208 research outputs found
Direct Dynamics Simulations Using Hessian-Based Predictor-Corrector Integration Algorithms
In previous research [J. Chem. Phys.111, 3800 (1999)] a Hessian-based integration algorithm was derived for performing direct dynamics simulations. In the work presented here, improvements to this algorithm are described. The algorithm has a predictor step based on a local second-order Taylor expansion of the potential in Cartesian coordinates, within a trust radius, and a fifth-order correction to this predicted trajectory. The current algorithm determines the predicted trajectory in Cartesian coordinates, instead of the instantaneous normal mode coordinates used previously, to ensure angular momentumconservation. For the previous algorithm the corrected step was evaluated in rotated Cartesian coordinates. Since the local potential expanded in Cartesian coordinates is not invariant to rotation, the constants of motion are not necessarily conserved during the corrector step. An approximate correction to this shortcoming was made by projecting translation and rotation out of the rotated coordinates. For the current algorithm unrotated Cartesian coordinates are used for the corrected step to assure the constants of motion are conserved. An algorithm is proposed for updating the trust radius to enhance the accuracy and efficiency of the numerical integration. This modified Hessian-based integration algorithm, with its new components, has been implemented into the VENUS/NWChem software package and compared with the velocity-Verlet algorithm for the H2COâH2+CO, O3+C3H6, and Fâ+CH3OOH chemical reactions
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Tipping the balance: theoretical interrogation of divergent extended heterolytic fragmentations.
Herein we interrogate a type of heterolytic fragmentation reaction called a 'divergent fragmentation' using density functional theory (DFT), natural bond orbital (NBO) analysis, ab initio molecular dynamics (AIMD), and external electric field (EEF) calculations. We demonstrate that substituents, electrostatic environment and non-statistical dynamic effects all influence product selectivity in reactions that involve divergent fragmentation pathways. Direct dynamics simulations reveal an unexpected post-transition state bifurcation (PTSB), and EEF calculations suggest that some transition states for divergent pathways can, in principle, be selectively stabilized if an electric field of the correct magnitude is oriented appropriately
Ab initio quantum direct dynamics simulations of ultrafast photochemistry with Multiconfigurational Ehrenfest approach
The Multiconfigurational Ehrenfest (MCE) method is a quantum dynamics technique which allows treatment of a large number of quantum nuclear degrees of freedom. This paper presents a review of MCE and its recent applications, providing a summary of the formalisms, including its ab initio direct dynamics versions and also giving a summary of recent results. Firstly, we describe the Multiconfigurational Ehrenfest version 2 (MCEv2) method and its applicability to direct dynamics and report new calculations which show that the approach converges to the exact result in model systems with tens of degrees of freedom. Secondly, we review previous âon the flyâ ab initio Multiple Cloning (AIMC-MCE) MCE dynamics results obtained for systems of a similar size, in which the calculations treat every electron and every nucleus of a polyatomic molecule on a fully quantum basis. We also review the Time Dependent Diabatic Basis (TDDB) version of the technique and give an example of its application. We summarise the details of the sampling techniques and interpolations used for calculation of the matrix elements, which make our approach efficient. Future directions of work are outlined
On-the-fly CASPT2 surface hopping dynamics
We report the development of programs for on-the-fly surface hopping dynamics
simulations in the gas and condensed phases on the potential energy surfaces
computed by multistate multireference perturbation theory (XMS-CASPT2) with
full internal contraction. On-the-fly nonadiabatic dynamics simulations are
made possible by improving the algorithm for XMS-CASPT2 nuclear energy gradient
and derivative coupling evaluation. The program is interfaced to a surface
hopping dynamics program, Newton-X, and a classical molecular dynamics package,
tinker, to realize such simulations. On-the-fly XMS-CASPT2 surface-hopping
dynamics simulations of 9H-adenine and an anionic GFP model chromophore
(para-hydroxybenzilideneimidazolin-5-one) in water are presented to demonstrate
the applicability of our program to sizable systems. Our program is implemented
in the bagel package, which is publicly available under the GNU General Public
License
Analytic gradients for state-averaged multiconfiguration pair-density functional theory
