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International audience; In the present theoretical work, we investigated the stationary points (minima and transition states) on the ground state potential energy surfaces of neutral and ionic 1,4-diazabicyclo[2.2.2]octane (DABCO) - Arn 0,+1 (n=1-4) clusters. As established in our systematic work on DABCO - Ar cluster (Mathivon et al., J Chem Phys 139:164306, 2013), the (R)MP2/aug-cc-pVDZ level is accurate enough for validating the prediction of stable forms. For n = 1 and 2, further computations at the MP2/aug-cc-pVTZ level confirm these assumptions. We show that some of the already known isomers of these heteroclusters derived using lower levels of theory are not realistic. More interestingly, our work reveals that DABCO is subject to slight deformations when binding to a small number of Ar atoms. Moreover, we computed the potential energy surfaces of the lowest singlet electronic states of DABCO - Arn (n=1-3) and of DABCO+ - Arn (n=1-3), and the transition moments for the Sp (p=1-3) ← S0 neutral transitions. These electronic states are found to be Rydberg in nature. The shape of their potentials is mainly repulsive with slight stabilization in the S2 potentials. Finally, the effects of microsolvation of DABCO in Ar clusters in ground and electronic excited states are discussed. The photophysical and photochemical dynamics of these electronic states may be complex.

Using state-of-the-art theoretical methods, we investigate the lowest electronic states of singlet and triplet spin multiplicities of HSNO. These computations are done using configuration interaction ab initio methods and the aug-cc-pV5Z basis set. One-dimensional cuts of the six-dimensional potential energy surfaces of these electronic states along the HS, SN stretches and HSN, SNO bending and torsion coordinates are calculated. Several avoided crossings and conical intersections are found. We computed also radiative lifetimes and spin-orbit couplings of these electronic states. Our work shows that the dynamics on these excited states is very complex, and suggest that multi-step mechanisms will populate the ground state via radiationless processes or lead to predissociation or intramolecular isomerization. For instance, these potentials are used to propose mechanisms for the IR, Vis, and UV light-induced cis-trans interconversions of HSNO and reactivity towards HS + NO and H + SNO products. Our findings are in good agreement with previous experimental studies on the photochemistry of HSNO. The atmospheric implication of HSNO is also discussed.

Accurate ab initio computations of structural and spectroscopic parameters for the HPS/HSP molecules and corresponding cations and anions have been performed. For the electronic structure computations, standard and explicitly correlated coupled cluster techniques in conjunction with large basis sets have been adopted. In particular, we present equilibrium geometries, rotational constants, harmonic vibrational frequencies, adiabatic ionization energies, electron affinities, and, for the neutral species, singlet-triplet relative energies. Besides, the full-dimensional potential energy surfaces (PESs) for HPS(x) and HSP(x) (x = -1,0,1) systems have been generated at the standard coupled cluster level with a basis set of augmented quintuple-zeta quality. By applying perturbation theory to the calculated PESs, an extended set of spectroscopic constants, including τ, first-order centrifugal distortion and anharmonic vibrational constants has been obtained. In addition, the potentials have been used in a variational approach to deduce the whole pattern of vibrational levels up to 4000 cm(-1) above the minima of the corresponding PESs.

The hydroxymethyl peroxy (HMOO) radical is a radical product from the oxidation of non-methane hydrocarbons. The present study provides theoretical prediction of critical spectroscopic features of this radical that should aid in its experimental characterization. Structure, rotational constants, and harmonic frequencies are presented for the ground and first excited electronic states of HMOO. The adiabatic transition energy for the Ã←X̃ process is 7360 cm(-1), suggesting that this transition, occurring in the mid to near infrared, is the most promising candidate for observing the radical spectroscopically. The band origin of the Ã←X̃ transition of HMOO is calibrated and benchmarked with the corresponding state of the HOO radical, which is experimentally and theoretically well characterized.

