A comparison of semi-empirical SCF–MO and ab initio energy surfaces for the Beckmann rearrangement

1989 ◽  
pp. 623-625 ◽  
Author(s):  
Peter A. Hunt ◽  
Henry S. Rzepa
Keyword(s):  
Ab Initio ◽  
Beckmann Rearrangement ◽  
Energy Surfaces ◽  
Semi Empirical

scholarly journals Dynamics of chemical reactions of astrophysical interest

1997 ◽  
Vol 178 ◽  
pp. 271-280 ◽  
Author(s):  
M. Alagia ◽  
N. Balucani ◽  
L. Cartechini ◽  
P. Casavecchia ◽  
G.G. Volpi
Keyword(s):  
Potential Energy ◽  
Ab Initio ◽  
Chemical Reactions ◽  
Potential Energy Surfaces ◽  
Experimental Results ◽  
Empirical Potential ◽  
Beam Scattering ◽  
Scattering Technique ◽  
Energy Surfaces ◽  
Semi Empirical

The dynamics of the astrophysically relevant reactions OH + H2,OH+CO,N(2D)+C2H2 and O(1D)+H2 are studied using the crossed beam scattering technique. Comparisons of the experimental results with those of dynamics calculations on ab initio and semi-empirical potential energy surfaces are discussed.

CALCULATION OF HO2 DENSITY OF STATES ON THREE POTENTIAL ENERGY SURFACES

2010 ◽  
Vol 09 (03) ◽  
pp. 653-665 ◽  
Author(s):  
H. ZHANG ◽  
S. C. SMITH
Keyword(s):  
Potential Energy ◽  
Ab Initio ◽  
Density Of States ◽  
Potential Energy Surfaces ◽  
Chem Phys ◽  
Many Body ◽  
Filter Diagonalization ◽  
Harmonic Density ◽  
Energy Surfaces ◽  
Semi Empirical

Density of states (DOS) in both bound and unimolecular dissociation regime for HO2 system have been calculated quantum mechanically by Lanczos homogeneous filter diagonalization (LHFD) method. Three potential energy surfaces are explored and the results are contrasted for the total angular momentum J = 0 density of states. While two ab initio potential energy surfaces (PESs) (TU PES, J Chem Phys, 115:3621 and XXZLG PES, J Chem Phys122:244) produce the DOSs which are in fairly good agreement, the semi-empirical double many-body expansion (DMBE) IV PES (J Phys Chem94:8073) generates the much higher DOSs in higher energy range. The quantum mechanical DOSs are also compared with Troe et al.'s results from harmonic density, semiclassical density and their early density of states on the same TU ab initio surface.

scholarly journals Efficient Computation of Free Energy Surfaces of Diels–Alder Reactions in Explicit Solvent at Ab Initio QM/MM Level

Molecules ◽  
2018 ◽  
Vol 23 (10) ◽  
pp. 2487 ◽  
Author(s):  
Pengfei Li ◽  
Fengjiao Liu ◽  
Xiangyu Jia ◽  
Yihan Shao ◽  
Wenxin Hu ◽  
...  
Keyword(s):  
Free Energy ◽  
Ab Initio ◽  
Umbrella Sampling ◽  
Diels Alder ◽  
Efficient Computation ◽  
Acceptance Ratio ◽  
Step Procedure ◽  
Explicit Solvent ◽  
Energy Surfaces ◽  
Semi Empirical

For Diels–Alder (DA) reactions in solution, an accurate and converged free energy (FE) surface at ab initio (ai) quantum mechanical/molecular mechanical (QM/MM) level is imperative for the understanding of reaction mechanism. However, this computation is still far too expensive. In a previous work, we proposed a new method termed MBAR+wTP, with which the computation of the ai FE profile can be accelerated by several orders of magnitude via a three-step procedure: (I) an umbrella sampling (US) using a semi-empirical (SE) QM/MM Hamiltonian is performed; (II) the FE profile is generated using the Multistate Bennett Acceptance Ratio (MBAR) analysis; and (III) a weighted Thermodynamic Perturbation (wTP) from the SE Hamiltonian to the ai Hamiltonian is performed to obtain the ai QM/MM FE profile using weight factors from the MBAR analysis. In this work, this method is extended to the calculations of two-dimensional FE surfaces of two Diels–Alder reactions of cyclopentadiene with either acrylonitrile or 1-4-naphthoquinone at ai QM/MM level. The accurate activation free energies at the ai QM/MM level, which are much closer to the experimental measurements than those calculated by other methods, indicate that this MBAR+wTP method can be applied in the studies of complex reactions in condensed phase with much-enhanced efficiency.

