Sylvio Canuto

The dipole polarizability of Li

We have calculated the lowest excitation energies and the static dipole polarizability of Li− as well as the electron affinity of Li using current quantum-chemical methods, including full CI. We have studied the convergence of the polarizability with the size of the orbital basis set. We find that electron correlation has a dramatic effect on the polarizability reducing it to roughly half its SCF value. MBPT calculations through fourth-order and perturbative propagator methods give an unsatisfactory description of the electron correlation whereas polarization propagator methods based on a coupled-cluster reference state give a polarizability in fair agreement with the full CI result.

Ab initio studies of the photodissociation in the first excited states of à 1A1 and ã 3A1 of PH3

HF and CI calculations were carried out to explain the predissociation process in the first excited Au20091A1 state of PH3 that is observed experimentally. It is found that the mechanism of the predissociation involves Rydberg‐valence transformation leading to the production of PH2(X 2B1, A 2A1) and H(2S). This process is similar to that found in the first excited 1A2′′ state of NH3; however, the thermodynamics of the two processes are different. A corresponding study for the PH3(a 3A1) reveals a major difference from the 3A2′′ of NH3 due to the absence of the Rydberg character of the PH3(a 3A1) at the equilibrium geometry. In addition, we report HF calculations on the equilibrium bond distances and the angles, the vibrational frequencies and the barriers to inversion of the neutral ground state of PH3(X 1A1), its first singlet and triplet excited states (A 1A1 and a 3A1, respectively), and the ground state ion (X 2A1).

Many-body perturbation theory and coupled-cluster calculations of the ground-state structure of CO3

Abstract Many-body perturbation theory and coupled-cluster calculations are performed on CO 3 in C 2v and D 3h symmetries. The relative energy location of the 1 A 1 (C 2v and 1 A′ 1 (D 3h ) closed-shell states is found to be sensitive to electron-correlation corrections. At the highest level of calculation considered (commonly known as CCD + ST (CCD)), the results indicate that the 1 A 1 (C 2v state lies below the 1 A′ 1 (D 3h state by 0.19 eV, at their corresponding optimized SCF geometries. The calculated infrared spectrum, using the C 2v structure, is found to be in good agreement with the experimental result.

A comparison of theoretical models for interpreting the photoelectron spectrum of borazine

Abstract Ab initio and INDO/S calculations were carried out on borazine and the borazine radical cation. The first three bands of the photoelectron spectra are well predicted from the molecular orbital structure of the neutral; discrepancies for the higher-energy bands are resolved by a configuration interaction study of the radical cation.

Coupled-cluster calculation of the static polarisabilities and hyperpolarisabilities of magnesium

Abstract The dipole and the quadrupole polarisabilities, the dipole-dipole-quadrupole and second dipole hyperpolarisabilities of the magnesium atom are calculated with an extensive basis set by using the coupled-cluster model within the finite-field approximation. At the highest level, the single-reference coupled-cluster double-excitation model corrected to include contributions of single and triple excitations in the so-called CCD+ST(CCD) scheme, the results are 70.89, 810.0, −8.02×10 3 and 1.00×10 5 a.u., respectively, in good agreement with previous single-reference low-order perturbation theory results.

Isotropic magnetic shielding constants of retinal derivatives in aprotic and protic solvents.

We investigate the nuclear isotropic shielding constants σ((13)C) and σ((17)O) of isomers of retinoic acid and retinal in gas-phase and in chloroform, acetonitrile, methanol, and water solutions via Monte Carlo simulation and quantum mechanics calculations using the GIAO-B3LYP∕6-311++G(2d,2p) approach. Electronic solute polarization effects due to protic and aprotic solvents are included iteratively and play an important role in the quantitative determination of oxygen shielding constants. Our MP2∕6-31G+(d) results show substantial increases of the dipole moment of both retinal derivatives in solution as compared with the gas-phase results (between 22% and 26% in chloroform and between 55% and 99% in water). For the oxygen atoms the influence of the solute polarization is mild for σ((17)O) of hydroxyl group, even in protic solvents, but it is particularly important for σ((17)O) of carbonyl group. For the latter, there is a sizable increase in the magnitude with increasing solvent polarity. For the carbon atoms, the solvent effects on the σ((13)C) values are in general small, being more appreciable in carbon atoms of the polyene chain than in the carbon atoms of the β-ionone ring and methyl groups. The results also show that isomeric changes on the backbones of the polyene chains have marked influence on the (13)C chemical shifts of carbon atoms near to the structural distortions, in good agreement with the experimental results measured in solution.

On the spherical quadratic Zeeman problem in hydrogen

Abstract The factorized wavefunction approach to perturbation theory is applied to the spherical quadratic Zeeman problem in hydrogen. The ground and excited 2p states are considered. The results are in good agreement with other available results and provide a good upper bound to the complete Zeeman energy shifts. Transition strengths are calculated in good agreement with available results and the quantitative role of the residual non-spherical perturbation is briefly analyzed.

Another approach to the spherical Stark problem in hydrogen

Abstract The factorized wave function method of Hall is successfully applied to the spherical Stark problem in hydrogen. The results are in excellent agreement as compared to stabilization technique and accurate Pade approximants. This is the case even for large values of the field strength where the Rayleigh-Schrodinger perturbation theory becomes strongly divergent.

The virial theorem and the factorized wavefunction approach to perturbation theory. Application to the spherical Stark and spherical quadratic Zeeman problems in hydrogen

Abstract The factorized wavefunction approach to perturbation theory is applied to the spherical Stark and spherical quadratic Zeeman problems in hydrogen. The approximate solutions satisfy the virial theorem and the fulfillment of the Hellmann-Feynman theorem is also verified. Accurate energy shifts and other relevant expectation values are obtained. The exact low-order coefficients of the RSPT series are recovered from succesive derivatives ofthe calculated energy shifts.

Electronic states of silabenzene

Abstract Characterization of the electronic structural features of silabenzene has been attempted using INDO/S-CI calculations. The absorption spectrum is interpreted and various aspects related to absorption and emission are discussed.