ALBERT

Notes: Isothermal-isobaric Monte Carlo calculations were used to obtain predictions of the elastic coefficients and derived engineering moduli and Poisson ratios for crystalline hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The elastic coefficients were computed using the strain fluctuation formula due to Rahman and Parrinello [J. Chem. Phys. 76, 2662 (1982)]. Calculations were performed as a function of temperature (218 K≤T≤333 K) and hydrostatic pressure (0 GPa≤p≤4 GPa). The predicted values of the moduli and Poisson ratios under ambient conditions are in accord with general expectations for molecular crystals and with a very recent, unpublished determination for RDX. The moduli exhibit a sensitive pressure dependence whereas the Poisson ratios are relatively independent of pressure. The temperature dependence of the moduli is comparable to the precision of the results. However, the crystal does exhibit thermal softening for most pressures. An additional product of the calculations is information about the pressure-volume-temperature (pVT) equation of state. We obtain near-quantitative agreement with experiment for the case of hydrostatic compression and reasonable, but not quantitative, correspondence for thermal expansion. The results indicate a significant dependence of the thermal expansion coefficients on hydrostatic pressure. © 2000 American Institute of Physics.

Notes: A simple method for calculating vibrational transition probabilities in three-dimensional atom–diatomic collisions, which we call the classical approach quantum encounter (CAQE) method, is presented. It consists of using suitably initiated classical trajectories as a means of generating ensembles of phase-space points corresponding to the hard part of collisions in which the repulsive core of the interaction potential is sampled; these are then used as input to standard expressions for computing quantum vibrational transition probabilities. The method is applied to study the temperature dependence of the vibrational deactivation of HCl(v=1) in collisions with Ar atoms. It can be generalized, however, to treat vibrational energy exchange in encounters between two polyatomic molecules. Strengths and weaknesses of the method are discussed and comparisons are made with a preceding simpler theory wherein the classical dynamics of the approach was replaced by statistical approximations.

Notes: The unimolecular dissociation reactions of the 2-chloroethyl radical involving C–H and C–Cl bond fissions are investigated using classical trajectories and two variational transition-state theory methods on the same potential-energy surface. The transition-state theory methods employed are the efficient microcanonical sampling-transition state theory method, previously used to study the decomposition dynamics of disilane and 1,2-difluoroethane, and a J-conserving variant of this method that introduces constraining equations in the efficient microcanonical sampling procedure, such that the sampling is restricted to phase-space points associated with both a constant value of the system energy and total angular momentum.The results demonstrate that the unimolecular dissociation of the 2-chloroethyl radical is well described by statistical theories that assume an equal weight for all energetically accessible phase-space points. The results obtained from the statistical calculations form upper bounds to the trajectory-computed rate coefficients as expected for a statistical system. In addition, there is no evidence of mode-specific dynamics present in the trajectory results. The statistical behavior of the 2-chloroethyl radical stands in sharp contrast to the dissociation dynamics of disilane and 1,2-difluoroethane which have previously been shown to exhibit pronounced nonstatistical effects. It is shown that the existence of nonstatistical behavior cannot, in general, be qualitatively predicted from energy considerations alone.Comparison of the 2-chloroethyl radical, 1,2-difluoroethane, and disilane results again demonstrates that the existence of an energy decay rate out of a given bond that is fast relative to the unimolecular reaction rate is not a sufficient condition to guarantee statistical dynamics. It is found that the statistical behavior observed for 2-chloroethyl is due, in large part, to an increase in the potential coupling between the dissociating atom and the beta-carbon that occurs as the bond breaks. This coupling is associated with the conversion of the C–C single bond to a C(large-closed-square)C double bond upon C–Cl or C–H bond fission in 2-chloroethyl. It is concluded that unimolecular reactions will tend to exhibit nonstatistical dynamics if (1) the internal energy is close to the dissociation threshold, (2) motion along the reaction coordinate does not produce large energetic changes in one of more bonds in the remainder of the molecule, and (3) there exists a formation coordinate for the activated reactant that is strongly coupled to the dissociation coordinate but only weakly coupled to the other internal coordinates of the molecule.