Analytic gradients are important for efficient calculations of stationary points on potential energy surfaces, for interpreting spectroscopic observations, and for efficient direct dynamics simulations. For excited electronic states, as are involved in UVâVis spectroscopy and photochemistry, analytic gradients are readily available and often affordable for calculations using a state-averaged complete active space self-consistent-field (SA-CASSCF) wave function. However, in most cases, a post-SA-CASSCF step is necessary for quantitative accuracy, and such calculations are often too expensive if carried out by perturbation theory or configuration interaction. In this work, we present the analytic gradients for multiconfiguration pair-density functional theory based on SA-CASSCF wave functions, which is a more affordable alternative. A test set of molecules has been studied with this method, and the stationary geometries and energetics are compared to values in the literature as obtained by other methods. Excited-state geometries computed with state-averaged pair-density functional theory have similar accuracy to those from complete active space perturbation theory at the second-order
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Computational Design of a Tetrapericyclic Cycloaddition and the Nature of Potential Energy Surfaces with Multiple Bifurcations
An ambimodal transition state (TS) that leads to formation of four different pericyclic reaction products ([4 + 6]-, [2 + 8]-, [8 + 2]-, and [6 + 4]-cycloadducts) without any intervening minima has been designed and explored with DFT computations and quasiclassical molecular dynamics. Direct dynamics simulations propagated from the ambimodal TS show the evolution of trajectories to give the four cycloadducts. The topography of the PES is a key factor in product selectivity. A good correlation is observed between geometrical resemblance of the products to the ambimodal TS (measured by the RMSD) and the ratio of products formed in the dynamics simulationsWe are grateful to the National Science Foundation (CHE1764328 to K.N.H.) for financial support of this research and
for access to XSEDE and UCLA Hoffman 2 for computer time
and for this study. A.M.S. thanks the Madrid Government
(Comunidad de Madrid-Spain) under the Multiannual Agreement with Universidad AutĂłnoma de Madrid in the line
Support to Young Researchers, in the context of the V PRICIT
(SI3-PJI-2021-00463) and âMinisterio de EducaciĂłn Cultura y
Deporteâ for funding (CAS18/00458
Influence of O2 and N2 on the conductivity of carbon nanotube networks
We have performed experiments on single-wall carbon nanotube (SWNT) networks
and compared with density-functional theory (DFT) calculations to identify the
microscopic origin of the observed sensitivity of the network conductivity to
physisorbed O2 and N2. Previous DFT calculations of the transmission function
for isolated pristine SWNTs have found physisorbed molecules have little
influence on their conductivity. However, by calculating the four-terminal
transmission function of crossed SWNT junctions, we show that physisorbed O2
and N2 do affect the junction's conductance. This may be understood as an
increase in tunneling probability due to hopping via molecular orbitals. We
find the effect is substantially larger for O2 than for N2, and for
semiconducting rather than metallic SWNTs junctions, in agreement with
experiment.Comment: 6 pages, 5 figures, 1 tabl
Potential energy surfaces for the HBr+ + CO2 â Br + HOCO+ reaction in the HBr+ 2Î 3/2 and 2Î 1/2 spin-orbit states
Quantum mechanical (QM) + molecular mechanics (MM) models are developed to represent
potential energy surfaces (PESs) for the HBr+ + CO2 â Br + HOCO+ reaction with HBr+ in the
2Î 3/2 and 2Î 1/2 spin-orbit states. The QM component is the spin-free PES and spin-orbit coupling
for each state is represented by a MM-like analytic potential fit to spin-orbit electronic structure
calculations. Coupled-cluster single double and perturbative triple excitation (CCSD(T)) calculations
are performed to obtain âbenchmarkâ reaction energies without spin-orbit coupling. With zero-point
energies removed, the âexperimentalâ reaction energy is 44 ± 5 meV for HBr+(2Î 3/2) + CO2 â
Br(2P3/2) + HOCO+, while the CCSD(T) value with spin-orbit effects included is 87 meV. Electronic
structure calculations were performed to determine properties of the BrHOCO+ reaction intermediate
and [HBr· · ·OCO]+ van der Waals intermediate. The results of different electronic structure methods
were compared with those obtained with CCSD(T), and UMP2/cc-pVTZ/PP was found to be a
practical and accurate QM method to use in QM/MM direct dynamics simulations. The spin-orbit
coupling calculations show that the spin-free QM PES gives a quite good representation of the shape
of the PES originated by 2Î 3/2HBr+. This is also the case for the reactant region of the PES for
2Î 1/2 HBr+, but spin-orbit coupling effects are important for the exit-channel region of this PES. A
MM model was developed to represent these effects, which were combined with the spin-free QM
PES
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