Sture Nordholm

Efficient microcanonical sampling for a preselected total angular momentum

Expressions for the molecular momentum density of states as a function of spatial configuration in an angular momentum resolved microcanonical ensemble are derived. These expressions are then used to formulate an efficient sampling scheme for the generation of spatial configurations or full phase space vectors in an ensemble where both energy and angular momentum are predetermined. Applications to simple diatomic (OH) and triatomic (H2O) molecular models are presented.

Quantized vibrational densities of states and ergodic energy transfer in molecular collisions

Abstract A simple method of generating quantized ro-vibrational densities of states based only on the thermodynamic data is presented. The result is a smooth function accurate over a wide energy range. Its simple power-law form allows the ergodic collision theory limits of energy-transfer moments to be calculated analytically, revealing the nature and magnitude of the quantum effects.

Molecular dynamics study of energy transfer in binary collisions of water molecules

Collisional energy transfer between two water molecules, one highly energized (reactant) and another thermally equilibrated (medium) molecule, has been studied by classical molecular dynamics simulation over a range of excitation energies and medium temperatures. The focus is on the dependence of the energy transfer efficiency on the excitation energy, the medium temperature, and the gross features as well as the details of the interaction between the molecules. High quality interaction potentials based on experimental data or quantum chemical calculations are used and the results are compared with those obtained by simpler potentials constructed from Lennard‐Jones pair potentials and point charges. The dipolar contribution to the interaction is varied and the molecules are partially or fully deuterated. The strong electrostatic interaction is found to yield efficient energy transfer for small impact parameters but also a large cross section for water collisions. The energy transfer efficiency is sensitiv...

A classical molecular dynamics study of the intramolecular energy transfer of model trans-stilbene

Abstract An approximate anharmonic potential energy surface for the S1 state of trans-stilbene has been constructed. This model surface, which is based on the potential of toluene and ethene, was optimised to the available trans-stilbene S1 state experimental frequencies and the trans → cis isomerisation barrier height. The surface was employed in classical trajectory studies where the main goal was to investigate the internal energy transfer of the isolated trans-stilbene molecule. Although the applicability of classical dynamics for large molecules at low vibrational energies is uncertain, we find that the beating patterns that have been observed for the isolated photoexcited molecule are qualitatively reproduced by the classical trajectories. The simulated quasiperiodic beating patterns persist at elevated molecular thermal energies where zero point energy effects are approximately taken into account. The simulation data indicate the presence of a bottleneck to intramolecular energy redistribution that is also robust with respect to increased molecular thermal energies. Similarly, the RRKM rate coefficient that is based on our surface overestimates the isomerisation rates that are observed experimentally. The experimental data that have been obtained for the isolated molecule and for the molecule under low pressure thermal conditions are discussed in the light of these results.

A simulation study of energy transfer in methyl isocyanide-inert gas collisions

Abstract The collisional energy transfer between inert gas atoms and highly excited methyl isocyanide (CH3NC) molecules has been studied by comparing the results of classical trajectory scattering calculations to ergodic-limit estimates obtained by Monte Carlo simulation. The dependence of the average energy transferred per collision on initial target (CH3NC) energy and medium (He, Ne, Ar, or Xe) temperature has been investigated. The energy transferred has been separated into rational and vibrational contributions to the total, with the not-unexpected result being that vibrational energy transfer is inefficient when compared to rotation. The implications of the near vibrational adiabaticity of the collisions have been addressed by extensions of the Ergodic Collision Theory. Additional investigations were performed to explore the sensitivity of the energy transfer to the strength of the interaction between reactant and medium particles, and to study the convergence of dynamical results and statistical predictions for increasing strengths of intermolecular interaction. A physically-grounded functional form for the energy transfer kernel P(E′|E) based on the statistical analysis is proposed, and is found to accurately reproduce the results of trajectory calculations.

Quantum effects on ion–dipole capture rate coefficients

A recent theoretical study of the ion–dipole capture process using classical variational and effective potential methods is extended to the quantum regime. Capture rate coefficients are calculated for the ion–dipole potential energy surface utilizing a model where the vibrations are frozen, the rotations are quantized and the translational motion is classical. Results from a simple adiabatic capture theory, variational transition state theory, and an effective potential method are presented and compared with the corresponding classical rate coefficients and with results from classical trajectory calculations for H+3 ions colliding with HCl, HCN, and CS. Comparison is also made with other theoretical and experimental results.