International audience; Franzreb and Williams at Arizona State University detected recently the MgO2+ molecular species in the gas phase. Here we report a very detailed theoretical investigation of the low-lying electronic states of this dication including their potentials, spin-orbit, rotational and radial couplings. Our results show that the potential energy curves of the dicationic electronic states have deep potential wells. This confirms that this dication does exist in the gas phase; it is a thermodynamically stable molecule in its ground state, and it has several excited long-lived metastable states. The potential energy curves are used then to predict a set of spectroscopic parameters for the bound states of MgO2+. We have also incorporated these potentials, rotational and radial couplings in dynamical calculations to derive the cross sections for the charge transfer Mg2+ + O -> Mg+ + O+ reaction in the 1-10(3) eV collision energy domain via formation-decomposition of the MgO2+ dication. Our work shows the role of MgO2+ in the Earth ionosphere and more generally in atmospheric processes in solar planets, where this reaction efficiently participates in the predominance of Mg+ cations in these media compared to Mg and Mg2+.

Ab initio molecular orbital methods have been employed to determine the molecular structure, vibrational frequencies, and stability of the H(2)O(2)-H(2)O complex. These parameters were determined using coupled-cluster methods with Dunning correlation consistent basis sets. Vertical excitation energies for the lowest four excited states are calculated with the multireference internally contracted configuration interaction methods and coupled cluster equation of motion methods. No significant difference was observed between the first vertical excited state energies of H(2)O(2) within the H(2)O(2)-H(2)O complex.

This work reports the results of high level ab initio calculations of the OC-HCO(+) complex and the SC-HCS(+) complex and their hydrogen migration transition states. Geometry optimizations are performed at the CCSD(T)/aug-cc-pV5Z level of theory. Subsequent frequency calculations are carried out at the CCSD(T)/aug-cc-pVQZ level of theory. Additional geometry optimizations and harmonic frequency calculations for all the species involved in this study have been done with the explicitly correlated CCSD(T)-F12 method with the aug-cc-pVTZ and VTZ-F12 basis set. The geometries, rotational constants, harmonic vibrational frequencies, and energetics of the species involved in the complex are reported. These methods result in accurate computational predictions that have mean deviations for bond lengths, rotational constants, and vibrational frequencies of 0.001 Å, 163 MHz, and 46 cm(-1), respectively. These results provide essential spectroscopic properties for the complexes that can facilitate both laboratory and interstellar observations, and they also provide a comparison between oxygen and sulfur complex observability based on thermodynamic stability.

Using first-principle methods, we characterized Ar(m)BeO(q+) (0 ≤ m ≤ 3 and 0 ≤ q ≤ 2) and BeO(q+) (3 ≤ q ≤ 5) multicharged ions (MCIs). This includes their structural parameters, relative stabilities and vibrational wavenumbers. Our calculations confirm the existence of the ArBeO complex. In addition, we found (meta-)stable neutral and ionic complexes such as ArBeO(+), ArBeO(2+), Ar(2)BeO, Ar(2)BeO(+), Ar(2)BeO(2+), Ar(3)BeO, Ar(3)BeO(+) and Ar(3)BeO(2+). The analysis of the structural parameters and of the electron density differences shows that a strong perturbation in the electronic structure of the BeO(q+) (q = 1,2) moiety occurs upon complexation, resulting in a major increase in covalency of these multicharged ions (MCIs). The consequences of solvation of MCIs in argon matrices are pointed out. More generally, the effects on the spectroscopy of MCIs trapped on cold matrices are discussed.

International audience; The potential energy curves and the spin-orbit couplings for all the electronic states correlating with the lowest dissociation asymptote of S2O have been calculated by accurate ab initio approaches, including full-valence complete active space self-consistent field and internally contracted multi-reference configuration interaction. One-dimensional cuts of the three-dimensional potential energy surfaces are presented for linear and bent geometries, providing useful energetic information on the different paths of formation of S2O, starting from various fragments.

International audience; First-order calculations of spin-orbit constants, dipole moments and carbon-sulphur distances have been performed for HCnS (n = 1-12) radicals in the 2 electronic ground state. It is found that these molecular properties alternate with even or odd numbers of carbon atoms in the chains and the spin-orbit constant ASO is around -300 cm-1 for n even and about +120 cm-1 for n odd throughout the series. This agrees with the experimentally determined value of about -270 cm-1 for HC2S, but the theoretically predicted ASO values are much larger than the values given for HC3S and HC4S from a fit of their millimeter-wave spectra. Values that were too low were also assumed in the analysis of the rotational spectra of n = 4-8.

