Quasiclassical Trajectory Study of the Si + SH Reaction on an Accurate Double Many-Body Expansion Potential Energy Surface.

An accurate potential energy surface (PES) for the HSiS system based on MRCI+Q calculations extrapolated to the complete basis set limit is presented. Modeled with the double many-body expansion (DMBE) method, the PES provides an accurate description of the long-range interactions, including electrostatic and dispersion terms decaying as R-4, R-5, R-6, R-8, R-10 that are predicted from dipole moments, quadrupole moments, and dipolar polarizabilities, which are also calculated at the MRCI+Q level. The novel PES is then used in quasiclassical trajectory calculations to predict the rate coefficients of the Si + SH → SiS + H reaction, which has been shown to be a major source of the SiS in certain regions of the interstellar medium. An account of the zero-point energy leakage based on various nonactive models is also given. It is shown that the reaction is dominated by long-range forces, with the mechanism Si + SH → SiSH → SSiH → SiS + H being the most important one for all temperatures studied. Although SSiH corresponds to the global minimum of the PES, the contribution from the direct reaction Si + SH → SSiH → SiS + H is less than 0.5% for temperatures higher than 500 K. The rovibrational distributions of the products are calculated using the momentum Gaussian binning method and show that as the temperature is increased the average vibrational quantum number decreases while the rotational distribution spreads up to larger values.

Accurate DMBE potential-energy surface for CNO(2A″) and rate coefficients in C(3P)+NO collisions.

A realistic double many-body expansion potential energy surface (PES) is developed for the 2A″ state of the carbon-nitrogen-oxygen (CNO) system based on MRCI-F12/cc-pVQZ-F12 ab initio energies. The new PES reproduces the fitted points with chemical accuracy (root mean square deviation up to 0.043 eV) and explicitly incorporates long range energy terms that can accurately describe the electrostatic and dispersion interactions. Thermal rate coefficients were computed for the C(3P) + NO(2Π) reaction for temperatures ranging from 15 K to 10 000 K, and the values are compared to previously reported results. The differences are rationalized, and the major importance of long range forces in predicting the rate coefficients for barrierless reactions is emphasized.

Stability of neutral molecular polynitrogens: energy content and decomposition mechanisms.

The potential application of all-nitrogen molecules as high energy density materials (HEDMs) has been attracting considerable scientific effort. If stable enough to be synthesized and stored, these systems may be used as a green source of energy. However, it is very difficult to obtain these structures under mild experimental conditions. Theoretical chemistry may aid in the search for polynitrogens that are more likely to have experimental usability. The barriers towards decomposition are an effective way to assess their stability, but these have not been thoroughly studied. Most of the previous effort in this direction focus on a single N x case, each employing different accuracy levels, and the decomposition of caged structures has been little explored. Here we explore the stability and decomposition of several neutral molecular polynitrogens of different sizes and shapes using a common and accurate theoretical framework in order to compare among them, search for patterns and identify potential candidates for synthesis. We focus especially on new caged geometries, and our results indicate that the prismatic ones can be expected to present higher energy densities and be very stable with respect to unimolecular decomposition. It is shown that the energy content can be clearly stratified between chain, ring, cage and prismatic cage structures.

A method for predicting basins in the global optimization of nanoclusters with applications to AlxCuy alloys.

The problem of obtaining the geometrical configuration of a molecule that minimizes its potential energy is a very complicated one for a series of applications, ranging from determining the structure of biological macromolecules to nanoclusters of atoms. Global optimization tools are available for this task, and many of them are based in performing successive local optimizations, where the starting geometries for these steps are determined by an intelligent algorithm. Here we develop a method to save computing time in the optimization of nanoclusters by predicting if a given minimum has been previously visited during local optimization steps. Our application to Cu-Al nanoalloys indicates that it is possible to save a substantial amount of computational cost. The application also reveals new promising AlxCuy clusters and explain their stabilities in terms of the jellium model.

Accurate Potential Energy Surface for Quartet State HN2 and Interplay of N(4S) + NH(X̃3Σ-) versus H + N2(A3Σu+) Reactions.

A global potential energy surface for the lowest quartet state of HN2 is reported for the first time from accurate multireference ab initio calculations extrapolated to the complete basis set limit using the double many-body expansion method. All its stationary points are characterized, with the lowest quartet of HN2 predicted to have a bent global minimum 36 kcal mol-1 below the N(4S) + NH(X̃3Σ-) asymptote, from which it is barrierlessly achievable. The entire set of calculated ab initio points has been fitted for energies up to 1000 kcal mol-1 above the global minimum with an RMSD of 0.89 kcal mol-1, a gap comprising all identified stationary points. Special care is taken in modeling the involved long-range forces and cusps caused by crossing seams. The novel PES prompts for the calculation of rate constants for several unexplored reactions that are relevant for combustion, plasma, and atmospheric chemistry.

