1,721,083 research outputs found
Exploring unvisited regions to investigate solution properties: The backyard of H3O+ and its aggregates
No full textInteractions between bare and tri-solvated (Eigen) hydronium with water and methanol is investigated with ab initio methods. Unexplored configurations on the oxygen side of H3O+ are covered to improve our understanding of the solvation energetics. Results indicate that the interaction between the charged and polar species, with oxygen atoms oriented toward H3O+ oxygen (‘back to back’ or ‘O-side’ to ‘O-side’ interaction), is similar to or stronger than a hydrogen bond. The repulsion along the O–O directrix onset in the range 2.2–2.7 Å, indicating H2O short-range O–O contacts with (H3O+) as possible despite water coordination
Macroscopic evidences for non-Rice-Ramsperger-Kassel effects in the reaction between H3O+ and D2O: The occurrence of nonstatistical isotopic branching ratio
No full textThe dynamics of the isotopic scrambling in the energized and metastable complex D2O-H3O+ has been studied using classical molecular dynamics (MD) trajectories starting from regions of phase space corresponding to an already formed collisional complex. The simulations cover the range of internal energies spanned by gas phase collision experiments. Rate constants for the isotopic exchange and the complex dissociation have been computed; the isotopic branching ratio R=[HD2O+]/[H2DO+] has also been obtained from MD simulations and found to deviate substantially from an equivalent prediction based on a previously proposed kinetic scheme. This finding suggests the possibility that details of the reaction dynamics play a role in defining the isotopic branching ratio. The analysis of trajectory results indicated a relatively long lifetime for the collisional complex and the presence of multiple time scales for the exchange process, with a large fraction of the exchange events being separated only by a single oxygen-oxygen vibration or half of it. The occurrence of these fast consecutive jumps and their different probabilities as a function of the relative direction between first and second jumps suggest the presence of ballistic motion in the complex following each reactive event. This can be explained on the basis of overlapping regions in phase space and it is used to provide an explanation of the difference between kinetic and MD branching ratios. (c) 2007 American Institute of Physics
Discretization error-free estimate of low temperature statistical dissociation rates in gas phase: Applications to Lennard-Jones clusters X(13-n)Y(n) (n=0-3)
No full textIn this work, an improved approach for computing cluster dissociation rates using Monte Carlo (MC) simulations is proposed and a discussion is provided on its applicability as a function of environmental variables (e.g., temperature). With an analytical transformation of the integrals required to compute variational transition state theory (vTST) dissociation rates, MC estimates of the expectation value for the Dirac delta delta(q(rc)-q(c)) have been made free of the discretization error that is present when a prelimit form for delta is used. As a by-product of this transformation, the statistical error associated with is reduced making this step in the calculation of vTST rates substantially more efficient (by a factor of 4-2500, roughly). The improved MC procedure is subsequently employed to compute the dissociation rate for Lennard-Jones clusters X(13-n)Y(n) (n=0-3) as a function of temperature (T), composition, and X-Y interaction strength. The X(13-n)Y(n) family has been previously studied as prototypical set of systems for which it may be possible to select and stabilize structures different from the icosahedral global minimum of X(13). It was found that both the dissociation rate and the dissociation mechanism, as suggested by the statistical simulations, present a marked dependence on n, T, and the nature of Y. In particular, it was found that a vacancy is preferentially formed close to a surface impurity when the X-Y interaction is weaker than the X-X one whatever the temperature. Differently, the mechanism was found to depend on T for stronger X-Y interactions, with vacancies being formed opposite to surface impurities at higher temperature. These behaviors are a reflex of the important role played by the surface fluctuations in defining the properties of clusters. (c) 2008 American Institute of Physics
Higher order diffusion Monte Carlo propagators for linear rotors as diffusion on a sphere: Development and application to O(2)@He(n)
