Gilberte Chambaud

A variational method for the calculation of spin-rovibronic levels of Renner-Teller triatomic molecules

This paper reports an extension of our method, introduced several years ago, for the variational treatment of the rovibronic levels of Renner-Teller triatomic molecules. A more complete treatment now introduces the effect of electron spin. Thus new terms enter the Hamiltonian because (a) the total-angular-momentum operator is replaced by - Ľ - S in the kinetic-energy operator, and (b) spin-orbit effects must be introduced. Here we include the latter through a semi-empirical form AĽ · S . Expansion functions having the correct symmetry (Σg +, Σu +, Σg -, Σu -), for D∞h molecules are derived. Hence the present approach takes into account the full dimensionality, anharmonicity and rotation-vibration coupling, as well as the coupling of all relevant angular momenta. This is particularly important for open-shell molecules and/or for electronically degenerate states. For the 2Πg electronic ground state of the CO2 + ion the spin-rotation and spin-orbit split Renner-Teller levels have been calculated using three-...

Ab initio Hartree-Fock instabilities in closed-shell molecular systems

The Hartree-Fock instability of twelve polyatomic systems is studied at theab initio level. It is found that all the systems with at least one double bond, exhibit a non-singlet instability. On the other hand instabilities of singlet type as well as instabilities of non-real type appear only in a small number of cases. The existence of these instabilities is discussed with respect to the location of low-lying excited states and to the weight of ionic structure.

Theoretical study of the Renner-Teller à 2A1-[Xtilde] 2B1 system of NH2

Three-dimensional potential energy functions of the A 2B1-[Xtilde] 2A1 bent-bent Renner-Teller system of NH2 have been generated and used in calculations of the rovibronic energy levels (J ⩽ 9>/2) by a variational approach accounting for anharmonicity, rotation-vibration, electronic angular momenta and electron-spin coupling effects. For low values of N, the theoretical rovibronic term values up to about 20 000 cm-1 agree with available experimental data to within a few wavenumbers. For the first time, all existing rovibronic states for N = 0 and 1 are given and assigned up to 18 000 cm-1. For higher levels including stretching modes the Fermi polyads have been used in the assignments.

Bent valence excited states of CO2

The conical intersection regions on the potential energy functions of the valence excited 1,3Σu−, 1,3Δu, 3Σu+, and 1,3Πg states of CO2 have been investigated by ab initio calculations. Using large scale multireference configuration interaction the ordering of the lowest valence excited states of CO2 has been calculated to be 3B2, 3A2, 1A2 followed by 1B2. All these states have bent equilibrium structures and lie energetically below their dissociation asymptotes. The near equilibrium parts of the potential energy functions have been mapped in three dimensions by multiconfiguration self‐consistent field calculations. The 1,3B2 and 1,3A2 states differ in their equilibrium angles (118° and 127°, respectively), and have much longer equilibrium distances (around 1.26 A) than the electronic ground state. Anomalously low values of ca. 800 cm−1 have been calculated for the wave number of the antisymmetric stretching vibrations of the 1A2, 3B2, and 1B2 states. The crossings between the bent valence excited states i...

Electronic structure and spectroscopy of monohalides of metals of group I-B

The potential energy curves of the low lying electronic states of nine molecular compounds MX (M = Cu, Ag, An; X = F, Cl, Br) have been calculated by ab initio multi-reference configuration interaction (MRCI) and coupled-cluster (CC) methods using scalar-relativistic energy-consistent pseudopotentials. The electronic structures of these compounds have been analysed and compared. New spin-orbit (SO) pseudopotentials for Cu, Ag and Au have been optimised and applied for determining the SO splitting of the correlated states. In this way, accurate spectroscopic data have been derived that lead to new assignments and predictions for electronic states unobserved so far.

