Lipeng Sun

Use of a single trajectory to study product energy partitioning in unimolecular dissociation: Mass effects for halogenated alkanes

A single trajectory (ST) direct dynamics approach is compared with quasiclassical trajectory (QCT) direct dynamics calculations for determining product energy partitioning in unimolecular dissociation. Three comparisons are made by simulating C(2)H(5)F-->HF + C(2)H(4) product energy partitioning for the MP26-31G(*) and MP26-311 + + G(**) potential energy surfaces (PESs) and using the MP26-31G(*) PES for C(2)H(5)F dissociation as a model to simulate CHCl(2)CCl(3)-->HCl + C(2)Cl(4) dissociation and its product energy partitioning. The trajectories are initiated at the transition state with fixed energy in reaction-coordinate translation E(t) (double dagger). The QCT simulations have zero-point energy (ZPE) in the vibrational modes orthogonal to the reaction coordinate, while there is no ZPE for the STs. A semiquantitative agreement is obtained between the ST and QCT average percent product energy partitionings. The ST approach is used to study mass effects for product energy partitioning in HX(X = F or Cl) elimination from halogenated alkanes by using the MP26-31G(*) PES for C(2)H(5)F dissociation and varying the masses of the C, H, and F atoms. There is, at most, only a small mass effect for partitioning of energy to HX vibration and rotation. In contrast, there are substantial mass effects for partitioning to relative translation and the polyatomic products vibration and rotation. If the center of mass of the polyatomic product is located away from the C atom from which HX recoils, the polyatomic has substantial rotation energy. Polyatomic products, with heavy atoms such as Cl atoms replacing the H atoms, receive substantial vibration energy that is primarily transferred to the wag-bend motions. For E(t) (double dagger) of 1.0 kcalmol, the ST calculations give average percent partitionings to relative translation, polyatomic vibration, polyatomic rotation, HX vibration, and HX rotation of 74.9%, 6.8%, 1.5%, 14.4%, and 2.4% for C(2)H(5)F dissociation and 39.7%, 38.1%, 0.2%, 16.1%, and 5.9% for a model of CHCl(2)CCl(3) dissociation.

Ab initio direct dynamics trajectory simulation of C2H5F-->C2H4 + HF product energy partitioning.

Direct dynamics classical trajectory simulations were performed to study product energy partitioning in C(2)H(5)F-->C(2)H(4)+HF dissociation. The intrinsic reaction coordinate potential energy curve, reaction energetics, and transition state (TS) properties were calculated for this reaction at different levels of electronic structure theory, and MP2/6-31G( *) was chosen as a meaningful and practical method for performing the direct dynamics. The trajectories show that the HF bond, uncoupled from the other degrees of freedom, is formed within the first 10 fs as the system moves from the TS towards products. The populations of the HF vibration states, determined from the simulations, decrease monotonically as found from experiments. However, the simulations populations for the low and high energy vibration states are larger and smaller, respectively, than the experimental results. The HF rotational temperature found from the simulations is in agreement with experiment. Increasing the TSs excess energy gives higher rotational temperatures for both C(2)H(4) and HF. Energy is partitioned to the products from both the excess energy in the TS and the potential energy release in the exit channel. Partitioning from these two energy sources is distinguished by varying the TSs excess energy. An analysis of the simulations energy disposal shows that the fractions of the excess energy partitioned to relative translation, C(2)H(4) vibration, C(2)H(4) rotation, HF vibration, and HF rotation, are 0.17, 0.64, 0.076, 0.067, and 0.046, respectively, and are in good agreement with previous simulations on empirical potentials and experiments. The partitioning found for the potential energy release is 81%, <0.05%, 5%, 11%, and 3% to relative translation, C(2)H(4) vibration, C(2)H(4) rotation, HF vibration, and HF rotation. This result is substantially different than the deduction from experiments, which summarizes the partitioning as 20%, 45%, 24%, and <12% to relative translation, C(2)H(4) vibration+rotation, HF vibration, and HF rotation. Possible origins of the difference between the simulations and experiments in the release of the potential energy is discussed.

Comparisons of classical and Wigner sampling of transition state energy levels for quasiclassical trajectory chemical dynamics simulations

Quasiclassical trajectory calculations are compared, with classical and Wigner sampling of transition state (TS) energy levels, for C(2)H(5)F( not equal)-->HF+C(2)H(4) product energy partitioning and [Cl...CH(3)...Cl](-) central barrier dynamics. The calculations with Wigner sampling are reported here for comparison with the previously reported calculations with classical sampling [Y. J. Cho et al., J. Chem. Phys. 96, 8275 (1992); L. Sun and W. L. Hase, J. Chem. Phys. 121, 8831 (2004)]. The C(2)H(5)F( not equal) calculations were performed with direct dynamics at the MP2/6-31G( *) level of theory. Classical and Wigner sampling give post-transition state dynamics, for these two chemical systems, which are the same within statistical uncertainties. This is a result of important equivalences in these two sampling methods for selecting initial conditions at a TS. In contrast, classical and Wigner sampling often give different photodissociation dynamics [R. Schinke, J. Phys. Chem. 92, 3195 (1988)]. Here the sampling is performed for a vibrational state of the ground electronic state potential energy surface (PES), which is then projected onto the excited electronic states PES. Differences between the ground and the excited PESs may give rise to substantially different excitations of the vibrational and dissociative coordinates on the excited state PES by classical and Wigner sampling, resulting in different photodissociation dynamics.

