Articles you may be interested inWe develop a full-dimensional global analytical potential energy surface (PES) for the F − + CH 3 F reaction by fitting about 50 000 energy points obtained by an explicitly correlated composite method based on the second-order Møller-Plesset perturbation-F12 and coupled-cluster singles, doubles, and perturbative triples-F12a methods and the cc-pVnZ-F12 [n = D, T] basis sets. The PES accurately describes the (a) back-side attack Walden inversion mechanism involving the pre-and post-reaction (b) ion-dipole and (c) hydrogen-bonded complexes, the configuration-retaining (d) front-side attack and (e) double-inversion substitution pathways, as well as (f) the proton-abstraction channel. The benchmark quality relative energies of all the important stationary points are computed using the focal-point analysis (FPA) approach considering electron correlation up to coupled-cluster singles, doubles, triples, and perturbative quadruples method, extrapolation to the complete basis set limit, core-valence correlation, and scalar relativistic effects. The FPA classical(adiabatic) barrier heights of (a), (d), and (e) are −0.45(−0.61), 46.07(45.16), and 29.18(26.07) kcal mol −1 , respectively, the dissociation energies of (b) and (c) are 13.81(13.56) and 13.73(13.52) kcal mol −1 , respectively, and the endothermicity of (f) is 42.54(38.11) kcal mol −1 . Quasiclassical trajectory computations of cross sections, scattering (θ) and initial attack (α) angle distributions, as well as translational and internal energy distributions are performed for the F − + CH 3 F(v = 0) reaction using the new PES. Apart from low collision energies (E coll ), the S N 2 excitation function is nearly constant, the abstraction cross sections rapidly increase with E coll from a threshold of ∼40 kcal mol −1 , and retention trajectories via double inversion are found above E coll = ∼30 kcal mol −1 , and at E coll = ∼50 kcal mol −1 , the front-side attack cross sections start to increase very rapidly. At low E coll , the indirect mechanism dominates (mainly isotropic backward-forward symmetric θ distribution and translationally cold products) and significant long-range orientation effects (isotropic α distribution) and barrier recrossings are found. At higher E coll , the S N 2 reaction mainly proceeds with direct rebound mechanism (backward scattering and hot product translation). C 2015 AIP Publishing LLC. [http://dx
We test the accuracy of various standard, explicitly correlated F12, and composite ab initio methods with different correlation consistent basis sets for high-dimensional potential energy surface (PES) developments, thereby providing a practical guidance for reaction dynamics studies. Relative potential energies are computed at 15 geometries covering the energy range and configuration space of chemical importance for each of the six prototypical polyatomic reactions, X + CH4 → HX + CH3 [X = F, O, Cl] and X(-) + CH3Y → Y(-) + CH3X [X/Y = F/F, OH/F, F/Cl]. The average accuracies of the Hartree-Fock and MP2 methods are 1500-8000 and 400-1000 cm(-1), respectively. The standard CCSD(T) method provides errors of 900-1400 and 250-450 cm(-1) with aug-cc-pVDZ and aug-cc-pVTZ basis sets, respectively. The explicitly correlated CCSD(T)-F12 method reduces the corresponding errors to about 200 and 100 cm(-1); thus, we recommend using the F12 methods for PES developments. For F12 computations, the cc-pVnZ-F12 [n = D and T] basis sets usually, but not always, perform better than the corresponding aug-cc-pVnZ bases. We do not find clear preference between the F12a and F12b methods for PES developments. Composite methods are advocated instead of standard CCSD(T) because for example, one can obtain CCSD(T)/aug-cc-pVnZ quality results on the expense of MP2/aug-cc-pVnZ [n = T and Q] computations. The post-CCSD(T), the core correlation, and the scalar relativistic effects are found to be ∼100, 80-130, and 10-50 cm(-1), respectively. The all-electron CCSD(T)/aug-cc-pCVnZ relative energies differ from the complete-basis-set limit by about 1000, 300, 100, and 50 cm(-1) for n = D, T, Q, and 5, respectively.
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