The Experts below are selected from a list of 237 Experts worldwide ranked by ideXlab platform
Lionel M. Raff - One of the best experts on this subject based on the ideXlab platform.
-
theoretical investigation of the dissociation dynamics of vibrationally excited Vinyl Bromide on an ab initio potential energy surface obtained using modified novelty sampling and feedforward neural networks ii numerical application of the method
Journal of Chemical Physics, 2007Co-Authors: M Malshe, Lionel M. Raff, M. G. Rockley, Martin T. Hagan, Paras M. Agrawal, R. KomanduriAbstract:A previously reported method for conducting molecular dynamics simulations of gas-phase chemical dynamics on ab initio potential-energy surfaces using modified novelty sampling and feedforward neural networks is applied to the investigation of the unimolecular dissociation of Vinyl Bromide. The neural network is fitted to a database comprising the MP4(SDQ) energies computed for 71 969 nuclear configurations using an extended basis set. Dissociation rate coefficients and branching ratios at an internal excitation energy of 6.44eV for all six open reaction channels are reported. The distribution of vibrational energy in HBr formed in three-center dissociation is computed and found to be in excellent accord with experimental measurements. Computational requirements for the electronic structure calculations, neural network training, and trajectory calculations are given. The weight and bias matrices required for implementation of the neural network potential are made available through the Supplementary Material.
-
Theoretical investigation of the dissociation dynamics of vibrationally excited Vinyl Bromide on an ab initio potential-energy surface obtained using modified novelty sampling and feed-forward neural networks.
The Journal of chemical physics, 2006Co-Authors: D. I. Doughan, Lionel M. Raff, M. G. Rockley, Martin T. Hagan, Paras M. Agrawal, R. KomanduriAbstract:The reaction dynamics of vibrationally excited Vinyl Bromide have been investigated using classical trajectory methods on a neural network potential surface that is fitted to an ab initio database of 12 122 configuration energies obtained from electronic structure calculations conducted at the MP4(SDQ) level of theory using a 6-31G(d,p) basis set for the carbon and hydrogen atoms and Huzinaga's (43334334) basis set augmented with split outer s and p orbitals (4332143214) and a polarization f orbital with an exponent of 0.5 for the bromine atom. The sampling of the 12-dimensional configuration hyperspace of Vinyl Bromide prior to execution of the electronic structure calculations is accomplished by combining novelty-sampling methods, chemical intuition, and trajectory sampling on empirical and neural network surfaces. The final potential is obtained using a two-layer feed-forward neural network comprising 38 and 1 neurons, respectively, with hyperbolic tangent sigmoid and linear transfer functions in the hidden and output layers, respectively. The fitting is accomplished using the Levenberg-Marquardt algorithm with early stopping and Bayesian regularization methods to avoid overfitting. The interpolated potentials have a standard deviation from the ab initio results of 0.0578 eV, which is within the range generally regarded as "chemical accuracy" for the purposes of electronic structure calculations. It is shown that the potential surface may be easily and conveniently transferred from one research group to another. The files required for transfer of the Vinyl Bromide surface can be obtained from the Electronic Physics Auxiliary Publication Service. Total dissociation rate coefficients for Vinyl Bromide are obtained at five different excitation energies between 4.50 and 6.44 eV. Branching ratios into each of the six open reaction channels are computed at 24 vibrational energies in the range between 4.00 and 6.44 eV. The distribution of vibrational energies in HBr formed via three-center dissociation from Vinyl Bromide is determined and compared with previous theoretical and experimental results. It is concluded that the combination of ab initio electronic structure calculations, novelty sampling with chemical intuition and trajectories on empirical analytic surfaces, and feed-forward neural networks provides a viable framework in which to execute purely ab initio molecular-dynamics studies on complex systems with multiple open reaction channels.
-
trajectory investigations of the dissociation dynamics of Vinyl Bromide on an ab initio potential energy surface
Journal of Physical Chemistry A, 2001Co-Authors: Asif Rahaman, Lionel M. RaffAbstract:The reaction dynamics of vibrationally excited Vinyl Bromide have been investigated using classical trajectory methods on a global, analytic potential-energy hypersurface that is developed primaril...
