Adiabatic Ground State Research Articles

Transition state structure, energetics, and rate constants for the CH 4 + F( 2P) → CH 3 + HF reaction

The CH 4 + F( 2P) → CH 3 + HF abstraction reaction was investigated using the BH&HLYP, MP2, and CCSD(T) approaches, and two series of correlation consistent basis sets. At the CCSD(T)/cc-pVTZ level, the classical barrier height (Δ V #), and the vibrationally adiabatic ground-state barrier height ( Δ V a G # ) are equal to 1.8 and 0.5 kcal/mol, respectively. CCSD(T)/cc-pVQZ calculations give a value for the enthalpy of reaction at 0 K ( Δ H 0 0 ) of −31.5 kcal/mol (Expt. −32.1 kcal/mol). Rate constants calculated with the zero-order interpolated variational transition state (IVTST-0) method are in agreement with experiment. The activation energy computed as local two-points slopes of Arrhenius plots in the temperature range of 200–400 K, equal to 0.59 kcal/mol, is also in good agreement with experiment (0.53 kcal/mol).

Ab Initio and Direct Dynamics Studies of the Reaction of Singlet Methylene with Acetylene and the Lifetime of the Cyclopropene Complex

The energetics of the (1)CH(2) + C(2)H(2) --> H + C(3)H(3) reaction are accurately calculated using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. The reaction enthalpy (0 K) is predicted to be -20.33 kcal/mol. This reaction has no classical barrier in either the entrance or exit channel. However, there are several stable intermediates-cyclopropene (c-C(3)H(4)), allene (CH(2)CCH(2)), and propyne (CH(3)CCH)-along the minimum energy path. These intermediates with zero-point energy corrections lie below the reactants by 87.11 (c-C(3)H(4)), 109.69 (CH(2)CCH(2)), and 110.78 kcal/mol (CH(3)CCH). The vibrationally adiabatic ground-state (VAG) barrier height for c-C(3)H(4) isomerization to allene is obtained as 45.2 kcal/mol, and to propyne as 37.2 kcal/mol. In addition, the (1)CH(2) + C(2)H(2) reaction is investigated utilizing the dual-level "scaling all correlation" (SAC) ab initio method of Truhlar et al., i.e., the UCCSD(SAC)/cc-pVDZ theory. Results show that the reaction occurs via long-lived complexes. The lifetime of the cyclopropene intermediate is obtained as 3.2 +/- 0.4 ps. It is found that the intermediate propyne can be formed directly from reactants through the insertion of (1)CH(2) into a C-H bond of C(2)H(2). However, compared to the major mechanism in which the propyne is produced through a ring-opening of the cyclopropene complex, this reaction pathway is much less favorable. Finally, the theoretical thermal rate constant exhibits a negative temperature dependence, which is in excellent agreement with the previous results. The temperature dependence is consistent with the earlier RRKM results but weaker than the experimental observations at high temperatures.

Quantum phase transitions in antiferromagnetic planar cubic lattices

Motivated by its relation to an $\cal{NP}$-hard problem, we analyze the ground state properties of anti-ferromagnetic Ising-spin networks embedded on planar cubic lattices, under the action of homogeneous transverse and longitudinal magnetic fields. This model exhibits a quantum phase transition at critical values of the magnetic field, which can be identified by the entanglement behavior, as well as by a Majorization analysis. The scaling of the entanglement in the critical region is in agreement with the area law, indicating that even simple systems can support large amounts of quantum correlations. We study the scaling behavior of low-lying energy gaps for a restricted set of geometries, and find that even in this simplified case, it is impossible to predict the asymptotic behavior, with the data allowing equally good fits to exponential and power law decays. We can therefore, draw no conclusion as to the algorithmic complexity of a quantum adiabatic ground-state search for the system.

