An extended hybrid density functional (X3LYP) with improved descriptions of nonbond interactions and thermodynamic properties of molecular systems

We derive here the form for the exact exchange energy density for a density that decays with Gaussian-type behavior at long range. This functional is intermediate between the B88 and the PW91 exchange functionals. Using this modified functional to match the form expected for Gaussian densities, we propose the X3LYP extended functional. We find that X3LYP significantly outperforms Becke three parameter Lee–Yang–Parr (B3LYP) for describing van der Waals and hydrogen bond interactions, while performing slightly better than B3LYP for predicting heats of formation, ionization potentials, electron affinities, proton affinities, and total atomic energies as validated with the extended G2 set of atoms and molecules. Thus X3LYP greatly enlarges the field of applications for density functional theory. In particular the success of X3LYP in describing the water dimer (with Re and De within the error bars of the most accurate determinations) makes it an excellent candidate for predicting accurate ligand–protein and ligand–DNA interactions

Evidence for O-atom exchange in the O(^1D) + N_2O reaction as the source of mass-independent isotopic fractionation in atmospheric N_2O

Recent experiments have shown that in the oxygen isotopic exchange reaction for O(^1D) + CO_2 the elastic channel is approximately 50% that of the inelastic channel [Perri et al., 2003]. We propose an analogous oxygen atom exchange reaction for the isoelectronic O(^1D) + N_2O system to explain the mass-independent isotopic fractionation (MIF) in atmospheric N_2O. We apply quantum chemical methods to compute the energetics of the potential energy surfaces on which the O(^1D) + N_2O reaction occurs. Preliminary modeling results indicate that oxygen isotopic exchange via O(^1D) + N_2O can account for the MIF oxygen anomaly if the oxygen atom isotopic exchange rate is 30–50% that of the total rate for the reactive channels

Reply to comment by Röckmann and Kaiser on "Evidence for O-atom exchange in the O(^1D) + N_2O reaction as the source of mass-independent isotopic fractionation in atmospheric N_2O"

Based upon the authors’ questioning of the existence of the C_(2v) intermediate, we have reviewed our evidence for the existence of this state. It now appears that this state was in fact an artifact of our calculation [Yung et al., 2004], and was a saddle point rather than a true minimum. Our desire to provide a timely response to this criticism has kept us from determining exactly what minimum structure will be obtained by a full minimization at the level of theory employed. However, it is clear that the C_(2v) symmetry of the compound is broken in such a way that the two N-O bonds are no longer equivalent. We are grateful to the authors for helping us resolve this issue

A candidate LiBH4 for hydrogen storage: Crystal structures and reaction mechanisms of intermediate phases

First-principles calculation and x-ray diffraction simulation methods have been used to explore crystal structures and reaction mechanisms of the intermediate phases involved in dehydriding of LiBH4. LiBH4 was found to dehydride via two sequential steps: first dehydriding through LiBH, followed by the dehydriding of LiBH through LiB. The first step, which releases 13.1 wt. % hydrogen, was calculated to have an activation barrier of 2.33 eV per formula unit and was endothermic by 1.28 eV per formula unit, while the second step was endothermic by 0.23 eV per formula unit. On the other hand, if LiBH4 and LiBH each donated one electron, possibly to the catalyst doped on their surfaces, it was found that the barrier for the first step was reduced to 1.50 eV. This implies that the development of the catalyst to induce charge migration from the bulk to the surface is essential to make LiBH4 usable as a hydrogen storage material in a moderate temperature range, which is also important to stabilize the low-temperature structure of Pnma (no. 62) LiBH on dehydrogenation. Consequently, the high 13.1 wt. % hydrogen available from the dehydriding of LiBH4 and LiBH and their phase stability on Pnma when specific catalysts were used suggest that LiBH4 has good potential to be developed as the hydrogen storage medium capable of releasing the Department of Energy target of 6.5 wt. % for a hydrogen fuel cell car in a moderate temperature range

Mechanism of the Stoddart-Heath Bistable Rotaxane Molecular Switch

We use quantum mechanics to characterize the structure and current−voltage performance of the Stoddart−Heath rotaxane-based programmable electronic switch. We find that the current when the ring is on the DNP is 37−58 times the current when the ring is on the TTF, in agreement with experiment (ratio of 10−100). This establishes the basis for iterative experimental−theoretical efforts to optimize systems for molecule-based electronics which we illustrate by predicting the effect of adding a group such as CN to the rotaxane

Atomistic simulations of adiabatic coherent electron transport in triple donor systems

A solid-state analogue of Stimulated Raman Adiabatic Passage can be implemented in a triple well solid-state system to coherently transport an electron across the wells with exponentially suppressed occupation in the central well at any point of time. Termed coherent tunneling adiabatic passage (CTAP), this method provides a robust way to transfer quantum information encoded in the electronic spin across a chain of quantum dots or donors. Using large scale atomistic tight-binding simulations involving over 3.5 million atoms, we verify the existence of a CTAP pathway in a realistic solid-state system: gated triple donors in silicon. Realistic gate profiles from commercial tools were combined with tight-binding methods to simulate gate control of the donor to donor tunnel barriers in the presence of cross-talk. As CTAP is an adiabatic protocol, it can be analyzed by solving the time independent problem at various stages of the pulse - justifying the use of time-independent tight-binding methods to this problem. Our results show that a three donor CTAP transfer, with inter-donor spacing of 15 nm can occur on timescales greater than 23 ps, well within experimentally accessible regimes. The method not only provides a tool to guide future CTAP experiments, but also illuminates the possibility of system engineering to enhance control and transfer times.Comment: 8 pages, 5 figure