Investigation of the Structure and Spectroscopy of H 5 + Using Diffusion Monte Carlo (original) (raw)
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The Journal of Chemical Physics, 2008
Diffusion Monte Carlo computations, with and without importance sampling, of the zero-point properties of H 5 + and its isotopomers using a recent high accuracy global potential energy surface are presented. The global minimum of the potential possesses C 2v symmetry, but the calculations predict a D 2d geometry for zero-point averaged structure of H 5 + with one H atom "in the middle" between two HH diatoms. The predicted zero-point geometries of the deuterated forms have H in the middle preferred over D in the middle and for a nonsymmetric arrangement of D atoms the preferred arrangement is one which maximizes the number of D as the triatomic ion. We speculate on the consequences of these preferences in scattering of H 2 +H 3 + and isotopomers at low energies, such as those in the interstellar medium.
The Journal of Chemical Physics, 2010
In this work a reliable full nine-dimensional potential energy surface for studying the dynamics of H 5 + is constructed, which is completely symmetric under any permutation of the nuclei. For this purpose, we develop a triatoms-in-molecules method as an extension of the more common diatoms-in-molecules one, which allows a very accurate description of the asymptotic regions by including correctly the charge-induced dipole and quadrupole interactions. Moreover, this treatment provides a semiquantitative description of all the topological features of the global potential compared with coupled cluster results. In particular, the hop of the proton between two H 2 fragments produces a double well in the potential. This resonant structure involving the five atoms produces a stabilization, lowering the barrier, and the triatoms-in-molecules yields to a barrier significantly higher than the ab initio results. Therefore, to improve the triatomics-in-molecules potential surface, two five-body terms are added, which are fitted to more than 110 000 coupled-cluster ab initio points. The global potential energy surface thus obtained in this work has an overall root mean square error of 0.079 kcal/mol for energies below 27 kcal/mol above the global well. The features of the potential are described and compared with previous available surfaces.
Vibrational spectrum of the H5+ molecule using quantum Monte Carlo
2006
In this article we present a caracterization of the vibrational spectrum of the H5+ molecule using the correlation function quantum Monte Carlo (CFQMC) method and a genetic algorithm study of the topology of the potential energy surface used in this work. The vibrational modes associated with the H3+ - H2 torsion and stretching posses very flat minima. As a consequence the fundamental frequencies corresponding to these modes are poorly described in the harmonic approximation. The vibrational frequencies obtained in this work are in good agreement with the available experimental data as well as other computational methods found in literature. In our genetic algorithm study of the potential energy surface using cartesian coordinates we have found some unexpected minima. A careful analysis shows that some of these minima are described by the same curviliniar coordinates in which the potential is described. However, they represent nonequivalent molecular geometries.
Ja n 20 06 Vibrational spectrum of the H + 5 molecule using quantum Monte Carlo
2008
In this article we present a caracterization of the vibrational spectrum of the H+5 molecule using the correlation function quantum Monte Carlo (CFQMC) method and a genetic algorithm study of the topology of the potential energy surface used in this work. The vibrational modes associated with the H+3 -H2 torsion and stretching posses very flat minima. As a consequence the fundamental frequencies corresponding to these modes are poorly described in the harmonic approximation. The vibrational frequencies obtained in this work are in good agreement with the available experimental data as well as other computational methods found in literature. In our genetic algorithm study of the potential energy surface using cartesian coordinates we have found some unexpected minima. A careful analysis shows that some of these minima are described by the same curviliniar coordinates in which the potential is described. However, they represent nonequivalent molecular geometries.
Signatures of Large-Amplitude Vibrations in the Spectra of H5+ and D5+
The Journal of Physical Chemistry Letters, 2012
H5+ is a weakly bound molecular ion, which is formed from the reaction of H3+ and H2 and that has a very rich vibrational spectrum. In this work, diffusion Monte Carlo (DMC) approaches are used to explore the nature of vibrationally excited states of the proton-transfer mode in H5+. On the basis of these calculations, alternative assignments of the recently reported infrared multiphoton dissociation spectra of H5+ and D5+ [ J. Phys. Chem. Lett. 2012, 3, 3160−3166] are suggested. In the proposed assignments, progressions of transitions in the proton-transfer mode with up to nine quanta of excitation are invoked. Reduced dimensional calculations of the spectra of H5+ and D5+ are used to provide an understanding of why such high overtones should be observable through absorption spectroscopy. Implications of how excitations of this mode can provide insights into the H3+ + H2 reaction are also discussed.