Quasiclassical Study of the C(3P) + NO(X2Π) and O(3P) + CN(X2Σ+) Collisional Processes on an Accurate DMBE Potential Energy Surface.

The predicted rate constants for C + NO and O + CN collisions in three potential energy surfaces (PESs) for the 2A' state of the CNO molecule are compared using quasiclassical trajectories. Different temperature dependencies are obtained for the C + NO reaction, which are explained in terms of the long-range properties of the PESs. Recommended values and mechanistic details are also reported. For O + CN collisions, a better agreement between the theoretical results is found, except for temperatures below 100 K.

A genetic algorithm survey on closed-shell atomic nitrogen clusters employing a quantum chemical approach.

The DFT potential energy hypersurfaces of closed-shell nitrogen clusters up to ten atoms are explored via a genetic algorithm (GA). An atom-atom distance threshold parameter, controlled by the user, and an "operator manager" were added to the standard evolutionary procedure. Both B3LYP and PBE exchange-correlation functionals with 6-31G basis set were explored using the GA. Further evaluation of the structures generated were performed through reoptimization and vibrational analysis within MP2 and CCSD(T) levels employing larger correlation consistent basis set. The binding energies of all stable structures found are calculated and compared, as well as their energies relative to the dissociation into N2, [Formula: see text] and [Formula: see text] molecules. With the present approach, we confirmed some previously reported polynitrogen structures and predicted the stability of new ones. We can also conclude that the energy surface profile clearly depends on the calculation method employed.

Accurate Explicit-Correlation-MRCI-Based DMBE Potential-Energy Surface for Ground-State CNO.

We report a new global double many-body expansion potential energy surface for the ground state of the CNO(2A') manifold, calculated by the explicit correlation multireference configuration interaction method. The functional form was accurately fitted to 3701 ab initio points with a root mean squared deviation of 0.99 kcal mol-1. All stationary points reported in previous forms are systematically described and improved, in addition to three new ones and a characterization of an isomerization transition state between the CNO and NCO minima. The novel proposed form gives a realistic description of both short-range and long-range interactions and hence is commended for dynamics studies.

The effect of intersystem crossings in N((2)D) + H2 collisions.

The transitions between quartet and doublet states of the NH2 molecule are studied for the first time, allowing the evaluation of the N((4)S) + H2 reactive channel. High level ab initio calculations of the spin-orbit coupling are performed over the whole configurational space of the NH2 molecule and fitted to a proposed analytic form. Quasiclassical trajectories coupled with the surface hopping method are employed to calculate reaction cross section and rate constants. The reaction is largely affected by the initial rovibrational states of H2, while the formation of long-lived complexes enhances the reaction probability.

Modeling cusps in adiabatic potential energy surfaces.

A method for modeling cusps on adiabatic potential energy surfaces without the need for any adiabatic-to-diabatic transformation is presented and shown to be successfully applied to the (2)A″ state of NO2. The more complicated case of a system with permutationally equivalent crossing seams is also examined and illustrated by considering the two first (2)A' states of the nitrogen trimer.

Exploring the utility of many-body expansions: a consistent set of accurate potentials for the lowest quartet and doublet states of the azide radical with revisited dynamics.

Starting from reliable double many-body expansion potentials calibrated with energies calculated at the state-of-the-art ab initio level but of varying accuracies, it is shown that a single scaling parameter suffices to bring them into consistency while mimicking key experimental data and leaving mostly unaffected crossing seams of the conical type. This emerges as a valuable asset of their underlying formalism. Use of the novel potentials for studying the dynamics of the N(4S/²D) + N2(¹Σ(g)⁺) reactions yields results in excellent agreement with the most accurate available data.

Growth analysis of sodium-potassium alloy clusters from 7 to 55 atoms through a genetic algorithm approach.

The potential energy hypersurface associated with sodium-potassium alloy clusters is explored via an enhanced genetic algorithm, where two different operators are added to the standard evolutionary procedure. Based on the recent result that the empirical Gupta many-body potential yields reasonable results for clusters with more than seven atoms, we have employed this function in the evaluation of the energies. Agglomerates from seven to the well-established 55-atom structure are studied, and their second-order energy difference and excess energies are calculated. It is found that the most stable alloys (compared to the homonuclear counterparts) are found with the proportion of sodium atoms in the range of 30 to 40%. The experimental propensity of core-shell segregation is successfully predicted by the current approach.

Electronic Quenching in N((2)D) + N2 Collisions: A State-Specific Analysis via Surface Hopping Dynamics.

The electronic quenching reaction N((2)D) + N2 → N((4)S) + N2 is studied using the trajectory surface hopping method and employing two doublet and one quartet accurate potential energy surfaces. State-specific properties are analyzed, such as the dependence of the cross section on the initial quantum state of the reactants, vibrational energy transfer, and rovibrational distribution of the product N2 molecule in thermalized conditions. It is found that rotational energy on the reactant N2 molecule is effective in promoting the reaction, whereas vibrational excitation tends to reduce the reaction probability. For initial states and collision energy thermalized in an initial bath, it is found that the products are "hotter", both vibration and rotation wise.