International audience; The piezoelectric constants of aluminum nitride and zinc oxide wurtzite-type nanowires have been calculated by first-principles approach and compared with the data obtained for bulk structures. The methods adopted here include the Hartree-Fock and density functional theory procedures in their periodic formulation. The piezoelectric response is seen to be higher in nanowires than in bulk. In zinc oxide wires, it is found that the piezoelectric constant is significantly enhanced by partial substitution of zinc by mercury.

International audience; The theoretical calculations of dipole moments provide reliable compact information required for the evaluation of radiative transition probabilities in the electronic states of small molecular ions. The comparison of the computed line intensities and dipole derivatives with available experimental data in neutral molecules demonstrates that they can be calculated ab initio with a precision of a few percent, hence with an accuracy sufficient for most of the experimental applications.

In high precision Penning trap mass spectrometry the cyclotron frequency of a polarizable ion is perturbed due to the Stark interaction with the motional electric field. For polar diatomic molecular ions, which have adjacent rotational levels of opposite parity, these shifts can be particularly large---especially for the lowest rotational levels, which are those occupied by ions stored for many hours in cryogenic Penning traps. In order to provide corrections to precision atomic mass measurements, we consider the calculation of orientation polarizabilities of $\mathrm{C}{\mathrm{O}}^{+}$ and the positive ions of the first and second row diatomic hydrides, $\mathrm{Li}{\mathrm{H}}^{+}$ to $\mathrm{Ar}{\mathrm{H}}^{+}$. Dipole moments for these ions have been calculated using the restricted coupled cluster method with perturbative triples and large basis sets. Using these dipoles and an effective Hamiltonian, we have obtained rotational-state dependent polarizabilities of the open-shell diatomic ions $\mathrm{C}{\mathrm{O}}^{+}$, $\mathrm{N}{\mathrm{H}}^{+}$, $\mathrm{O}{\mathrm{H}}^{+}$, $\mathrm{F}{\mathrm{H}}^{+}$, $\mathrm{P}{\mathrm{H}}^{+}$, $\mathrm{S}{\mathrm{H}}^{+}$, and $\mathrm{Cl}{\mathrm{H}}^{+}$. Results are given for those rotational levels that are significantly populated at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, for magnetic fields up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. For the remaining first and second row hydride cations, polarizabilities at the magnetic fields of interest can be obtained from a simple formula valid for closed-shell molecules. Conversely, in cases where the polarizability shifts can be measured, our results enable experimental determination of dipole moments.

The low-energy electronic states of NSO anion and its SNO isomeric form for the singlet, triplet, and quintet spin multiplicities have been investigated by accurate ab initio approaches and large atomic basis sets. One-dimensional cuts of the three-dimensional potential energy surfaces (PESs) along selected interatomic distances and bending angles for these states have been calculated to assess the formation and stability of NSO(-) and SNO(-) in the gas phase. Results show that these anions have two low-energy states (X̃(1)A(') and 1(3)A″) that are bound and stable with respect to electron detachment. Owing to the energetic position of the dissociating asymptotes of the neutral and anionic species, several electronic excited states are suggested to be stable with respect to the electron autodetachment process in the long-range parts of the potentials before reaching the molecular region. The nature of the PESs in these regions and their implications and effects on the formation of SNO(-) from atomic and molecular fragments are discussed. This information is essential for a better understanding of the potential role of these species in diverse media.

International audience; Using coupled-cluster approach full six-dimensional analytic potential energy surfaces for two cyclic SiC3 isomers [C-C transannular bond (I) and Si-C transannular bond (II)] have been generated and used to calculate anharmonic vibrational wave functions. Several strong low-lying anharmonic resonances have been found. In both isomers already some of the fundamental transitions cannot be described within the harmonic approximation. Adiabatic electron affinities and ionization energies have been calculated as well. The Franck-Condon factors for the photodetachment processes c-SiC3-(I)-> c-SiC3(I) and c-SiC3-(II)-> c-SiC3(II) are reported. (c) 2006 American Institute of Physics.