Simulation of the infrared predissociation spectra of H_{5}^{+}
Physical Review A, 2012
ABSTRACT A quantum study of the bound states and infrared predissociation spectra of H5 + is done using a recently proposed global and accurate potential-energy surface [ Aguado et al. J. Chem. Phys. 133 024306 (2010)]. The bound states are calculated for seven degrees of freedom using an iterative Lanczos method, yielding a dissociation energy in very good agreement with the available experimental data. The predissociation states are described by a wave-packet treatment considering a shared-proton model, in which the three-dimensional motion of the central atom is described using non-Jacobi bond coordinates. The justification of this model is that the change in the electric dipole moment is larger as a function of the motion of the central atom, responsible for the proton transfer in the H3 ++H2→H2+H3 + reaction. The electric dipole moment is calculated at the level of coupled-cluster theory with single and double excitations and fitted in nine dimensions to an analytical function. With it, the infrared predissociation spectrum is simulated, yielding a reasonable agreement with recent measurements.
The Journal of Chemical Physics, 2005
We report quantum diffusion Monte Carlo ͑DMC͒ and variational calculations in full dimensionality for selected vibrational states of H 5 O 2 + using a new ab initio potential energy surface ͓X. Huang, B. Braams, and J. M. Bowman, J. Chem. Phys. 122, 044308 ͑2005͔͒. The energy and properties of the zero-point state are focused on in the rigorous DMC calculations. OH-stretch fundamentals are also calculated using "fixed-node" DMC calculations and variationally using two versions of the code MULTIMODE. These results are compared with infrared multiphoton dissociation measurements of Yeh et al. ͓L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 ͑1989͔͒. Some preliminary results for the energies of several modes of the shared hydrogen are also reported.
The Journal of Chemical Physics, 2010
The potential energy surface of H 5 + is characterized using density functional theory. The hypersurface is evaluated at selected configurations employing different functionals, and compared with results obtained from ab initio CCSD͑T͒ calculations. The lowest ten stationary points ͑minima and saddle-points͒ on the surface are located, and the features of the short-, intermediate-, and long-range intermolecular interactions are also investigated. A detailed analysis of the surface's topology, and comparisons with extensive CCSD͑T͒ results, as well as a recent ab initio analytical surface, shows that density functional theory calculations using the B3͑H͒ functional represent very well all aspects studied on the H 5 + potential. These include the tiny energy difference between the minimum at 1-C 2v configuration and the 2-D 2d one corresponding to the transition state for the proton transfer between the two equivalent C 2v minima, and also the correct asymptotic behavior of the long-range interactions. The calculated binding energy and dissociation enthalpies compare very well with previous benchmark coupled-cluster ab initio data, and with experimental data available. Based on these results the use of such approach to perform first-principles molecular dynamics simulations could provide reliable information regarding the dynamics of protonated hydrogen clusters.
Ab initio potential energy and dipole moment surfaces for H5O2 +
The Journal of chemical physics, 2005
Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H(5)O(2) (+). Tens of thousands of coupled-cluster [CCSD(T)] and second-order Moller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H(5)O(2) (+) PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm(-1) for the entire data set up to 110 000 cm(-1). This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and sy...
The role of large-amplitude motions in the spectroscopy and dynamics of H5+
The Journal of Chemical Physics, 2014
Protonated hydrogen dimer, H₅⁺, is the intermediate in the astrochemically important proton transfer reaction between H₃⁺ and H2. To understand the mechanism for this process, we focus on how large amplitude motions in H₅⁺ result in scrambling of the five hydrogen atoms in the collision complex. To this end, the one-dimensional zero-point corrected potential surfaces were mapped out as functions of reaction coordinates for the H₃⁺ + H2 collision using minimized energy path diffusion Monte Carlo [C. E. Hinkle and A. B. McCoy, J. Phys. Chem. Lett. 1, 562 (2010)]. In this study, the previously developed approach was extended to allow for the investigation of selected excited states that are expected to be involved in the proton scrambling dynamics. Specifically, excited states in the shared proton motion between the two H2 groups, and in the outer H2 bending motions were investigated. Of particular interest is the minimum distance between H₃⁺ and H2 at which all five hydrogen atoms become free to exchange. In addition, this diffusion Monte Carlo-based approach was used to determine the zero-point energy E0, the dissociation energy D0, and excitation energies associated with the vibrational motions that were investigated. The evolution of the wave functions was also studied, with a focus on how the intramolecular vibrations in H₅⁺ evolve into motions of H₃⁺ or H2. In the case of the proton scrambling, we find that the relevant transition states become fully accessible at separations between H₃⁺ and H2 of approximately 2.15 Å, a distance that is accessed by the excited states of H₅⁺ with two or more quanta in the shared proton stretch. The implications of this finding on the vibrational spectroscopy of H₅⁺ are also discussed.