-
theoretical investigations of intramolecular energy transfer rates and pathways for Vinyl Bromide on an ab initio potential energy surface
Journal of Physical Chemistry A, 2001Co-Authors: Asif Rahaman, Lionel M. RaffAbstract:The dynamics of intramolecular energy transfer in Vinyl Bromide have been investigated using projection methods and results obtained from classical trajectories computed on a global potential-energy surface (PES1) that has been fitted to a large database of energies obtained from ab initio electronic structure calculations at the MP4 level of theory with extended basis sets and experimental thermochemical data. Similar calculations on two additional potential surfaces derived from PES1 are also reported. The intramolecular energy transfer dynamics obtained on these surfaces are compared with one another and with previously reported results using an empirical hybrid-type surface (EPS) fitted to thermochemical, structural, kinetic, and spectroscopic data and the results of a limited number of electronic structure calculations for the transition states. The temporal variation of the average vibrational mode energies are computed from the projected mode kinetic energy profiles using the equipartition theorem....
-
Intramolecular energy transfer rates for Vinyl Bromide and deuterium-substituted Vinyl Bromides from power spectrum line splittings
The Journal of Chemical Physics, 1997Co-Authors: Ran Pan, Lionel M. RaffAbstract:Continuous frequency modulated (CFM) line splittings are used to determine the energy transfer rate coefficients for the local C–Br and C=C vibrational modes in Vinyl Bromide and the C–H stretching modes in doubly deuterium-substituted Vinyl Bromides. The global potential developed by Abrash et al. is employed in all calculations. Energy transfer rate coefficients are extracted from the fine structure spacing of the numerically computed power spectrum of the bond coordinates. The consistency of the averaged individual rate coefficients is evaluated by comparison with results obtained from local mode energy decay curves. It is found that the total intramolecular vibrational relaxation (IVR) rate coefficients for all modes investigated are large relative to the unimolecular decomposition rate. However, previous studies show that IVR is not globally rapid so statistical behavior of the unimolecular reaction is not expected. It is shown that near overlapping resonances in the power spectrum make it difficult ...
Myung Soo Kim - One of the best experts on this subject based on the ideXlab platform.
-
Photodissociation yield spectroscopy of Vinyl Bromide cation generated by mass-analyzed threshold ionization: vibrational spectroscopy and decay dynamics in the (~)B state.
The Journal of chemical physics, 2007Co-Authors: Mina Lee, Myung Soo KimAbstract:A new technique [mass-analyzed threshold ionization (MATI)-photodissociation yield spectroscopy] to probe bound excited states of a cation was developed, which measures photodissociation yield of the cation generated by mass-analyzed threshold ionization. A vibrational spectrum of Vinyl Bromide cation in the (~)B state was obtained using this technique. Optical resolution in the low vibrational energy range of the spectrum was far better than in conventional MATI spectra. The origin of the (~)B state was found at 2.2578+/-0.0003 eV above the first ionization onset. Almost complete vibrational assignment was possible for peaks appearing in the spectrum. Analysis of time-of-flight profiles of C(2)H(3) (+) product ion obtained with different laser polarization angles suggested that photoexcited Vinyl Bromide cation remained in the (~)B state for several hundred picoseconds prior to internal conversion to the ground state and dissociation therein.
-
mass analyzed threshold ionization study of Vinyl Bromide cation in the first excited electronic state using vacuum ultraviolet radiation generated by four wave mixing in hg
Journal of Chemical Physics, 2005Co-Authors: Mina Lee, Myung Soo KimAbstract:The vibrational spectrum of the Vinyl Bromide cation in the first excited electronic state AA′2 was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. The use of an improved vacuum-ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for MATI signals. From the MATI spectrum, the ionization energy to the AA′2 state of the cation was determined to be 10.9150±0.0006eV. Nearly complete vibrational assignments for the MATI peaks were possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the density-functional theory (DFT) and time-dependent DFT/B3LYP levels with the 6-311+G(df,p) basis set.
-
vacuum ultraviolet mass analyzed threshold ionization spectroscopy of Vinyl Bromide franck condon analysis and vibrational assignment
Journal of Chemical Physics, 2003Co-Authors: Mina Lee, Myung Soo KimAbstract:Vibrational spectrum of Vinyl Bromide cation in the ground electronic state was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. From MATI spectrum, ionization energy to the ground state of the cation was determined to be 9.8171±0.0006 eV (79 180±5 cm−1). Almost complete vibrational assignments for the peaks in the MATI spectrum were possible by utilizing vibrational frequencies and Franck–Condon factors calculated at the Becke three parameter Lee–Yang–Parr (B3LYP)/6-311++G(df,pd) level. Franck–Condon analysis for one-photon MATI spectra is especially useful because calculations of only the ground electronic states are involved while that for two-photon MATI spectra requires excited state calculations.