Theoretical study of the electronic states of

Three-dimensional potential energy functions (PEFs) have been generated for the X, a, and b states of using the internally contracted multireference configuration interaction approach. Analytic forms of the PEFs were employed in calculations of the vibrational energy levels, vibrational wavefunctions and Franck-Condon factors for the hypothetical direct ionization process . For the state the Renner-Teller problem has been solved and the pattern of the bending levels analysed. The collinear charge separation path yielding has been calculated for 14 electronic states. The electronic ground state of was found to have a barrier height of 1.4 eV, in good agreement with the experimentally detected onset of this charge separation process. The shapes of the close-lying potential energy functions indicate that for energies higher than about 4 eV above the electronic ground state, dissociation processes from these states will be accompanied by complicated coupling effects.

Theoretical potential energy function and rovibronic spectrum of CO+2(X 2Πg)

For the electronic ground state of CO+2 the three‐dimensional potential energy, electric dipole, and transition moment functions have been calculated from highly correlated multireference configuration interaction electronic wave functions. Along the antisymmetric stretching displacements the shape of the potential energy functions is found to be very sensitive to the electron correlation effect. Using a modified theoretical potential energy function rovibronic energy levels have been calculated variationally by the method of Carter and Handy. In this approach, anharmonicity, rotation–vibration, electronic angular momenta, and electron spin coupling effects have been accounted for. The vibronic band origins agree to within about 10 to 20 cm−1 with the available experimental data, and the rotational levels agree to within 0.01 cm−1 for low J values. Additional vibrational band origins have been predicted for energies up to 3200 cm−1. The anomalously low frequency of the antisymmetric stretching mode and it...

First-principles based multiscale model of piezoelectric nanowires with surface effects

A continuum model of nanowires incorporating surface piezoelectricity is proposed which extends the electric enthalpy energy with surface terms. The corresponding equations are solved by a numerical method using finite elements technique. A methodology is introduced to compute the surface piezoelectric coefficients by first-principles calculations through the Berry phase theory. We provide the e33s, e31s, and e15s piezoelectric coefficients of (101¯0) surfaces for hexagonal wurtzite nanowires made of GaN, ZnO, and AlN. The effective piezoelectric coefficient along the axis of the nanowire is found to increase when the diameter decreases, for the three studied materials. Finally, the solution of the continuum model is compared with large-size first-principles calculations on piezoelectric nanowires.

Rovibronic spectrum of the N3 radical in the X 2Πg state

Based on ab initio electronic structure calculations, the three‐dimensional potential energy functions for the electronic ground state X 2Πg of N3 have been generated and used in beyond Born–Oppenheimer calculations of the rovibronic energy levels by a variational approach accounting for anharmonicity, rotation–vibration, electronic angular momenta, and electron spin coupling effects. The vibronic levels (J=P) for energies up to 4300 cm−1 are given. The few experimentally known vibronic energy differences have been reproduced with an accuracy of better than 10 cm−1. For several vibronic levels also the rovibronic levels are reported. The electron–nuclear motion and anharmonic coupling effects have been analyzed. The strength of the Born–Oppenheimer breakdown depends strongly on the rovibronic symmetry. Like in the isoelectronic CO+2 ion, the anharmonic coupling in the vibronic states occurs mainly within the blocks of Fermi polyads for (2ν1+ν2) =constant. In low lying states the anharmonic coupling effect...

Theoretical study of the electronic states of AlO and AlO

Abstract The spectroscopic constants of the excited doublet and quartet states of aluminium oxide have been calculated from potential energy functions obtained by the ab initio MRCI approach. Such constants are given also for several states that have not yet been observed. The characteristics of the calculated electric dipole and transition moment functions are discussed in connection with the ionicity and avoided crossing regions of the excited states. The spin–orbit interactions between electronic states have been calculated for those cases where this type of coupling was assumed to be responsible for pertubations of the spectra. Also for the bound electronic ground state of the AlO − anion the spectroscopic constants are given and its photoelectron spectrum is discussed.