning of numerous proteins by allosteric interactions. Hemoglobin from Scapharca inaequivalvis (HbI) is a homodimeric protein that can serve as a minimal unit for studying cooperativity. We investigated the structural changes in HbI after carbon monoxide dissociation using time-resolved resonance Raman spectroscopy and observed structural rearrangements in the Fe-proximal histidine bond, the position of the heme in the pocket, and the hydrogen bonds between heme and interfacial water upon ligand dissociation. Some of the spectral changes were different from those observed for human adult hemoglobin due to differences in subunit assembly and quaternary changes. The structural rearrangements were similar for the singly and doubly dissociated species but occurred at different rates. The rates of the observed rearrangements indicated that they occurred synchronously with subunit rotation and are influenced by intersubunit coupling, which underlies the positive cooperativity of HbI., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Lanthanide (Ln) oxide clusters have complex electronic structures arising from the partially occupied Ln 4f subshell. New anion photoelectron (PE) spectra of Sm x Ce 3-x O y - (x = 0-3; y = 2-4) along with supporting results of density functional theory (DFT) calculations suggest interesting x and y-dependent Sm 4f subshell occupancy with implications for Sm-doped ionic conductivity of ceria, as well as the overall electronic structure of the heterometallic oxides. Specifically, the Sm centers in the heterometallic species have higher 4f subshell occupancy than the homonuclear Sm 3 O y - /Sm 3 O y clusters. The higher 4f subshell occupancy both weakens Sm-O bonds and destabilizes the 4f subshell relative to the predominantly O 2p bonding orbitals in the clusters. Parallels between the electronic structures of these small cluster systems with bulk oxides are explored. In addition, unusual changes in the excited state transition intensities, similar to those observed previously in the PE spectra of Sm 2 O - and Sm 2 O 2 - [J. O. Kafader et al., J. Chem. Phys. 146, 194310 (2017)], are also observed in the relative intensities of electronic transitions to excited neutral state bands in the PE spectra of Sm x Ce 3-x O y - (x = 1-3; y = 2, 4). The new spectra suggest that the effect is enhanced with lower oxidation states and with an increasing number of Sm atoms, implying that the prevalence of electrons in the diffuse Sm 6s-based molecular orbitals and a more populated 4f subshell both contribute to this phenomenon. Finally, this work identifies challenges associated with affordable DFT calculations in treating the complex electronic structures exhibited by these systems, including the need for a more explicit treatment of strong coupling between the neutral and PE.

In multicomponent liquid mixtures, the diffusion flow of chemical species can be induced by temperature gradients, which leads to a separation of the constituent components. This cross effect between temperature and concentration is known as thermodiffusion or the Ludwig-Soret effect. The performance of boundary driven non-equilibrium molecular dynamics along with the enhanced heat exchange (eHEX) algorithm was studied by assessing the thermodiffusion process in n-pentane/n-decane (nC 5 -nC 10 ) binary mixtures. The eHEX algorithm consists of an extended version of the HEX algorithm with an improved energy conservation property. In addition to this, the transferable potentials for phase equilibria-united atom force field were employed in all molecular dynamics (MD) simulations to precisely model the molecular interactions in the fluid. The Soret coefficients of the n-pentane/n-decane (nC 5 -nC 10 ) mixture for three different compositions (at 300.15 K and 0.1 MPa) were calculated and compared with the experimental data and other MD results available in the literature. Results of our newly employed MD algorithm showed great agreement with experimental data and a better accuracy compared to other MD procedures.

In an unprecedented experimental investigation, a ternary and a four component hydrocarbon mixture at high pressure have been studied in a nearly convection free environment to understand the thermodiffusion process. A binary mixture has also been investigated in this environment. Experimental investigations of the three mixtures have been conducted in space onboard the spacecraft FOTON-M3 thereby isolating the gravity-induced convection that otherwise interferes with thermodiffusion experiments on Earth. The experimental results have also been used to test a thermodiffusion model that has been calibrated based on the results of previous experimental investigations. It was found that with an increase in the number of components in the mixtures, the performance of the thermodiffusion model deteriorated. Computational analysis was also made to estimate the possible sources of errors. Simulations showed that the vibrations of the spacecraft could influence the estimates of thermodiffusion factors. It was also found that they are sensitive to slight variations in the temperature of the mixture.

We discuss hybrid quantum-mechanics/molecular-mechanics (QM/MM) and quantum mechanics/quantum mechanics (QM/QM) generalizations to our recently developed quantum wavepacket ab initio molecular dynamics methodology for simultaneous dynamics of electrons and nuclei. The approach is a synergy between a quantum wavepacket dynamics, ab initio molecular dynamics, and the ONIOM scheme. We utilize this method to include nuclear quantum effects arising from a portion of the system along with a simultaneous description of the electronic structure. The generalizations provided here make the approach a potentially viable alternative for large systems. The quantum wavepacket dynamics is performed on a grid using a banded, sparse, and Toeplitz representation of the discrete free propagator, known as the "distributed approximating functional." Grid-based potential surfaces for wavepacket dynamics are constructed using an empirical valence bond generalization of ONIOM and further computational gains are achieved through the use of our recently introduced time-dependent deterministic sampling technique. The ab initio molecular dynamics is achieved using Born-Oppenheimer dynamics. All components of the methodology, namely, quantum dynamics and ONIOM molecular dynamics, are harnessed together using a time-dependent Hartree-like procedure. We benchmark the approach through the study of structural and vibrational properties of molecular, hydrogen bonded clusters inclusive of electronic, dynamical, temperature, and critical quantum nuclear effects. The vibrational properties are constructed through a velocity/flux correlation function formalism introduced by us in an earlier publication.

We have used finite temperature ab initio molecular dynamics simulations in conjunction with computation of critical quantum nuclear effects to probe the differences between single-photon argon tagged action spectral results and infrared multiple-photon dissociation experiments for a proton bound molecular ion system. We find that the principal difference between the results in these experimental techniques is essentially that of cluster temperature. The multiple-photon dissociation experiments conducted using room temperature ions reflect a larger degree of conformational freedom compared to the colder single-photon argon tagged action spectral results. Our ab initio molecular dynamics simulation techniques accurately capture the effects of conformational sampling, adequately reproduce both spectra, and can be utilized to assign the dynamically averaged finite temperature spectra.

Ion-water-cluster properties are investigated both through the multistate empirical valence bond potential and a polarizable model. Equilibrium properties of the ion-water clusters H+(H2O)100, Na+(H2O)100, Na+(H2O)20, and Cl-(H2O)17 in the temperature region 100-450 K are explored using a hybrid parallel basin-hopping and tempering algorithm. The effect of the solid-liquid phase transition in both caloric curves and structural distribution functions is investigated. It is found that sodium and chloride ions largely reside on the surface of water clusters below the cluster melting temperature but are solvated into the interior of the cluster above the melting temperature, while the solvated proton was found to have significant propensity to reside on or near the surface in both the liquid- and solid-state clusters.