Active Learning Accelerates Ab Initio Molecular Dynamics on Pericyclic Reactive Energy Surfaces

2020 ◽  
Author(s):  
Shi Jun Ang ◽  
Wujie Wang ◽  
Daniel Schwalbe-Koda ◽  
Simon Axelrod ◽  
Rafael Gomez-Bombarelli
Keyword(s):  
Molecular Dynamics ◽  
Potential Energy ◽  
Transition State ◽  
Potential Energy Surface ◽  
Ab Initio ◽  
Energy Surface ◽  
Computational Cost ◽  
Reactive Trajectories ◽  
Dynamics Simulations ◽  
Energy Surfaces

<div>Modeling dynamical effects in chemical reactions, such as post-transition state bifurcation, requires <i>ab initio</i> molecular dynamics simulations due to the breakdown of simpler static models like transition state theory. However, these simulations tend to be restricted to lower-accuracy electronic structure methods and scarce sampling because of their high computational cost. Here, we report the use of statistical learning to accelerate reactive molecular dynamics simulations by combining high-throughput ab initio calculations, graph-convolution interatomic potentials and active learning. This pipeline was demonstrated on an ambimodal trispericyclic reaction involving 8,8-dicyanoheptafulvene and 6,6-dimethylfulvene. With a dataset size of approximately</div><div>31,000 M062X/def2-SVP quantum mechanical calculations, the computational cost of exploring the reactive potential energy surface was reduced by an order of magnitude. Thousands of virtually costless picosecond-long reactive trajectories suggest that post-transition state bifurcation plays a minor role for the reaction in vacuum. Furthermore, a transfer-learning strategy effectively upgraded the potential energy surface to higher</div><div>levels of theory ((SMD-)M06-2X/def2-TZVPD in vacuum and three other solvents, as well as the more accurate DLPNO-DSD-PBEP86 D3BJ/def2-TZVPD) using about 10% additional calculations for each surface. Since the larger basis set and the dynamic correlation capture intramolecular non-covalent interactions more accurately, they uncover longer lifetimes for the charge-separated intermediate on the more accurate potential energy surfaces. The character of the intermediate switches from entropic to thermodynamic upon including implicit solvation effects, with lifetimes increasing with solvent polarity. Analysis of 2,000 reactive trajectories on the chloroform PES shows a qualitative agreement with the experimentally-reported periselectivity for this reaction. This overall approach is broadly applicable and opens a door to the study of dynamical effects in larger, previously-intractable reactive systems.</div>

Relation between Magnetic, Spectroscopic and Structural Properties of Binuclear Copper(II) Complexes of Pentadentate Schiff-base Ligand, Semi-empirical and ab-initio Calculations

2003 ◽  
Vol 58 (5-6) ◽  
pp. 363-372 ◽  
Author(s):  
Y. Elerman ◽  
H. Kara ◽  
A. Elmali
Keyword(s):  
Ab Initio ◽  
Coupling Constants ◽  
Antiferromagnetic Coupling ◽  
X Ray Diffraction ◽  
Hartree Fock ◽  
Ligand Orbital ◽  
Rhf Calculations ◽  
The Difference ◽  
Semi Empirical

The synthesis and characterization of [Cu2(L1)(3,5 prz)] (L1=1,3-Bis(2-hydroxy-3,5-chlorosalicylideneamino) propan-2-ol) 1 and of [Cu2(L2)(3,5 prz)] (L2=1,3-Bis(2-hydroxy-bromosalicylideneamino) propan-2-ol) 2 are reported. The compounds were studied by elemental analysis, infrared and electronic spectra. The structure of the Cu2(L1)(3,5 prz)] complex was determined by x-ray diffraction. The magnetochemical characteristics of these compounds were determined by temperaturedependent magnetic susceptibility measurements, revealing their antiferromagnetic coupling. The superexchange coupling constants are 210 cm−1 for 1 and 440 cm−1 for 2. The difference in the magnitude of the coupling constants was explained by the metal-ligand orbital overlaps and confirmed by ab-initio restricted Hartree-Fock (RHF) calculations. In order to determine the nature of the frontier orbitals, Extended Hückel Molecular Orbital (EHMO) calculations are also reported.

scholarly journals Global ab initio potential energy surfaces for the O2(Σ3g−)+O2(Σ3g−) interaction

2010 ◽  
Vol 133 (12) ◽  
pp. 124311 ◽  
Author(s):  
Massimiliano Bartolomei ◽  
Estela Carmona-Novillo ◽  
Marta I. Hernández ◽  
José Campos-Martínez ◽  
Ramón Hernández-Lamoneda
Keyword(s):  
Potential Energy ◽  
Ab Initio ◽  
Potential Energy Surfaces ◽  
Energy Surfaces
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