Exploring the Multiple Reaction Pathways for the H + cyc-C3H6 Reaction

Reaction pathways for the hydrogen atom plus cyclopropane (cyc-C3H6) reaction are studied using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. For this activated reaction, results reveal two reaction mechanisms, a direct H-abstraction and a H-addition/ring-opening. The hydrogen-abstraction reaction yields the H2 and cyclopropyl (cyc-C3H5) radical products. The vibrationally adiabatic ground-state (VAG) barrier height is predicted to be 13.03 kcal/mol. The isomerization barrier height from the product cyclopropyl to allyl radical is 21.98 kcal/mol via a cyc-C3H5 ring-opening process. In addition, the H-addition and ring-opening mechanism will lead to an n-C3H7 radical, which can result in a variety of products such as CH3 + C2H4, H + CH3CHCH2, and H2 + C3H5, etc. The VAG barrier height of the H-addition reaction is 16.49 kcal/mol, which is slightly higher than that of the direct H-abstraction reaction. Although the H + cyc-C3H6 → CH4 + CH2CH reaction is exoergic by 11.90 kcal/mol, this reaction is unlikely due to a high barrier of 43.05 kcal/mol along the minimum energy path.

Hydrogen–antihydrogen interaction: spectacular breakdown of the adiabatic approximation

The adiabatic correction for the hydrogen–antihydrogen interaction in the ground state has been calculated using the Born–Handy method. The variational wavefunction was a linear combination of explicitly correlated Gaussian functions. It has been found that the correction grows rapidly for the nuclear separation approaching the critical distance. It is hypothesized that it becomes infinite at the point where the bound-state solution of the leptonic Schrödinger equation with fixed nuclei changes to a continuum wavefunction. This effect is nonphysical and means that there is no reasonable way to extend the adiabatic ground-state potential energy curve below the critical distance in a strict manner.

Charge Transfer in the Electron Donor—Acceptor Complex BH3NH3

AbstractFor Abstract see ChemInform Abstract in Full Text.

Charge transfer in the electron donor-acceptor complex BH3NH3.

As a simple yet strongly binding electron donor-acceptor (EDA) complex, BH(3)NH(3) serves as a good example to study the electron pair donor-acceptor complexes. We employed both the ab initio valence bond (VB) and block-localized wave function (BLW) methods to explore the electron transfer from NH(3) to BH(3). Conventionally, EDA complexes have been described by two diabatic states: one neutral state and one ionic charge-transferred state. Ab initio VB self-consistent field (VBSCF) computations generate the energy profiles of the two diabatic states together with the adiabatic (ground) state. Our calculations evidently demonstrated that the electron transfer between NH(3) and BH(3) falls in the abnormal regime where the reorganization energy is less than the exoergicity of the reaction. The nature of the NH(3)-BH(3) interaction is probed by an energy decomposition scheme based on the BLW method. We found that the variation of the charge-transfer energy with the donor-acceptor distance is insensitive to the computation levels and basis sets, but the estimation of the amount of electron transferred heavily depends on the population analysis procedures. The recent resurgence of interest in the nature of the rotation barrier in ethane prompted us to analyze the conformational change of BH(3)NH(3), which is an isoelectronic system with ethane. We found that the preference of the staggered structure over the eclipsed structure of BH(3)NH(3) is dominated by the Pauli exchange repulsion.

Global ab initio potential energy surfaces for the lowest three doublet states (1 2A′, 2 2A′, and 1 2A″) of the BrH2 system

Global adiabatic potential energy surfaces (PESs) of the lowest three doublet states (1 A2A′, 2 2A′, and 1 2A″) for the BrH2 system have been calculated using the multireference configuration interaction (MRCI) method including the Davidson’s correction (Q) with the aug-cc-pVTZ basis set. Spin–orbit effects were considered on the basis of the Breit–Pauli Hamiltonian using the MRCI wave functions. The calculated adiabatic energies were fitted to the analytical functional form of many-body expansion. The barrier heights of the H+HBr→H2+Br abstraction and H+H′Br→H′+HBr exchange reactions on the ground-state PES were calculated to be 1.28 and 11.71 kcal mol−1, respectively, both of which are slightly smaller than the values obtained in the previous work [G. C. Lynch, D. G. Truhlar, F. B. Brown, and J.-G. Zhao, J. Phys. Chem. 99, 207 (1995)]. The fits for the 1 2A′, 2 2A′, and 1 2A″ PESs were successful within an accuracy of 0.1 kcal mol−1 in the important regions of PESs such as the transition states and van der Waals wells. Thermal rate constants for the abstraction and exchange reactions and their isotopic variants were calculated with the fitted 1 2A′ PES using the improved canonical variational transition-state theory with the least-action adiabatic ground-state approximation method. The calculated rate constants were found to agree better with experiment than those obtained by Lynch et al.