The neutral tetrasulfur and the trisulfur and tetrasulfur anion radicals were studied by density functional theory (DFT) employing the hybrid functionals B3LYP, PBE0 and PB95 and large basis sets. The spectral data were obtained by time-dependent density functional theory (TD-DFT). The calculated absorption wavelengths for the calculated vertical electronic transitions in the visible region were compared with the wavelengths of the absorption maxima of ultramarines and of synthetic sulfur compounds measured in matrices or in solution. The chromophore of ultramarine blue was considered as a test case and the chromophore of ultramarine red studied in several model calculations. The calculated transition energies and intensities favour the cis tetrasulfur chromophore of red ultramarine. The excellent agreement between the theoretical and experimental absorption wavelengths of the color bands supports earlier assignments. The electronic structure of cis and trans tetrasulfur is discussed in terms of ylidic and biradical resonance structures by natural resonance theory. The singlet ground states display RKS/UKS instability. The broken-symmetry UKS structure is of lower energy than the RKS structure, which does, however, not influence the molecular and electronic structure significantly.

International audience; A double minimum six-dimensional Potential energy surface (PES) is determined in symmetry coordinates for the most stable rhombic (D-2h) B-4 isomer in its (1)A(g) electronic ground state by fitting to energies calculated ab initio. The PES exhibits a barrier to the D-4h square structure of 255 cm(-1). The vibrational levels (J=0) are calculated variationally using an approach which involves the Watson kinetic energy operator expressed in normal coordinates. The pattern of about 65 vibrational levels up to 1600 cm-1 for all stable isotopomers is analyzed. Analogous to the inversion in ammonia-like molecules, the rhombus rearrangements lead to splittings of the vibrational levels. In B-4 it is the B-1g (D-4h mode which distorts the square molecule to its planar rhombic form. The anharmonic fundamental vibrational transitions of B-11(4) are calculated to be (splittings in parentheses): G(O) = 2352(22) cm(-1), v(1)(A(1g)) - 1136(24) cm(-1,) v(2)(B-1g)=209(144) cm(-1) v(3)(B-2g)=1198(19)cm(-1), v(4)(B-2u) = 271(24) cm(-1), and v(5) (E-u) = 1030( 166) cm(-1) (D-4h notation). Their variations in all stable isotoporners were investigated. Due to the presence of strong anharmonic resonances between the B-1g in-plane distortion and the B-2u, out-of-plane bending modes. the hiaher overtones and combination levels are difficult to assign unequivocally.

We have compared different strategies for ab initio quantum chemistry density matrix renormalization group treatments. The two starting orbital blocks include all valence and active orbitals of the reference complete active space self consistent field wave function. To generate the remaining blocks, we propose following the order of the contributions to the correlation energy: a posteriori using approximate occupation numbers or a priori, using a Moller–Plesset type of arguments, by explicit evaluation of second-order interactions. We have compared two different schemes for orbital localization to identify the important and less important orbital interactions and simplify the generation of the orbital blocks. To truncate the expansion we have compared two approaches, keeping constant the number m of components or the threshold λ to fix the residue of the expansion at each step. The extrapolation of the energies is found to provide accurate estimates of the full configuration interaction energy, making the expansion independent on the actual values of the two parameters m and λ. We propose to generate the factors for the two blocks from ground and excited eigenvectors of the Hamiltonian matrix.

A new implementation of the density matrix renormalization group is presented for ab initio quantum chemistry. Test computations have been performed of the dissociation energies of the diatomics Be2, N2, HF. A preliminary calculation on the Cr2 molecule provides a new variational upper bound to the ground state energy.

High level ab initio electronic structure calculations at different levels of theory have been performed on HNP and HPN neutrals, anions, and cations. This includes standard coupled cluster CCSD(T) level with augmented correlation-consistent basis sets, internally contacted multi-reference configuration interaction, and the newly developed CCSD(T)-F12 methods in connection with the explicitly correlated basis sets. Core-valence correction and scalar relativistic effects were examined. We present optimized equilibrium geometries, harmonic vibrational frequencies, rotational constants, adiabatic ionization energies, electron affinities, vertical detachment energies, and relative energies. In addition, the three-dimensional potential energy surfaces of HNP(-1,0,+1) and of HPN(-1,0,+1) were generated at the (R)CCSD(T)-F12b∕cc-pVTZ-F12 level. The anharmonic terms and fundamentals were derived using second order perturbation theory. For HNP, our best estimate for the adiabatic ionization energy is 7.31 eV, for the adiabatic electron affinity is 0.47 eV. The higher energy isomer, HPN, is 23.23 kcal∕mol above HNP. HPN possesses a rather large adiabatic electron affinity of 1.62 eV. The intramolecular isomerization pathways were computed. Our calculations show that HNP(-) to HPN(-) reaction is subject to electron detachment.