Chemical Sciences
Calculating the instability of nitrogen salts
Can the combination of a stable polynitrogen cation, such as N5+ , with a stable polynitrogen anion, such as N3– , lead to a stable ionic nitrogen allotrope, such as N5+ N3– ? Dixon et al. used high-level ab initio molecular orbital theory to conclusively determine whether the lattice energies of N5+ N3– and N5+ N5– are sufficient to stabilize them as solids. Their calculations showed that neither salt can be stabilized and that both should decompose spontaneously into N3 radicals and N2 . This conclusion was experimentally confirmed for the N5+ N3– salt.
D. A. Dixon, D. Feller, K. O. Christe, W. W. Wilson, A. Vij, V. Vij, H. D. Brooke Jenkins, R. M. Olson, and M. S. Gordon, “Enthalpies of formation of gas phase N3 , N3– , N5+ , and N5– from ab initio molecular orbital theory, stability predictions for N5+ N3– and N5+ N5– , and experimental evidence for the instability of N5+ N3– ,” J. Am. Chem. Soc. (in press). BES, BER, SciDAC, DARPA, AFOSR, NSF
A more accurate calculation of silicon dicarbide
Unlike many typical chemical systems, in which errors due to incomplete treatment of the one-electron basis and electron correlation are balanced and largely cancelled at modest levels of theory, silicon dicarbide has a barrier to linearity that requires both prodigious basis sets and high-order correlation treatments for accurate predictions. Kenny et al. calculated molecular partial-wave expansions for the T-shaped and linear forms of SiC2 using the MP2-R12/A method. Their results are the first of sufficient accuracy to call for revisitation of the spectroscopically determined barrier of 5.4 ± 0.6 kcal mol–1, moving it upward to 6.3 kcal mol–1.
J. P. Kenny, W. D. Allen, and H. F. Schaefer, “Complete basis set limit studies of conventional and R12 correlation methods: The silicon dicarbide (SiC2 ) barrier to linearity,” J. Chem. Phys. 118, 7353 (2003). BES, SciDAC
How electron impact excites a molecule
A major goal of studying electron-molecule collisions is to explore the mechanisms
that control the flow of energy from electronic to nuclear degrees of freedom.
McCurdy et al. have completed the first calculation on vibrational excitation
of a molecule (CO2 ) via coupled resonant states (Figure 4). The
multidimensional nuclear dynamics on these coupled surfaces was necessary
to explain quantitatively, for the first time, the experimental observations
on this system. These methods have been applied in calculations on electron-impact
dissociation of water, which is one of the principal mechanisms suspected
to initiate radiation damage of biological tissue in environments with ionizing
radiation.
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Figure 4 |
C. W. McCurdy, W. A. Isaacs, H.-D. Meyer, and T. N. Rescigno, “Resonant
vibrational excitation of CO2 by electron impact: Nuclear dynamics
on the coupled components of the 2
u
resonance,” Phys. Rev. A 67, 042708 (2003). BES
Designing nano-scale shock absorbers
Carbon nanotubes are the fundamental building blocks in many nanosystems. Rivera et al. performed molecular dynamics computational “experiments” on double-walled carbon nanotubes of varying lengths and diameters in which they pulled the inner nanotube out of the outer nanotube to several different distances and allowed the nanotube to retract and subsequently oscillate. They found that the oscillation is damped (hence acting like a nano shock absorber) and can be predicted by a very simple mechanical model that takes into account the sliding friction between the surfaces. The oscillation was around 1 GHz, consistent with prior experimental and theoretical calculations.
J. L. Rivera, C. McCabe, and P. T. Cummings, "Oscillatory behavior of double nanotubes under extension: A simple nanoscale damped spring," Nano Letters 3, 1001 (2003). BES, NSF
Solving the Schrödinger equation for large systems
Exact solutions of the Schrödinger equation within a given one-particle basis set, lie at the heart of quantum chemistry, providing invaluable benchmarks by which the accuracy of more approximate methods may be judged. Chan and Head-Gordon used a newly developed density matrix renormalization group algorithm to compute exact solutions of the Schrödinger equation for water at two geometries in a basis of 41 orbitals, demonstrating that it is now possible to obtain numerically exact solutions for systems considerably larger than can be contemplated using traditional full configuration interaction algorithms.
G. K. L. Chan and M. Head-Gordon, “Exact solution (within a triple zeta, double polarization basis set) of the electronic Schrödinger equation for water,” J. Chem. Phys. 118, 8551 (2003). BES
Quantum Monte Carlo study of the electronic excited states
Accurate computational predictions of molecular electronic excited states have proved more difficult to obtain than ground states. Ethylene is the prototypical p-electron system whose photochemical behavior is important in chemistry, biology, and technology. El Akramine et al. carried out a theoretical study of the transition between the ground state and the lowest triplet state of ethylene using the diffusion Monte Carlo method. The ground state atomization energy and heat of formation were consistent with experimental results, and the triplet-state atomization energy and heat of formation were also predicted.
O. El Akramine, A. C. Kollias, and W. A. Lester, Jr., “Quantum Monte Carlo study of the singlet-triplet transition in ethylene," J. Chem. Phys. 119, 1483 (2003). BES
Oxygen dissociation on copper steps
Copper and copper alloy catalysts are at the heart of important industrial processes, and copper also plays an important role in the microelectronics industry. Dissociation of dioxygen (O2) is the first step of oxygen chemistry on the surface of metals such as copper. Xu and Mavrikakis studied the adsorption and dissociation of dioxygen on copper steps using periodic self-consistent density functional theory calculations, and found that the adsorption of both atomic and molecular oxygen is enhanced on the stepped surface.
Y. Xu and M. Mavrikakis, "The adsorption and dissociation of O2 molecular precursors on Cu: The effect of steps," Surface Science 538, 219 (2003). BES, DOD, NSF, 3M
A new method for wave-packet propogation
Wang and Thoss have presented a method for wave-packet propagation that is based rigorously on a variational principle and is also sufficiently efficient to be applicable to complex dynamical problems with up to a few hundred degrees of freedom. This method can treat substantially more physical degrees of freedom than the original method, and thus significantly enhances the ability to carry out quantum dynamical simulations for complex molecular systems. The efficiency of the new formulation was demonstrated by converged quantum dynamical simulations for systems with a few hundred to a thousand degrees of freedom.
H. Wang and M. Thoss, “Multilayer formulation of the multiconfiguration time-dependent Hartree theory,” J. Chem. Phys. 119, 1289 (2003). BES, NSF, DAAD
Adding hydrogen to lean premixed natural gas
Hawkes and Chen performed direct numerical simulation of lean premixed methane-air and hydrogen-methane-air flames to understand how the addition of hydrogen can affect flame stability and pollutant formation in gas turbines. They found a greater rate of heat release in the enriched flame, which is explained by a difference in flame areas and influences of turbulent stretch on the local burning rates. They found that there may be a pollutant tradeoff between NO and CO emissions when using hydrogen-enriched fuels.
E. R. Hawkes and J. H. Chen, "Turbulent stretch effects
on hydrogen enriched lean premixed methane-air flames," 3rd Joint Meeting
of the U.S. Sections of the Combustion Institute, Chicago, IL (2003). BES