No full textExploiting the theoretical treatment of particles diffusing on corrugated surfaces and the isomorphism between the "particle on a sphere" and a linear molecule rotation, a new diffusion kernel is introduced to increase the order of diffusion Monte Carlo (DMC) simulations involving linear rotors. Tests carried out on model systems indicate the superior performances of the new rotational diffusion kernel with respect to the simpler alternatives previously employed. In particular, it is evidenced a second order convergence toward exact results with respect to the time step of dynamical correlation functions, a fact that guarantees an identical order for the diffusion part of the DMC projector. The algorithmic advantages afforded by the latter are discussed, especially with respect to the "a posteriori" and "on the fly" extrapolation schemes. As a first application to the new algorithm, the structure and energetics of O(2)@He(n) (n = 1-40) clusters have been studied. This was done to investigate the possible cause of the quenching of the reaction between O(2) and Mg witnessed upon increasing the size of superfluid He droplets used as a solvent. With the simulations on O(2) indicating a strong localization in the cluster core, the behaviour as a function of n is ascribed to the extremely fluxional comportment of Mg@He(n), which dwells far from the droplet center, albeit being solvated, when n is large. (C) 2011 American Institute of Physics. [doi:10.1063/1.3639190
Effect of the cluster angular momentum J and the projectile orbital momentum L on capture probability and postcollision dynamics
Full text linkIn this work, collisions between rotating atomic clusters composed of Lennard-Jones (LJ(n)) particles and an identical projectile have been investigated by means of trajectory simulations as a function of the cluster angular momentum J and internal energy E, and for different values of the projectile impact parameter b and relative velocity v(p). As expected, the collision (P(c)(b)) and capture [or sticking P(s)(b)] probabilities are found to decay below unity for values of b larger than the average surface radius of the cluster, with dP/db being strongly dependent on v(p). Both P(c)(b) and P(s)(b), however, appear to be largely insensitive to the modulus of the cluster angular momentum vertical bar J vertical bar and only weakly dependent on E for collisions involving target clusters with a lifetime tau > 100 ps. The latter findings are interpreted as indicating the absence of strong changes in the structure of the target as a function of vertical bar J vertical bar and E. The comparison between the dissociation lifetime (tau(dyn)) of the postcapture complexes (LJ*(n+1))obtained continuing trajectories after monomer capture and the one computed from the fragmentation of statistically prepared clusters (tau(stat)) supports the validity of a two-step capture-dissociation model; similarly, the comparison between the average amount of energy exchanged during trajectories (Delta E(dyn)) in the process LJ(n) + LJ --> LJ*(n+ 1) --> LJ(n) + LJ and the one predicted by statistical simulations (Delta E(stat)) suggests a fast statistical energy redistribution in the collisional complex even for very short tau(dyn) (e.g., 40 ps). In the case of projectiles aimed at the edge of the cluster [(grazing collisions, P(c)(b) < 1]; however, the time elapsed between formal collision and dissociation, tau(coll), is such that tau(coll) < tau(stat) and the trajectories indicate the presence of ballistic dynamics and of a weak energy exchange (Delta E(coll) < Delta E(dyn), with Delta E(coll) being the average energy exchanged during collisions). The relevance of these results to the study of gas phase nucleation and to the possibility of building a fully microcanonical framework for its description is discussed. (C) 2009 American Institute of Physics. [doi:10.1063/1.3239476
On possible simplifications in the theoretical description of gas phase atomic cluster dissociation
No full textIn this work, we investigate the possibility of describing gas phase atomic cluster dissociation by means of variational transition state theory (vTST) in the microcanonical ensemble. A particular emphasis is placed on benchmarking the accuracy of vTST in predicting the dissociation rate and kinetic energy release of a fragmentation event as a function of the cluster size and internal energy. The results for three Lennard-Jones clusters (LJ(n), n=8,14,19) indicate that variational transition state theory is capable of providing results of accuracy comparable to molecular dynamics simulations at a reduced computational cost. Possible simplifications of the master equation formalism used to model a dissociation cascade are also suggested starting from molecular dynamics results. In particular, it is found that the dissociation rate is only weakly dependent on the cluster total angular momentum J for the three cluster sizes considered. This would allow one to partially neglect the J-dependency of the kinetic coefficients, leading to a substantial decrease in the computational effort needed for the complete description of the cascade process. The impact of this investigation on the modeling of the nucleation process is discussed