A theoretical study of the potential energy surface for the reaction OH+CO→H+CO 2

Stationary points on the OH+CO→H+CO 2 potential energy surface are calculated using an extrapolated full coupled-cluster/complete basis set (FCC/CBS) method. The trans-HOCO intermediate is found to lie 30.10 kcal/mole below the OH + CO dissociation limit. For the forward reaction, the vibrationally adiabatic ground-state barrier height is predicted to be 1.03 kcal/mole. Finally, a many-body expansion potential energy surface for HOCO is obtained by fitting to the new ab initio results.

An Interpolated ab Initio Quantum Scattering Study of the Temperature Dependence of the CH3 + HBr → CH4 + Br Reaction

An interpolated ab initio quantum scattering (AIQS) study for the CH3 + HBr → CH4 + Br reaction is presented. The ab initio calculation of energy points has been done using the UMP2/6-311G(3df,d,p) level of theory. Interpolation between the energy points was done using a recently developed generalized discrete variable representation (GDVR) approach. The quantum scattering calculations were performed using a reduced dimensionality rotating line umbrella (RLU) model. The UMP2 results give a vibrationally adiabatic ground-state barrier height of 0.69 kcal/mol for the forward reaction. An unstable van der Waals complex CH3···HBr was also obtained. This complex plays only a minor role in the reaction dynamics. Calculated thermal rate coefficients show a strong nonlinear Arrhenius behavior. Below 550 K they have a negative temperature dependence, whereas a positive activation energy was obtained at higher temperatures. It was found that placing one quantum of energy in the vibrational mode of the H−Br bond str...

Theoretical exploration of ultrafast spectroscopy of small clusters

AbstractThe central issue in femtosecond (fs) time resolved spectroscopy of clusters is the investigation of geometric relaxation and internal vibrational redistribution (IVR) after optical excitation in a nonequilibrium configuration of nuclei by laser photoelectron excitation, and corresponding time delayed probing by multiphoton‐ionization. For this purpose, we have developed multistate ab initio molecular dynamics involving adiabatic ground and excited electronic states, as well as nonadiabatic coupling between them, using the time evolution of initial thermal ensemble in Wigner representation. The combination of ab initio quantum chemical methods, used for the adiabatic and nonadiabatic molecular dynamics “on the fly,” and the Wigner distribution approach for the description of the motion of the nuclei allowed us the accurate determination of pump‐probe and pump‐dump signals also under temperature dependent initial conditions. The connection between simulated pump‐probe signals and the underlying dynamics of nuclei involving adiabatic electronic ground states has been first established for the example of the Ag\documentclass{article}\pagestyle{empty}\begin{document}$_{3}^{-}$\end{document}/Ag3/Ag\documentclass{article}\pagestyle{empty}\begin{document}$_{3}^{+}$\end{document} systems, and compared with experimental negative‐to‐neutral‐to‐positive NeNePo pump‐probe signals. Our simulations reproduced the experimental NeNePo results and determined, in addition to the timescales of geometric relaxation, the conditions under which the resonant or dissipative IVR, as well as vibrational coherence, should be found in the experimental pump‐probe signals. This can be realized in the zero electron kinetic energy NeNePo‐ZEKE experiments, which are in progress. The above combination of methods has been recently extended to the analysis of the timescales as well as of the dynamics in excited electronic states of the nonstoichiometric NanFn−1 (n=2–4) clusters with the single excess valence electron. Our approach allows the simulation of femtosecond NeExPo‐pump‐probe and NeExNe‐pump‐dump signals, based on an analytic formulation which utilizes temperature dependent ground state initial conditions of neutral system (Ne); an ensemble of trajectories carried out either on the adiabatic electronic excited state (Ex), or on both the excited and the ground states through nonadiabatic coupling in connection with the fewest switching hopping algorithm for the investigation of the dynamics of the system; and either the cationic (Po) or the neutral ground state (Ne) for the probing step. The choice of the systems has been made in order to determine the timescales of processes involving (1) fast geometric relaxation leaving the bonding frame intact versus IVR, as during the adiabatic dynamics in the first excited state of Na4F3, being the smallest prototype of F‐colored centers in the bulk; and (2) the photo‐isomerization process through the conical intersection during nonadiabatic dynamics due to the long amplitude motion, as in the Na3F2 cluster after breaking of one metallic and one ionic bond, representing the first example of a five atomic cluster in the gas phase exhibiting conical intersection between the ground and the first excited state. In both cases, full complexity of the problem has been considered taking into account all degrees of freedom. The investigated systems represent important test cases for providing the conceptual framework of ultrafast dynamics in finite systems. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Quantum dynamics of the reaction of hydrogen atom with methane