R. Komanduri - One of the best experts on this subject based on the ideXlab platform.
-
theoretical investigation of the dissociation dynamics of vibrationally excited Vinyl Bromide on an ab initio potential energy surface obtained using modified novelty sampling and feedforward neural networks ii numerical application of the method
Journal of Chemical Physics, 2007Co-Authors: M Malshe, Lionel M. Raff, M. G. Rockley, Martin T. Hagan, Paras M. Agrawal, R. KomanduriAbstract:A previously reported method for conducting molecular dynamics simulations of gas-phase chemical dynamics on ab initio potential-energy surfaces using modified novelty sampling and feedforward neural networks is applied to the investigation of the unimolecular dissociation of Vinyl Bromide. The neural network is fitted to a database comprising the MP4(SDQ) energies computed for 71 969 nuclear configurations using an extended basis set. Dissociation rate coefficients and branching ratios at an internal excitation energy of 6.44eV for all six open reaction channels are reported. The distribution of vibrational energy in HBr formed in three-center dissociation is computed and found to be in excellent accord with experimental measurements. Computational requirements for the electronic structure calculations, neural network training, and trajectory calculations are given. The weight and bias matrices required for implementation of the neural network potential are made available through the Supplementary Material.
-
Theoretical investigation of the dissociation dynamics of vibrationally excited Vinyl Bromide on an ab initio potential-energy surface obtained using modified novelty sampling and feed-forward neural networks.
The Journal of chemical physics, 2006Co-Authors: D. I. Doughan, Lionel M. Raff, M. G. Rockley, Martin T. Hagan, Paras M. Agrawal, R. KomanduriAbstract:The reaction dynamics of vibrationally excited Vinyl Bromide have been investigated using classical trajectory methods on a neural network potential surface that is fitted to an ab initio database of 12 122 configuration energies obtained from electronic structure calculations conducted at the MP4(SDQ) level of theory using a 6-31G(d,p) basis set for the carbon and hydrogen atoms and Huzinaga's (43334334) basis set augmented with split outer s and p orbitals (4332143214) and a polarization f orbital with an exponent of 0.5 for the bromine atom. The sampling of the 12-dimensional configuration hyperspace of Vinyl Bromide prior to execution of the electronic structure calculations is accomplished by combining novelty-sampling methods, chemical intuition, and trajectory sampling on empirical and neural network surfaces. The final potential is obtained using a two-layer feed-forward neural network comprising 38 and 1 neurons, respectively, with hyperbolic tangent sigmoid and linear transfer functions in the hidden and output layers, respectively. The fitting is accomplished using the Levenberg-Marquardt algorithm with early stopping and Bayesian regularization methods to avoid overfitting. The interpolated potentials have a standard deviation from the ab initio results of 0.0578 eV, which is within the range generally regarded as "chemical accuracy" for the purposes of electronic structure calculations. It is shown that the potential surface may be easily and conveniently transferred from one research group to another. The files required for transfer of the Vinyl Bromide surface can be obtained from the Electronic Physics Auxiliary Publication Service. Total dissociation rate coefficients for Vinyl Bromide are obtained at five different excitation energies between 4.50 and 6.44 eV. Branching ratios into each of the six open reaction channels are computed at 24 vibrational energies in the range between 4.00 and 6.44 eV. The distribution of vibrational energies in HBr formed via three-center dissociation from Vinyl Bromide is determined and compared with previous theoretical and experimental results. It is concluded that the combination of ab initio electronic structure calculations, novelty sampling with chemical intuition and trajectories on empirical analytic surfaces, and feed-forward neural networks provides a viable framework in which to execute purely ab initio molecular-dynamics studies on complex systems with multiple open reaction channels.
Mina Lee - One of the best experts on this subject based on the ideXlab platform.
-
Photodissociation yield spectroscopy of Vinyl Bromide cation generated by mass-analyzed threshold ionization: vibrational spectroscopy and decay dynamics in the (~)B state.
The Journal of chemical physics, 2007Co-Authors: Mina Lee, Myung Soo KimAbstract:A new technique [mass-analyzed threshold ionization (MATI)-photodissociation yield spectroscopy] to probe bound excited states of a cation was developed, which measures photodissociation yield of the cation generated by mass-analyzed threshold ionization. A vibrational spectrum of Vinyl Bromide cation in the (~)B state was obtained using this technique. Optical resolution in the low vibrational energy range of the spectrum was far better than in conventional MATI spectra. The origin of the (~)B state was found at 2.2578+/-0.0003 eV above the first ionization onset. Almost complete vibrational assignment was possible for peaks appearing in the spectrum. Analysis of time-of-flight profiles of C(2)H(3) (+) product ion obtained with different laser polarization angles suggested that photoexcited Vinyl Bromide cation remained in the (~)B state for several hundred picoseconds prior to internal conversion to the ground state and dissociation therein.