Notes: The possibility of utilizing different types of power spectra obtained from classical trajectories as a diagnostic tool to identify the presence of nonstatistical dynamics is explored by using the unimolecular bond-fission reactions of 1,2-difluoroethane and the 2-chloroethyl radical as test cases. In previous studies, the reaction rates for these systems were calculated by using a variational transition-state theory and classical trajectory methods. A comparison of the results showed that 1,2-difluoroethane is a nonstatistical system, while the 2-chloroethyl radical behaves statistically. Power spectra for these two systems have been generated under various conditions. The characteristics of these spectra are as follows: (1) The spectra for the 2-chloroethyl radical are always broader and more coupled to other modes than is the case for 1,2-difluoroethane. This is true even at very low levels of excitation. (2) When an internal energy near or above the dissociation threshold is initially partitioned into a local C–H stretching mode, the power spectra for 1,2-difluoroethane broaden somewhat, but discrete and somewhat isolated bands are still clearly evident.In contrast, the analogous power spectra for the 2-chloroethyl radical exhibit a near complete absence of isolated bands. The general appearance of the spectrum suggests a very high level of mode-to-mode coupling, large intramolecular vibrational energy redistribution (IVR) rates, and global statistical behavior. (3) The appearance of the power spectrum for the 2-chloroethyl radical is unaltered regardless of whether the initial C–H excitation is in the CH2 or the CH2Cl group. This result also suggests statistical behavior. These results are interpreted to mean that power spectra may be used as a diagnostic tool to assess the statistical character of a system. The presence of a diffuse spectrum exhibiting a nearly complete loss of isolated structures indicates that the dissociation dynamics of the molecule will be well described by statistical theories. If, however, the power spectrum maintains its discrete, isolated character, as is the case for 1,2-difluoroethane, the opposite conclusion is suggested. Since power spectra are very easily computed, this diagnostic method may prove to be useful.

Notes: Classical trajectories have been employed in a study of the intramolecular dynamics and unimolecular decomposition of the 2-chloroethyl radical. A potential-energy surface was constructed by using the available experimental data and theoretical results. The following reaction channels were included in the study: ⋅CH2CH2Cl→CH2=CH2+⋅Cl, ⋅CH2CH2Cl→CH2=CHCl+⋅H. Mode-specific behavior was investigated by computing ensembles of trajectories for initial conditions (1) in which the normal-mode vibrations of the radical were assigned zero-point energies and a single C–H local stretch on the radical end of the system was excited, and (2) in which the normal modes were all excited so as to distribute the total energy uniformly throughout the radical. First-order rate coefficients were calculated both for the disappearance of the reactant and for the two chemically distinct reaction channels. The results do not indicate significant, if any, mode-specific effects. Energy transfer from and into local C–H stretching modes was studied. Relaxation of an initially excited C–H bond is observed to be irreversible and complete within about 0.6 ps.

Notes: Rigid molecule Monte Carlo simulations are used in a computational study of the isothermal, hydrostatic compression of crystalline hexahydro-1,3,5-trinitro-1,3,5-triazine and β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine for pressures in the range 0 GPa ≤p≤ 7.5 GPa. The purpose of the investigation is to assess the utility of simple intermolecular potential-energy functions, generally parameterized for ambient conditions of temperature and pressure, for studies of polyatomic molecular crystals under the extremes of pressure that are important in accident scenarios involving high explosives. The calculated results are found to be in good agreement with published x-ray diffraction data. © 1998 American Institute of Physics.

Notes: We examine methods for dealing with the flow of zero-point energy in classical trajectory simulations and identify some of the problems associated with their use. Fundamental issues which must be considered, both in assessing the extent of the zero-point energy problem and in the development of useful remedies, are discussed. © 1996 American Institute of Physics.

Notes: We have devised a semiclassical procedure based on the Makri–Miller [J. Chem. Phys. 91, 4026 (1989)] model for calculating the eigenvalue splitting in many-atom systems and have used it to calculate the ground-state splitting in several isotopomers of malonaldehyde. A potential-energy surface that includes all twenty-one vibrational degrees of freedom was constructed based on the available theoretical and experimental information. The results for calculations in which all atoms are allowed full three-dimensional motion are in good agreement with the experimentally measured values. Restricting the molecular motion to a plane leads to an increase in the splitting due to a decrease in the average height and width of the barrier to tunneling when the molecule is not allowed to vibrate transverse to the molecular plane. Low energy mode-specific excitations were used to study the sensitivity of the splitting to the motions of heavy atoms. The results show that the heavy atom motions have significant influence on the tunneling. This study demonstrates that simple semiclassical methods can be used to treat proton tunneling in large systems. © 1995 American Institute of Physics.