Time-independent quantum scattering calculations on the H+ CH 4 → H 2+ CH 3 reaction have been performed by using a four-dimensional rotating bond umbrella (RBU) model, based on a modified potential energy surface of Espinosa-Garcı́a and Corchado. The new surface gives a vibrationally adiabatic ground state barrier height of 14.1 kcal/mol. Calculated thermal rate constants are in good agreement with the experimental results. At room temperature the rate constant obtained is 8.16×10 −19 cm 3 molec −1 s −1 . It is found that the simple energy minimization approximation gives significantly small thermal rate constants for this reaction.

Ab initio QM/MM simulations with a molecular orbital‐valence bond (MOVB) method: application to an SN2 reaction in water

A mixed molecular orbital and valence bond (MOVB) method is described in combined ab initio QM/MM simulations of the SN2 reaction of Cl− + CH3Cl → ClCH3 + Cl− in water. The method is based on the construction of individual charge-localized, diabatic states using a block-localized wave function approach, followed by configuration interaction calculations to obtain the adiabatic potential energy surface. To examine the performance of the MOVB method, modern ab initio VB calculations were performed. The MOVB gas phase results are found to be in reasonable agreement in the overall potential energy surface in comparison with Hartree–Fock, MP2, and ab initio VB calculations. The only exception is that the activation energy is predicted to be about 4 kcal/mol higher in MOVB than in other methods. This is attributed to the configuration interaction procedure, which does not further optimize orbital coefficients in MOVB calculations, and it emphasizes the importance of orbital optimization in these calculations. The adiabatic ground-state potential surface can also be approximate by other quantum chemical models in Monte Carlo QM/MM simulations. At the HF/6-31G(d) level, the calculated activation free energy of 26 kcal/mol in water is in good agreement with experiment and with previous computational results. Importantly, the MOVB method allows for the solvent reaction coordinate to be used to define the reaction path in ab initio QM/MM simulations. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1458–1469, 2000

Molecular hydrogen exposed to a suddenly turned-on strongelectric field or low-frequency laser

The behaviour of molecular hydrogen in a dc or (low-frequency)laser field is studied in the limit of a sudden switch-on of thefield. Compared to the case of an adiabatically slow turn-on itis found that around the equilibrium internuclear distance thepopulation of the adiabatic ground state remains dominant evenfor very strong fields. However, this picture changes atinternuclear distances corresponding to field-induced (avoided)crossings with an excited state. At these distances the groundand excited states are populated with similar probabilities. Itis concluded that even in the limit of an instantaneous turn-onof the field it is sufficient to consider the ionization rate ofat most two electronic states.

Direct ab initio quantum scattering for the H2+OH→H+H2O reaction using Møller–Plesset fourth order perturbation theory

We present a realistic ab initio quantum scattering (AIQS) calculation for the chemical reaction H2+OH→H+H2O using the rotating bond approximation (RBA), based on a dual-level MP4//SAC (the fourth order Møller–Plesset perturbation theory/scaling all correlation) method with the 6-311+G(d,p) basis set. The MP4//SAC results give a vibrationally adiabatic ground state barrier height of 0.2722 eV. Calculated thermal rate constants are in good agreement with experimental measurements.

Global potential energy surfaces for the H3+ system. Analytical representation of the adiabatic ground-state 1 1A′ potential

Adiabatic global potential energy surfaces, for singlet and triplet states of A′ and A″ symmetries, were computed for an extensive grid for a total of 8469 conformations of H3+ system at full configuration interaction ab initio level and using an extended basis set that has also been optimized for excited states. An accurate (root-mean-square error lower than 20 cm−1) global fit to the ground-state potential is obtained using a diatomics-in-molecules approach corrected by several symmetrized three-body terms with a total of 96 linear parameters and 3 nonlinear parameters. This produces an accurate global potential which represents all aspects of ground-state H3+ including the absolute minimum, the avoided crossing and dissociation limits, satisfying the correct symmetry properties of the system. The rovibrational eigenstates have been calculated up to total angular momentum J=20 using hyperspherical coordinates with symmetry adapted basis functions. The infrared spectra thus reproduced is within 1 cm−1 with respect to the experimental values for several transitions.