-
mass analyzed threshold ionization study of Vinyl Bromide cation in the first excited electronic state using vacuum ultraviolet radiation generated by four wave mixing in hg
Journal of Chemical Physics, 2005Co-Authors: Mina Lee, Myung Soo KimAbstract:The vibrational spectrum of the Vinyl Bromide cation in the first excited electronic state AA′2 was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. The use of an improved vacuum-ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for MATI signals. From the MATI spectrum, the ionization energy to the AA′2 state of the cation was determined to be 10.9150±0.0006eV. Nearly complete vibrational assignments for the MATI peaks were possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the density-functional theory (DFT) and time-dependent DFT/B3LYP levels with the 6-311+G(df,p) basis set.
-
vacuum ultraviolet mass analyzed threshold ionization spectroscopy of Vinyl Bromide franck condon analysis and vibrational assignment
Journal of Chemical Physics, 2003Co-Authors: Mina Lee, Myung Soo KimAbstract:Vibrational spectrum of Vinyl Bromide cation in the ground electronic state was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. From MATI spectrum, ionization energy to the ground state of the cation was determined to be 9.8171±0.0006 eV (79 180±5 cm−1). Almost complete vibrational assignments for the peaks in the MATI spectrum were possible by utilizing vibrational frequencies and Franck–Condon factors calculated at the Becke three parameter Lee–Yang–Parr (B3LYP)/6-311++G(df,pd) level. Franck–Condon analysis for one-photon MATI spectra is especially useful because calculations of only the ground electronic states are involved while that for two-photon MATI spectra requires excited state calculations.
Tucker Carrington - One of the best experts on this subject based on the ideXlab platform.
-
Using neural networks, optimized coordinates, and high-dimensional model representations to obtain a Vinyl Bromide potential surface
Journal of Chemical Physics, 2008Co-Authors: Sergei Manzhos, Tucker CarringtonAbstract:We demonstrate that it is possible to obtain good potentials using high-dimensional model representations (HDMRs) fitted with neural networks (NNs) from data in 12 dimensions and 15 dimensions. The HDMR represents the potential as a sum of lower-dimensional functions and our NN-based approach makes it possible to obtain all of these functions from one set of fitting points. To reduce the number of terms in the HDMR, we use optimized redundant coordinates. By using exponential neurons, one obtains a potential in sum-of-products form, which greatly facilitates quantum dynamics calculations. A 12-dimensional (reference) potential surface for Vinyl Bromide is first refitted to show that it can be represented as a sum of two-dimensional functions. To fit 3d functions of the original coordinates, to improve the potential, a huge amount of data would be required. Redundant coordinates avoid this problem. They enable us to bypass the combinatorial explosion of the number of terms which plagues all HDMR and multim...
-
Using neural networks, optimized coordinates, and high-dimensional model representations to obtain a Vinyl Bromide potential surface.
The Journal of chemical physics, 2008Co-Authors: Sergei Manzhos, Tucker CarringtonAbstract:We demonstrate that it is possible to obtain good potentials using high-dimensional model representations (HDMRs) fitted with neural networks (NNs) from data in 12 dimensions and 15 dimensions. The HDMR represents the potential as a sum of lower-dimensional functions and our NN-based approach makes it possible to obtain all of these functions from one set of fitting points. To reduce the number of terms in the HDMR, we use optimized redundant coordinates. By using exponential neurons, one obtains a potential in sum-of-products form, which greatly facilitates quantum dynamics calculations. A 12-dimensional (reference) potential surface for Vinyl Bromide is first refitted to show that it can be represented as a sum of two-dimensional functions. To fit 3d functions of the original coordinates, to improve the potential, a huge amount of data would be required. Redundant coordinates avoid this problem. They enable us to bypass the combinatorial explosion of the number of terms which plagues all HDMR and multimode-type methods. We also fit to a set of approximately 70,000 ab initio points for Vinyl Bromide in 15 dimensions [M. Malshe et al., J. Chem. Phys. 127, 134105 (2007)] and show that it is possible to obtain a surface in sum-of-products form of quality similar to the quality of the full-dimensional fit. Although we obtain a full-dimensional surface, we limit the cost of the fitting by building it from fits of six-dimensional functions, each of which requires only a small NN.