Improved importance sampling distribution for rate constant calculation
Full text linkAn efficient method to compute the thermal rate constant for rare events within the correlation function C(t) approach is presented. This method, which is based on a modification of the sampling function used to evaluate the dynamical correlation function C(t), can be applied to high-dimensional systems having a rough energy landscape without previous knowledge on the transition states location. In this work, the sampling of a Boltzmann-like distribution for the linear momenta with a lower inverse temperature (beta*= 1/kT*) than the correct one (beta=1/kT) is proposed as a way to increase the number of reactive trajectories. The mismatch between the beta* and beta distributions is then corrected by a reweighting procedure which allows one to obtain the exact correlation function C(t). The efficiency of this scheme in computing the rate of a particle jumping across the barrier of a simple 2D double well potential is improved by a factor 4-25 depending on the relative value of the original b and modified b* temperatures. When merged with the "puddle potential" method [S. A. Corcelli, J. A. Rohman, and J. C. Tully, J. Chem. Phys., 118, 1085 (2003)], the new importance sampling function improves the efficiency of the correlation function approach by a factor 16-800 with respect to the unbiased sampling. To test the method in a more challenging case, the previous model system was extended by adding six harmonically restrained particles, each one interacting with the diffusing particle. This model introduces both the possibility of energy exchange and a rougher energy landscape. The new sampling function alone is found to produce an improvement in efficiency of, at least, an order of magnitude when compared with the unbiased case; when merged with the puddle potential method, a 400-fold saving in computer time is found. (c) 2005 American Institute of Physics
Efficient calculation of low energy statistical rates for gas phase dissociation using umbrella sampling
No full textMonte Carlo (MC) simulations can be used to compute microcanonical statistical rates of gas phase dissociation reactions. Unfortunately, the MC approach may suffer from a slow convergence and large statistical errors for energies just above the dissociation threshold. In this work, umbrella sampling is proposed as a device to reduce the statistical error of MC rate constants. The method is tested by computing the classical dissociation rate for the reaction [H5O2+](*)-> H2O+H3O+ over the range of internal energy 38 < E <= 100 kcal/mol. Comparing with other literature methods, it is found that umbrella sampling reduces the computational effort by up to two orders of magnitude when used in conjunction with a careful choice of sampling distributions. The comparison between MC rate constants and classical Rice-Ramsperberg-Kassel harmonic theory shows that anharmonicity plays an important role in the dissociation process of the Zundel cation (H5O2+) at all energies. (c) 2006 American Institute of Physics
Toward tailoring metallocene structure for the control of comonomer reactivity in ethylene-styrene copolymerization
No full textThree-fragment counterpoise correction of potential energy curves for proton-transfer reactions
No full textWe present a thorough study of the effect of basis set choice and of the three-fragment counterpoise correction for the basis set superposition error on the shape of proton-exchange energy surfaces. This has been investigated by employing the correlated MP2 method and basis sets from cc-pVDZ to aug-cc-pVTZ quality. To understand the effect of the correction and the overall accuracy of the different atomic basis sets, and to discover the best-compromise basis set for large surface scans, we computed the shape difference function between corrected and uncorrected results for the HF2-, H3O2-, H5O2+, N2H5-, and N2H7+ systems. Our results show this function to strongly depend on the system, although larger corrections are consistently observed when the more basic fragments (NH2- and OH-) are involved. Suggestions on which basis set could be used for potential energy surface scans are also given
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