Stationary points for the OH− + CH3F → CH3OH + F− potential energy surface

Abstract Ab initio calculations at the HF, MP2, and CCSD(T) levels of theory, utilizing a range of basis sets including the large bases 6-311++G(2df,2pd) and aug-cc-pVTZ, are used to study the OH − +CH 3 F→CH 3 OH+F − potential energy surface (PES). Structures, vibrational frequencies, and energies are determined for the reactant and product asymptotic limits, the OH⋯CH 3 F ion–dipole potential minimum, the [OH⋯CH 3 ⋯F] − central barrier, and the CH 3 OH⋯F − hydrogen-bonded minimum. This PES does not have a post-reaction F − ⋯CH 3 OH minimum complementary to the pre-reaction OH − ⋯CH 3 F minimum. Except for the CH 3 OH⋯F − minimum, the large basis sets and MP2 theory give a consistent set of structures and frequencies for the stationary points. Neither the structure nor the vibrational frequencies of the CH 3 OH⋯F − minimum are converged by the MP2 and large basis set calculations. RHF theory does not describe the energy of the [OH⋯CH 3 ⋯F] − central barrier nor the reaction exothermicity, however, it does give OH − +CH 3 F→OH − ⋯CH 3 F and F − +CH 3 OH→CH 3 OH⋯F − well depths in good agreement with the CCSD(T) values. Overall good agreement is found between the MP2/6-31+G ∗ and much higher level CCSD(T) energies for the stationary points. The MP2 and CCSD(T) calculations give a reaction exothermicity and F − +CH 3 OH→CH 3 OH⋯F − well depth in good agreement with the experimental values.

Dynamics of Ar+CH4/Ni{111} collision-induced desorption

Classical trajectory simulations were used to study Ar+CH4/Ni{111} collision-induced desorption and compared with experiment. To perform the simulations, analytic potentials were determined for Ar/CH4 and CH4/Ni{111}. An accurate form for the former potential was derived by carrying out a series of ab initio calculations at various levels of theory, while previously published ab initio calculations were used to develop the latter CH4/Ni{111} potential. Overall the simulation and experimental desorption cross sections are in excellent agreement, except at small incident angles θi (with respect to the surface normal) and low initial Ar translational energies, Ei, where the simulation cross sections are approximately a factor of 2 too large. Most of the desorption occurs by trajectories in which Ar first strikes CH4, but for both large θi and Ei, a small fraction of the desorption occurs by trajectories in which Ar first strikes the Ni surface. Excitation of the CH4 vibrational modes is negligible and CH4 ro...

Effect of the Ar–Ni(s) potential on the cross section for Ar+CH4/Ni{111} collision-induced desorption and the need for a more accurate CH4/Ni{111} potential

In a previous paper [L. Sun, P. de Sainte Claire, O. Meroueh, and W. L Hase, J. Chem. Phys. 114, 535 (2001)], a classical trajectory simulation was reported of CH(4) desorption from Ni{111} by Ar-atom collisions. At an incident angle theta(i) of 60 degrees (with respect to the surface normal), the calculated collision-induced desorption (CID) cross sections are in excellent agreement with experiment. However, for smaller incident angles the calculated cross sections are larger than the experimental values and for normal collisions, theta(i)=0 degrees , the calculated cross sections are approximately a factor of 2 larger. This trajectory study used an analytic function for the Ar+Ni(s) intermolecular potential which gives an Ar-Ni{111} potential energy minimum which is an order of magnitude too deep. In the work reported here, the previous trajectory study is repeated with an Ar+Ni(s) analytic intermolecular potential which gives an accurate Ar-Ni{111} potential energy minimum and also has a different surface corrugation than the previous potential. Though there are significant differences between the two Ar+Ni(s) analytic potentials, they have no important effects on the CID dynamics and the cross sections reported here are nearly identical to the previous values. Zero-point energy motions of the surface and the CH(4)-Ni(s) intermolecular modes are considered in the simulation and they are found to have a negligible effect on the CID cross sections. Calculations of the intermolecular potential between CH(4) and a Ni atom, at various levels of theory, suggest that there are substantial approximations in the ab initio calculation used to develop the CH(4)+Ni{111} potential. The implication is that the differences between the trajectory and experimental CID cross sections may arise from an inaccurate CH(4)+Ni{111} potential used in the trajectory simulation.

Experiments with Parallelizing Tribology Simulations

Different parallelization methods vary in their system requirements, programming styles, efficiency of exploring parallelism, and the application characteristics they can handle. For different situations, they can exhibit totally different performance gains. This paper compares OpenMP, MPI, and Strings for parallelizing a complicated tribology problem. The problem size and computing infrastructure is changed to assess the impact of this on various parallelization methods. All of them exhibit good performance improvements and it exhibits the necessity and importance of applying parallelization in this field.

Experiments with parallelizing a tribology application

Different parallelization methods vary in their system requirements, programming styles, efficiency of exploring parallelism, and the application characteristics they can handle. Different applications can exhibit totally different performance gains depending on the parallelization method used. The paper compares OpenMP, MPI, and Strings (a distributed shared memory) for parallelizing a complicated tribology problem. The problem size and computing infrastructure are changed and their impacts on the parallelization methods are studied. All of the methods studied exhibit good performance improvements. The paper exhibits the benefits that are the result of applying parallelization techniques to applications in this field.

Comparing various parallelizing approaches for tribology simulations

Different parallelization methods vary in their system requirements, programming styles, efficiency of exploring parallelism, and the application characteristics they can handle. For different situations, they can exhibit totally different performance gains. This chapter compares OpenMP, MPI, and Strings for parallelizing a complicated tribology problem. The problem size and computing infrastructure is changed to assess the impact of this on various parallelization methods. All of them exhibit good performance improvements and it exhibits the necessity and importance of applying parallelization in this field.