Simple estimation of thermal capture rates for ion-dipole collisions by canonical effective potential methods

Abstract Thermal capture rate coefficients are considered for collision partners which at long range interact by ion-dipole plus polarization potentials. The simple Langevin-Gioumousis-Stevenson theory is extended by mapping the true asymmetric multidimensional interaction potential onto an effective spherically symmetric potential obtained by analysis of canonical probability or flux equalities. Bound states are eliminated in the mapping as well as in the final rate coefficient. Capture rate coefficients are calculated for H3+ ions colliding with HCl, CS and HCN in a model where the ion is represented as a point charge and the target as a diatomic molecule. Corresponding calculations are carried out using canonical variational transition state theory. The theoretical results are compared with corresponding results obtained in classical trajectory calculations wherein the diatomic target (HCl, CS or HCN) is modeled as two point charges.

ENERGY TRANSFER IN COLLISIONS OF SMALL GAS PHASE CLUSTERS. COMPARISON OF MOLECULAR DYNAMICS AND STATISTICAL LIMIT ESTIMATES

Abstract Models of clusters are obtained by assigning Morse pairwise atomic interactions within the cluster and Lennard-Jones pairwise atomic interactions between clusters. Classical trajectory calculations and Monte Carlo sampling methods are then used to study energy transfer between a highly excited reactant cluster and a medium cluster and the statistical limits which apply to such energy transfer. The focus is on the complexity (size) dependence of the energy transfer efficiency. The impact parameter dependence of the average energy transferred per collision 〈 ΔE 〉, the cross section πb max 2 for the energy transfer and the dependence of 〈 ΔE 〉 on interaction strength are also studied. The initial energy of the target cluster and the temperature of the medium cluster are varied to allow comparison with the trends predicted by ergodic collision theory (ECT). Most trajectories are obtained for zero impact parameter b but the b -dependence of 〈 ΔE 〉 is studied and modeled by a simple functional form. The energy transfer kernel P ( E ′, E ) is examined. The shape found in the MD simulation for weak interaction collisions is well reproduced by an empirically constrained ECT model. When the interaction strength is enhanced to weak chemical bonding level P ( E ′, E ) is observed to split into a dominant ECT-like peak and an elastic central peak, i.e., a distinctly bimodal form with interesting mechanistic implications.

Complex formation in O+OH collisions: a two-step mechanism

Abstract A thorough theoretical analysis of O( 3 P) + OH( 2 Π) collisions by quasiclassical trajectory calculations and statistical rate coefficient estimation indicates that complex formation is of a two-step character. An oxygen atom approaching the diatomic molecule from the asymptotic uncoupled region first encounters a centrifugal barrier. Inside this barrier is a region of intermediate coupling strength, separated from an inner strong-coupling region by another potential barrier. The cross sections for complex formation and reaction to form O 2 + H show a strong dependence on the rotational quantum number. Despite these peculiarities the variational transition state theory and the canonical effective potential theory give a reasonable account of the rate coefficients.

Statistical theory of cluster cooling in rare gas. I. Energy transfer analysis for palladium clusters in helium

The cooling and heating of palladium clusters Pd13 and Pd55 by binary collisions with atoms of a surrounding helium gas are studied by means of molecular dynamics simulation. The efficiency of the collisional energy transfer is determined as a function of cluster and gas temperature and of cluster phase, the cluster being in either a solid or a liquid phase. A simple statistical analysis is presented for the energy transfer between a cluster and a rare gas atom. The analysis is based on an ergodic collision assumption of microcanonical relaxation in each collision. The deviation from this limiting law is collected in a collision efficiency factor which reflects incomplete energy redistribution during the lifetime of the collision complex. The thermal energy and change in heat capacity observed for the clusters at the freezing (melting) transition is accounted for by a parametrized density of states reflecting separate contributions from a solid and a molten structure. The same density of states is then us...