Berta Fernández

The benzene-argon complex: A ground and excited state ab initio study

Equilibrium dissociation energies De of the benzene–argon van der Waals complex are calculated in the ground state S0 and in the excited state S1 using integral-direct coupled cluster methods. The results confirm previous investigations of S0, showing that high quality correlation consistent basis sets and connected triple excitations are imperative for a good description of the van der Waals complex. We estimate the CCSD(T) dissociation energy De=389±2 cm−1 for the ground state S0. Using the CCSD linear response approach the frequency shift (redshift) δνe=19 cm−1 is obtained. Accurate spectroscopic structural data and frequency shifts δν0 for the 601 band of the S1←S0 transition are available for most of the benzene–rare gas atom complexes. However, the experimental determination of absolute dissociation energies of these complexes is connected with much larger uncertainties. The theoretical result agrees very well with the experimentally available redshift, showing that integral-direct coupled cluster m...

Ground state benzene–argon intermolecular potential energy surface

A highly accurate ab initio intermolecular potential energy surface for the benzene–argon van der Waals complex is evaluated using the coupled cluster singles and doubles model including connected triple excitations [CCSD(T)] model with an augmented correlation consistent polarized valence double zeta basis set extended with midbond functions. The vibrational energy levels obtained by full three-dimensional dynamical calculations are in excellent agreement with the available experimental data.

The effect of intermolecular interactions on the electric properties of helium and argon. I. Ab initio calculation of the interaction induced polarizability and hyperpolarizability in He2 and Ar2

The frequency-dependent interaction induced polarizabilities and second hyperpolarizabilities are calculated for He2 at the coupled cluster singles and doubles and full configuration interaction levels and for Ar2 at the coupled cluster singles and doubles level. The frequency-dependence is approximated by a power series to second-order in the frequency arguments using Cauchy moments and hyperpolarizability dispersion coefficients. Using large correlation consistent basis sets, results close to the basis set limit are obtained. The computed curves for the interaction induced (hyper-) polarizabilities are tabulated for a range of internuclear distances. The data are employed in a companion paper to make for the first time a direct comparison between the experimentally determined pressure dependence of an ESHG hyperpolarizability and ab initio calculated hyperpolarizability second virial coefficients.

Spin polarization in restricted electronic structure theory: Multiconfiguration self‐consistent‐field calculations of hyperfine coupling constants

It is shown that the isotropic hyperfine coupling constant Aiso associated with the Fermi contact interaction can be accurately calculated from conventional multiconfiguration self‐consistent‐field wave functions if, in addition to the expectation value of the contact operator, one also includes the terms originating from the response of the wave function. These response terms are nonvanishing only for perturbation operators that are nonsymmetric in spin space. Calculations of Aiso for N and BH2 illustrate the importance of the response terms and also indicate that a good estimate of the spin polarization of the core orbitals may be obtained from the response terms without correlating the core.

The effect of intermolecular interactions on the electric properties of helium and argon. II. The dielectric, refractivity, Kerr, and hyperpolarizability second virial coefficients

The dielectric, refractivity, Kerr, and hyperpolarizability second virial coefficients for the helium and argon gases are evaluated for a wide range of temperatures using a semiclassical approach and the high quality frequency-dependent interaction induced electric polarizabilities and second hyperpolarizabilities of the previous paper. For helium and argon we obtain satisfactory agreement with most of the experimental data for the dielectric and the refractivity second virial coefficients. Our results confirm that the helium gas second Kerr virial coefficient is very small at temperatures beyond 70 K. For argon we obtain a very good agreement with a recent experimental determination at 632.8 nm, whereas we suggest that previous experimental results for 458 nm might be inaccurate. The ESHG results indicate a possible disagreement between a recent experimental determination and the semiclassical ansatz for the second hyperpolarizability virial coefficients.

Benzene-argon S1 intermolecular potential energy surface

The benzene-argon S1 intermolecular potential energy surface is evaluated using coupled-cluster linear response methods and an augmented correlation-consistent polarized valence double-zeta basis set extended with midbond functions. As a result of the S1←S0 excitation, the well depth of the potential increases and the benzene-argon equilibrium distance is decreased by 0.065 A. Full three-dimensional vibrational calculations of the van der Waals vibrational energy levels, using the ab initio potential, are in good agreement with available experimental data.

Accurate intermolecular ground state potential of the Ar–N2 complex

The ground state potential energy surface of the Ar–N2 van der Waals complex is evaluated using the coupled cluster singles and doubles model, including connected triples corrections (CCSD(T)) and the aug-cc-pVTZ basis set extended with midbond functions. From the calculated ab initio potential the rovibronic spectroscopic properties are determined and compared with the available experimental data. Considerable improvement is obtained when four of the potential parameters are refined based on the Ar–14N2 rotational transition frequencies. The small discrepancies remaining demonstrate that the coupled cluster method can be used to predict the spectroscopic properties of van der Waals complexes.

Accurate ab initio rovibronic spectrum of the X 1Σg+ and B 1Σu+ states in Ar2

Using recently developed coupled-cluster response methods, accurate potential curves for the X 1Σg+ and B 1Σu+ electronic states of Ar2 are evaluated and the lowest rovibronic energy levels are calculated. The results are in very good agreement with the experimentally available spectroscopic data, and suggest a reassignment of the experimental vibrational numbering in the B 1Σu+ state.

Calculation of hyperfine coupling constants of the CN and CP ground state radicals

By carrying out a systematic basis set and electron correlation investigation, we have determined accurately the hyperfine coupling constants of the ground states of CN and CP. The basis set studies began with Dunning’s correlation consistent bases, after which systematic uncontractions and extensions with diffuse and tight functions were introduced until saturation was achieved. The basis set self‐consistent‐field (SCF) wave function results compare favorably with numerical Hartree–Fock (HF) results. The electron correlation study was based on extending systematically the active space of a complete valence orbital spin‐restricted multiconfiguration self‐consistent‐field (MCSCF) wave function and using the MCSCF restricted–unrestricted response function approach to obtain the hyperfine coupling constants.

MCSCF polarizability and hyperpolarizabilities of HCl and HBr

Abstract The frequency-dependent electric dipole polarizability ( α ) and first and second electric dipole hyperpolarizabilities ( β and γ ) of HCl and HBr were evaluated in the multiconfigurational self-consistent field response approximation. The effect of molecular vibration was investigated. Comparison with the available experimental and theoretical data shows a good agreement.