The H + H 2 exchange reaction constitutes an excellent benchmark with which to test dynamical theories against experiments. The H + D 2 (vibrational quantum number v = 0, rotational quantum number j = 0) reaction has been studied in crossed molecular beams at a collision energy of 1.28 electron volts, with the use of the technique of Rydberg atom time-of-flight spectroscopy. The experimental resolution achieved permits the determination of fully rovibrational state-resolved differential cross sections. The high-resolution data allow a detailed assessment of the applicability and quality of quasi-classical trajectory (QCT) and quantum mechanical (QM) calculations. The experimental results are in excellent agreement with the QM results and in slightly worse agreement with the QCT results. This theoretical reproduction of the experimental data was achieved without explicit consideration of geometric phase effects.

The hydrogen exchange reaction in its HϩD 2 (vϭ0,jϭ0)→HD(vЈϭ0,jЈ)ϩD isotopic variant has been investigated theoretically and experimentally at the collision energies 0.52 eV, 0.531 eV and 0.54 eV. A detailed comparison of converged quantum mechanical scattering calculations and state-to-state molecular beam experiments has allowed a direct assessment of the quality of the different ab initio potential energy surfaces used in the calculations, and strongly favors the newly refined version of the Boothroyd-Keogh-Martin-Peterson surface. The differences found in the calculations are traced back to slight differences in the topology of the potential energy surfaces.

The Kohn variational principle for the log-derivative matrix is used to calculate integral cross sections for H+D2 (v=0, j=0) to D+HD (v′=0,1,2, all j′) at the experimentally accessible collision energies of 0.55 and 1.3 eV. Comparison is made with experimental and theoretical studies in the literature. Product state relative rotational distributions, vibrational branching ratios, and energy partitioning fractions are all in good agreement with the recent experimental results of Rinnen, Kliner, and Zare. Absolute cross sections are compared with the experimental work of Levene et al. and Johnson et al. Our results agree very well with their experiments. It is found that the quasiclassical results of Blais and Truhlar compare well with the present exact quantum mechanical predictions in many respects, however, the product rotational distributions are ‘‘hotter’’ than the quantal ones.

A detailed comparison of quasiclassical trajectory (QCT) and quantum mechanical (QM) reaction probabilities and differential cross sections for the H + D 2 -+ HD + D reaction at the collision energies of 0.54 and 1.29 e V has been carried out using the same potential energy surface. The theoretical simulation of the recently published experimental results is also reported. The comparisons made here demonstrate the level of agreement between QCT and QM approaches, as well as between theory and experiment for this reaction.

Modern OpenMP threading techniques are used to convert the MPI-only Hartree-Fock code in the GAMESS program to a hybrid MPI/OpenMP algorithm. Two separate implementations that di er by the sharing or replication of key data structures among threads are considered, density and Fock matrices. All implementations are benchmarked on a super-computer of 3,000 Intel® Xeon Phi TM processors. With 64 cores per processor, scaling numbers are reported on up to 192,000 cores. e hybrid MPI/OpenMP implementation reduces the memory footprint by approximately 200 times compared to the legacy code. e MPI/OpenMP code was shown to run up to six times faster than the original for a range of molecular system sizes.