Non-linear response and hydrogen bond dynamics for electron solvation in methanol

Non-equilibrium and equilibrium adiabatic mixed quantum-classical molecular dynamics computer simulations of the solvation dynamics of an excess electron in methanol are reported. We develop the connection between the multiple time scales reflected in solvent response and individual physical phenomena, such as the radial collapse of the electron and structural relaxation of the hydrogen-bonding network of the solvent. The significant role of the latter aspect appears responsible for the breakdown of the linear response approximation for the relaxation of the adiabatic ground state energy of the excess electron.

Ab initio QM/MM simulations with a molecular orbital-valence bond (MOVB) method: application to an SN2 reaction in water

A mixed molecular orbital and valence bond (MOVB) method is described in combined ab initio QM/MM simulations of the SN2 reaction of Cl− + CH3Cl → ClCH3 + Cl− in water. The method is based on the construction of individual charge-localized, diabatic states using a block-localized wave function approach, followed by configuration interaction calculations to obtain the adiabatic potential energy surface. To examine the performance of the MOVB method, modern ab initio VB calculations were performed. The MOVB gas phase results are found to be in reasonable agreement in the overall potential energy surface in comparison with Hartree–Fock, MP2, and ab initio VB calculations. The only exception is that the activation energy is predicted to be about 4 kcal/mol higher in MOVB than in other methods. This is attributed to the configuration interaction procedure, which does not further optimize orbital coefficients in MOVB calculations, and it emphasizes the importance of orbital optimization in these calculations. The adiabatic ground-state potential surface can also be approximate by other quantum chemical models in Monte Carlo QM/MM simulations. At the HF/6-31G(d) level, the calculated activation free energy of 26 kcal/mol in water is in good agreement with experiment and with previous computational results. Importantly, the MOVB method allows for the solvent reaction coordinate to be used to define the reaction path in ab initio QM/MM simulations. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1458–1469, 2000

Reaction dynamics of chlorine atom with methane: Dual-level ab initio analytic potential energy surface and isotope effects

An analytic potential energy surface for the Cl+CH4⇌HCl+CH3 reaction in C3V symmetry has been obtained by fitting to 1136 energy points from a dual-level MP2/SAC (Mo/ller–Plesset second order perturbation/scaling all correlation) calculation using the 6-311G(2d,d,p) basis set. A zero-point energy correction is made to account for all modes not explicitly treated with the time-independent quantum scattering rotating line umbrella (RLU) model, which is used for the dynamics calculations. The effective potential gives a vibrationally adiabatic ground-state barrier height of 3.36 kcal/mol and an endothermicity (0 K) of 1.19 kcal/mol for the Cl+CH4 reaction, and 4.43 kcal/mol and 2.29 kcal/mol, respectively, for Cl+CD4. Thermal rate constants, tunneling and kinetic isotope effects have been investigated in detail. Calculated differential cross sections for Cl+CD4→DCl+CD3, with reactants and products in their vibrational ground states, show that the DCl product is strongly backward scattered. Further, ground state Cl+CD4 reacts to give the CD3 product predominantly unexcited at collision energies ranging from 0.15 eV to 0.25 eV. Generally, good agreement with experimental measurements and previous theoretical work is obtained.

A spectral transform minimum residual filter diagonalization method for interior eigenvalues of physical systems

A spectral transform technique is introduced into the minimum residual (MINRES) filter diagonalization (FD) algorithm for the computation of eigenvalues of large Hermitian matrices. It is a low storage method, i.e., only four real vectors are required to calculate all bound states of the system. In the MINRES FD step, the finite Krylov subspace is built up by a Lanczos iteration using a spectral transform operator which is expanded in a series of Chebyshev polynomials. A guided spectral transform method is suggested to achieve high efficiency of this new algorithm. As an example, all even parity bound states of NO2 have been calculated on the adiabatic ground state potential energy surface of NO2 by a single propagation using a hyperbolic tangent function guided filter operator. The results show that the method is accurate and highly efficient. A statistical analysis of the spectrum is also given.