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Vibrational mode selectivity has been investigated for the unimolecular decomposition of methylene nitramine (MN), CH2NNO2, by using classical trajectories computed on a (previously reported) potential-energy surface (PES) that is based on ab initio results. The PES allows for the two known primary decomposition pathways: (I) N–N bond scission to form H2CN and NO2 and (II) concerted molecular elimination giving HONO and HCN. Of particular interest in this study is the influence of selective vibrational mode excitations on the rates and branching of the decomposition via the competing reaction channels, which have barrier heights that differ by only 2 kcal/mol. Rates were computed for 57.7 kcal/mol above the zero-point energy for initial conditions corresponding to overtone excitations of each of the 15 normal modes. In each case, zero-point energy was assigned to each of the normal modes, and then one of the modes was assigned 57.7 kcal/mol excitation energy giving a total of 85.0 kcal/mol. One calculation was done at this energy with initial conditions corresponding to a microcanonical (statistical) distribution in which the energy was randomly distributed among all of the vibrational modes. Comparisons of the mode selective results with this statistical rate show that there is substantial enhancement of the decomposition rates for the two reactions for excitation of three of the normal modes (those with frequencies of 275, 442, and 753 cm−1). Excitation of the other 12 modes yielded rates in accord with the statistical rates for the 2 reactions. The eigenvectors of the normal modes of the molecule that give mode selectivity in the reaction rates project onto the reaction coordinates for the two pathways. One other mode (with frequency 606 cm−1) also projects strongly onto the reaction path for II), but there is no enhancement of the rate when it is excited. However, we found that energy transfers out of this mode to other modes not coupled to either reaction coordinate. The remaining 11 modes do not project to any significant extent onto either reaction coordinate. The results show that projection of a vibrational mode onto the reaction coordinate is necessary for mode selective reaction, but is not sufficient since energy transfer into the reaction coordinate must compete with energy flow to other modes that do not project onto (i.e., are uncoupled from) the reaction coordinates.

Scientific visualization methods have become the new standards for analyzing scientific datasets. However, while these visualization routines provide an excellent qualitative means for data analysis, many researchers still require quantitative information about their data. This paper describes how the marching cubes iso-surface algorithm can be modified to produce not only the qualitative information about the shape of a surface but also some quantitative information regarding the volume of space contained within or beneath that surface. The original marching cubes algorithm decomposes a dataset into cubes based on the grid structure provided. It then searches each cube, determining whether or not the surface intersects that particular cube. During this search, volume calculations can be performed for each cube or partial cube that is contained within or beneath the surface. The results of these calculations can then be summed to obtain a measurement for the volume of space that is within or beneath the surface. These additional tasks can easily be incorporated into the marching cubes algorithm, providing the researcher with a volumetric measurement to support the more qualitative visual information generally produced by an iso-surface algorithm.