There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

To make practical the molecular dynamics simulation of large scale reactive chemical systems (1000s of atoms), we developed ReaxFF, a force field for reactive systems. ReaxFF uses a general relationship between bond distance and bond order on one hand and between bond order and bond energy on the other hand that leads to proper dissociation of bonds to separated atoms. Other valence terms present in the force field (angle and torsion) are defined in terms of the same bond orders so that all these terms go to zero smoothly as bonds break. In addition, ReaxFF has Coulomb and Morse (van der Waals) potentials to describe nonbond interactions between all atoms (no exclusions). These nonbond interactions are shielded at short range so that the Coulomb and van der Waals interactions become constant as Rij → 0. We report here the ReaxFF for hydrocarbons. The parameters were derived from quantum chemical calculations on bond dissociation and reactions of small molecules plus heat of formation and geometry data for...

http://www.wag.caltech.edu/publications/sup/pdf/471.pdf

ReaxFF: A Reactive Force Field for Hydrocarbons

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Density functional theory for transition metals and transition metal chemistry

A new hybrid Hartree−Fock−density functional (HF-DF) model called the modified Perdew−Wang 1-parameter model for kinetics (MPW1K) is optimized against a database of 20 forward barrier heights, 20 r...

Adiabatic connection for kinetics

A new potential energy surface is reported for the gas-phase reaction Cl+CH4→HCl+CH3. It is based on the analytical function of Jordan and Gilbert for the analog reaction H+CH4→H2+CH3, and it is calibrated by using the experimental thermal rate coefficients and kinetic isotope effects. The forward and reverse thermal rate coefficients were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range, 200–2500 K. This surface is also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate coefficients. We find that excitation of C–H stretching modes and Cl–H stretching modes enhances the rate of both the forward and the reverse reactions, and excitation of the lowest frequency bending mode in the CH4 reactant also enhances the rate coefficient for the forward reaction. However, the vibrational excitation of t...

Potential energy surface, thermal, and state-selected rate coefficients, and kinetic isotope effects for Cl+CH4→HCl+CH3

We consider a new approach to reaction-path dynamics calculations in which the reaction path is calculated at a low level (LL) of theory and stationary point information from a high level (HL) of theory is used to interpolate corrections to energetic quantities, vibrational frequencies, and moments of inertia. Such a calculation is labeled XII/Y, where X denotes the high level and Y the low level. The theory is applied to the reaction OH + NH[sub 3] and three isotopomeric analogs. The highest-level optimization reported for the saddle point is QCISD(T)//MP2/aug-cc-pVTZ, which yields a classical barrier height of 3.65 kcal/mol. The rate constant is calculated at two levels, QCISD(T)//MP2/aug-cc-p VTZ[MP2/aug-cc-p VDZ] ///MP2/6-31G** and QCISD(T)//MP2/aug-cc-pVTZ[MP2/aug-cc-pVDZ] ///PM3-SRP; the calculated rare constant for the unsubstituted reaction is approximately invariant to the low level used in the dual-level scheme and agrees with experiment within a factor of 1.13 at 250 K and within a factor of 1.14 at 1500 K. 79 refs., 4 figs., 13 tabs.

Dual-Level Reaction-Path Dynamics (the /// Approach to VTST with Semiclassical Tunneling). Application to OH + NH3 .fwdarw. H2O + NH2

We report calculations of the reaction rates of O(3P) + CH4 → OH + CH3 and O(3P) + CD4 → OD + CD3 over the temperature range 300−2500 K. The calculations are based on variational transition state t...

Dual-level direct dynamics calculations of the reaction rates for a jahn-teller reaction : hydrogen abstraction from ch4 or cd4 by o(3p)

In recent years, many chemical reactions have been studied by means of the quasiclassical trajectory (QCT) method within the Gaussian binning (GB) procedure. The latter consists of "quantizing" the final vibrational actions in Bohr spirit by putting strong emphasis on the trajectories reaching the products with vibrational actions close to integer values. A major drawback of this procedure is that if N is the number of product vibrational modes, the amount of trajectories necessary to converge the calculations is ∼10(N)×larger than with the standard QCT method. Applying it to polyatomic processes is thus problematic. In a recent paper, however, Czako and Bowman propose to quantize the total vibrational energy instead of the vibrational actions [G. Czako and J. M. Bowman, J. Chem. Phys. 131, 244302 (2009)], a procedure called 1GB here. The calculations are then only ∼10 times more time consuming than with the standard QCT method, allowing thereby for considerable numerical saving. In this paper, we propose some theoretical arguments supporting the 1GB procedure and check its validity on model test cases as well as the prototype four-atom reaction OH+D(2)→HOD+D.

The method of Gaussian weighted trajectories. V. On the 1GB procedure for polyatomic processes

A modified and recalibrated potential energy surface for the gas-phase CH4+H→CH3+H2 reaction and its deuterated analogs is reported and tested, which is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and is calibrated with respect to updated experimental and theoretical stationary point (reactants, products, and saddle point) properties, and experimental forward thermal rate constants. The forward and reverse rate constants are calculated using variational transition-state theory with multidimensional tunneling effect over a wide temperature range, 300–2000 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of the tunneling in this reaction. Five sets of kinetic isotope effects are also calculated. In general, they agree with experimental values within the experimental errors. This surface is then used to analyze dynamical features, such as reaction-path curvature, the couplin...

Joaquin Espinosa-Garcia

Papers

Potential energy surface, thermal, and state-selected rate coefficients, and kinetic isotope effects for Cl+CH4→HCl+CH3

Dual-Level Reaction-Path Dynamics (the /// Approach to VTST with Semiclassical Tunneling). Application to OH + NH3 .fwdarw. H2O + NH2

Dual-level direct dynamics calculations of the reaction rates for a jahn-teller reaction : hydrogen abstraction from ch4 or cd4 by o(3p)

The method of Gaussian weighted trajectories. V. On the 1GB procedure for polyatomic processes

New analytical potential energy surface for the CH4+H hydrogen abstraction reaction: Thermal rate constants and kinetic isotope effects