Structures of Small Tantalum Cluster Anions: Experiment and Theory.

We present a study of the structural evolution of tantalum cluster anions Ta, 6 ≤ ≤ 13 using a combination of trapped ion electron diffraction (TIED) experiments with a variety of electronic structure methods. A genetic algorithm has been employed to establish a set of likely structures for each cluster, their geometries and energetics have been studied by density functional theory (DFT), random phase approximation, and two-component (2C) DFT methods, which include spin-orbit coupling. We find octahedral structures for Ta and Ta as well as structures based on the pentagonal bipyramid (Ta and Ta).

Can alumina particles be formed from Al hydroxide in the circumstellar media? A first-principles chemical study.

AlOH has been detected in the circumstellar envelope of an oxygen-rich supergiant star (VY CMa) and is an abundant Al-containing system. Water molecules have also been detected, even in a vibrationally excited state. The coalescence of AlOH units and other processes involving AlOH could be the source of alumina-type particles.

Tunneling above the crossover temperature.

Quantum mechanical tunneling of atoms plays a significant role in many chemical reactions. The crossover temperature between classical and quantum movement is a convenient preliminary indication of the importance of tunneling for a particular reaction. Here we show, using instanton theory, that quantum tunneling is possible significantly above this crossover temperature for specific forms of the potential energy surface.

A high-accuracy theoretical study of the CH(n)P systems n = 1-3.

We have performed high-level electronic structure computations on the most important species of the CH(n)P systems n = 1-3 to characterize them and provide reliable information about the equilibrium and vibrationally averaged molecular structures, rotational constants, vibrational frequencies (harmonic and anharmonic), formation enthalpies, and vertical excitation energies. Those chemical systems are intermediates for several important reactions and also prototypical phosphorus-carbon compounds; however, they are often elusive to experimental detection. The present results significantly complement their knowledge and can be used as an assessment of the experimental information when available.

Theoretical study of the chemiluminescence of the Al + H2O reaction.

We performed surface hopping simulations of Al + H(2)O collisions by a direct semiempirical method, reproducing the conditions of previous beam-gas experiments. We observed the formation of the HAlOH species, that dissociates to AlOH + H after a lifetime of about 0.6 ps.

Size, adsorption site, and spin effects in the reaction of Al clusters with water molecules: Al17 and Al28 as examples.

The first step of the reaction of two relatively large Alm clusters (m = 17, 28) with a few water molecules has been studied by electronic structure methods. The complexes Alm·(H2O)n (n = 1-2) have been characterized, and the saddle points corresponding to the first step in the reaction, namely, formation of HAlmOH·(H2O)n-1 systems, have been located. The Al28 cluster is special in the sense it has two electronic states, singlet and triplet, which are very close in energy and also have quite similar equilibrium structures.

The role of the excited electronic states in the C+ + H2O reaction.

The electronic excited states of the [COH2]+ system have been studied in order to establish their role in the dynamics of the C+ + H2O-->[COH]+ +H reaction, which is a prototypical ion-molecule reaction. The most relevant minima and saddle points of the lowest excited state have been determined and energy profiles for the lowest excited doublet and quartet electronic states have been computed along the fragmentation and isomerization coordinates. Also, nonadiabatic coupling strengths between the ground and the first excited state have been computed where they can be large.

A dynamical study of the Si(+) + H(2)O reaction.

A dynamical study of the Si(+) + H(2)O reaction has been carried out by means of a quasiclassical trajectory method that decomposes the reaction into a capture step, for which an accurate analytical potential is employed, and an unimolecular step, in which the evolution of the collision complex is studied through a direct dynamics BHandHLYP/6-31G(d,p) method. The capture rate coefficient has been computed for thermal conditions corresponding to temperatures ranging from 50 to 1000 K. It is concluded that the main reason why the reaction rate is about 10 times smaller than the capture rate (at T = 298 K) is the topology of the potential energy surface of the ground state.

Quasiclassical trajectories on a finite element density functional potential energy surface: The C++H2O reaction revisited.

A new method for the representation of potential energy surfaces (PESs) based on the p version of the finite element method is presented and applied to the PES of the [COH2]+ system in order to study the C++H2O-->[COH]++H reaction through the quasiclassical trajectory method. Benchmark ab initio computations have been performed on the most relevant stationary points of the PES through a procedure that incorporates basis set extrapolations, the contribution of the core correlation energy, and scalar relativistic corrections. The electronic structure method employed to compute the many points needed to construct the PES is a hybrid density functional approach of the B3LYP type with geometry-dependent parameters, which improves dramatically the performance with respect of the B3LYP method.

Accurately solving the electronic Schrodinger equation of small atoms and molecules using explicitly correlated (r12-)MR-CI. VIII. Valence excited states of methylene (CH2).

We compute the adiabatic transition energies of methylene (CH(2)) from the ground state to the lowest electronically excited valence states using the r(12)-MR-ACPF-2 method with a large basis set and an extended reference space. We recall that this method aims at reaching the basis-set and full configuration interaction (CI) limits simultaneously. Our best excitation energies, T(e) (T(0)), are 9.