Potential energy surface

About: Potential energy surface is a research topic. Over the lifetime, 11674 publications have been published within this topic receiving 307691 citations.

An ab initio derived torsional potential energy surface for (H2O)3. II. Benchmark studies and interaction energies

Abstract: A torsional potential energy surface for the cyclic water trimer was calculated at the level of second‐order Mo/ller–Plesset perturbation theory. For the construction of this ab initio surface, the first‐order wave function was expanded in a many‐electron basis which linearly depends on the interelectronic coordinates r12. The one‐electron basis of Gaussian orbitals was calibrated on the water monomer and dimer to ensure that the ab initio surface computed represents the (near‐ ) basis set limit for the level of theory applied. The positions of the free O—H bonds are described by three torsional angles. The respective three‐dimensional torsional space was investigated by 70 counterpoise corrected single‐point calculations for various values of these angles, providing a grid to fit an analytical representation of the potential energy surface. The four symmetry unique stationary points previously found at the Hartree–Fock and conventional Mo/ller–Plesset levels [Schutz et al., J. Chem. Phys. 99, 5228 (1993)] were studied in detail: Relative energies of the structures were calculated by applying second‐order Mo/ller–Plesset and coupled cluster methods; harmonic vibrational frequencies were calculated at the second‐order Mo/ller–Plesset level with a 6‐311++G(d,p) basis set at these stationary points. It is expected that the present torsional potential energy surface for the water trimer will play an important role in the understanding of the vibrational transitions observed by far‐infrared vibration–rotation–tunneling spectroscopy in terms of a nearly free pseudorotational interconversion on a cyclic vibrational–tunneling path.

WKB approximation for the reaction‐path Hamiltonian: Application to variational transition state theory, vibrationally adiabatic excited‐state barrier heights, and resonance calculations

Abstract: We present several kinds of calculations, for reaction rates, vibrationally adiabatic barrier heights, and resonance energies and widths, in which we use the WKB approximation for vibrational energies of stretches in the reaction‐path Hamiltonian. We consider both collinear and three‐dimensional atom‐transfer reactions. As compared to previous calculations employing the Morse approximation for vibrational energies of stretches, there is generally significant quantitative improvement in accuracy for ground‐state quantities and thermal reaction rates, and there is dramatic improvement in accuracy for excited‐state quantities. We also update our predictions for some three‐dimensional reaction rates calculated with an accurate ab initio potential energy surface.

Atomistic modelling of BaTiO3 based on first-principles calculations

Abstract: Interatomic potentials are determined in the framework of a shell model used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO3. The model is developed from first-principles calculations by mapping the potential energy surface for various ferroelectric distortions. The parameters are obtained by performing a fit of interatomic potentials to this energy surface. Several zero-temperature properties of BaTiO3, which are of central importance, are correctly simulated in the framework of our model. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations, showing that the non-trivial phase transition sequence of BaTiO3 is correctly reproduced. The lattice parameters and expansion coefficients for the different phases are in good agreement with experimental data, while the theoretically determined transition temperatures tend to be too small.

Stress-dependent molecular pathways of silica–water reaction

Abstract: Stress-corrosion of silica by water is studied by exploring the stress-dependent potential energy surface computed quantum mechanically at the level of molecular orbital theory. An ordered silica nanorod with clearly defined nominal tensile stress is constructed to model a structural unit of the stressed crack tip. Three competing hydrolysis reaction pathways are determined, each involving a distinct initiation step. Water dissociation, molecular chemisorption, and direct siloxane bond rupture dominate at low, intermediate, and high stress levels, respectively. A linear stress dependence in the thermodynamic driving force, not commonly considered in the criterion of brittle fracture initiation, is shown to originate from surface relaxation associated with bond rupture. This effect is particularly important in determining the Griffith condition of crack extension for nano-sized systems when spatial accommodation of foreign molecules is involved in the process of bond breaking. The physical origin of the stress dependence of kinetic barrier is revealed by a perturbation analysis of the minimum energy path parametrized by the continuous mechanical stress.

Accurate Adsorption Thermodynamics of Small Alkanes in Zeolites. Ab initio Theory and Experiment for H-Chabazite

Abstract: Heats of adsorption of methane, ethane, and propane in H-chabazite (Si/Al = 14.4) have been measured and entropies have been derived from adsorption isotherms. For these systems quantum chemical ab initio calculations of Gibbs free energies have been performed. The deviations from the experimental values for methane, ethane, and propane are below 3 kJ/mol for the enthalpy, and the Gibbs free energy. A hybrid high-level (MP2/CBS): low-level (DFT+dispersion) method is used to determine adsorption structures and energies. Vibrational entropies and thermal enthalpy contributions are obtained from vibrational partition functions for the DFT+dispersion potential energy surface. Anharmonic corrections have been evaluated for each normal mode separately. One-dimensional Schrodinger equations are solved for potentials obtained by (curvilinear) distortions of the normal modes using a representation in internal coordinates.