Accurate study of the two lowest singlet states of HN3: stationary structures and energetics at the MRCI complete basis set limit.

The two lowest singlet potential energy surfaces of the hydrazoic acid are explored using high-level quantum chemistry calculations, revealing isomeric forms, transition states, and a new path for the nitrogen ring-closing mechanism in the ground state. The reaction of cyc-N3 with hydrogen is shown to proceed through a barrierless path that dissociates to N2 + NH((1)Δ) without reaching the HN3 global minimum. Several intersections between the two states are localized for both N3 isomers near the N3 + H dissociation. The energies of stationary structures and dissociation asymptotes are obtained with the multireference configuration interaction method at the complete basis set limit. A comparison with recent experimental data regarding the H-N3 bond strength and the dissociation barrier in the excited state shows that the present results are of chemical accuracy (~1 kcal mol(-1)).

N(4S ∕2D)+N2: accurate ab initio-based DMBE potential energy surfaces and surface-hopping dynamics.

This work gives a full account of the N((4)S∕(2)D)+N(2)((1)Σ(g)(+)) interactions via accurate electronic structure calculations and study of the involved exchange reactions. A 2 × 2 diabatic representation of the potential energy surface is suggested for N(3)((2)A(')), which, combined with the two previously reported adiabatic forms for (2)A(") and another for (4)A("), completes the set of five global potentials required to study the title collisional processes. The trajectory results provide the first N((2)D)+N(2) rate constants, and allow a comparison with the ones for N((4)S)+N(2). Nonadiabatic effects are estimated by surface hopping, and the geometrical phase effect assessed by following the trajectories that encircle the crossing seam.

Ab initio based double-sheeted DMBE potential energy surface for N3(2A″) and exploratory dynamics calculations.

We report a global accurate double-sheeted potential energy surface for the lowest doublet states with (2)A″ symmetry of the N(3) radical using the double many-body expansion method. The functional form ensures by construction the degeneracy of the two adiabatic sheets along the D(3h) line and the corresponding cusp behavior. Calibrated from multireference configuration interaction energies, it reproduces all the predicted stationary structures on both sheets and ensures a correct description of the dissociation limits. A test quasiclassical trajectory study of N((2)D) + N(2) collisions is also reported in the lowest adiabatic sheet of the potential energy surface. The results commend it for both classical and quantum dynamics studies, while serving as a building block for the potential energy surfaces of larger nitrogen allotropes and azides.

Quasiclassical trajectory study of atom-exchange and vibrational relaxation processes in collisions of atomic and molecular nitrogen.

Quasiclassical trajectories have been integrated to study the exchange reaction of molecular nitrogen in collisions with atomic nitrogen for temperatures over the range of 1273 < or = T (K) < or = 10,000. A recently proposed potential energy surface for the ground A'' quartet state of the system has been employed. If compared to previous theoretical studies, the results of the present work show a higher reactivity due to a lower barrier, with a study of the effect of this height in the thermal rate constant being also performed. Vibrational energy transfer via chemical reaction and/or inelastic collisions are also studied.

Accurate double many-body expansion potential energy surface for N3((4)A'') from correlation scaled ab initio energies with extrapolation to the complete basis set limit.

A new global potential energy surface is reported for the (4)A'' ground electronic state of the N(3) system from double many-body expansion theory and an extensive set of accurate ab initio energies extrapolated to the complete basis set limit. It shows three equivalent metastable potential wells for C(2v) geometries that are separated from the three N((4)S) + N(2) asymptotes by energy barriers as predicted from previous ab initio work. The potential well and barrier height now predicted lie 42.9 and 45.9 kcal mol(-1) above the atom-diatom dissociation limit, respectively, being about 1 kcal mol(-1) lower than previous theoretical estimates. The ab initio calculations here reported predict also a (4)B(1)/(4)A(2) conical intersection and reveal a new minimum with D(3h) symmetry that lies 147 kcal mol(-1) above the atom-diatom asymptote. All major topographical features of the potential energy surface are accurately described by the DMBE function, including the weakly bound van der Waals minima at large atom-diatom separations.

Energy-switching potential energy surface for the water molecule revisited: A highly accurate singled-sheeted form.

A global ab initio potential energy surface is proposed for the water molecule by energy-switching/merging a highly accurate isotope-dependent local potential function reported by Polyansky et al. [Science 299, 539 (2003)] with a global form of the many-body expansion type suitably adapted to account explicitly for the dynamical correlation and parametrized from extensive accurate multireference configuration interaction energies extrapolated to the complete basis set limit. The new function mimics also the complicated Sigma/Pi crossing that arises at linear geometries of the water molecule.