Camilla Costa

A minimal basis bond-orbital investigation of the linear water dimer

The perturbative configuration interaction approach based on non-orthogonal bond-orbitals previously used for dealing with rotational barriers is applied to the study of the hydrogen bonding in the linear water dimer. First and second-order interaction energies are obtained in terms of static and transition charge distributions fully accounting for intermolecular overlap. Neglecting electron correlation, the second-order calculations include all single excitations from bonding to antibonding orbitals accounting for induction including exchange and giving results close to the corresponding supermolecular SCF-MOs in the same basis. Ab initio calculations using different gaussian minimal bases show that Clementis GTO basis MEDIUM is the most suitable for describing molecular interactions. Detailed component analysis of the energy up to second order is possible and reveals the main features of the intermolecular hydrogen bonding occurring between the water molecules.

Long-range second-order interactions and the shape of the He-HF and Ne-HF complexes

Direct ab initio calculation of long-range induction and dispersion interactions expended up to R−8 and evaluated at the SCF intermolecular separation for the complexes of He and Ne with HF shows that the angular shapes of such van der Waals atom-dipolar molecule dimers are determined by the absolute minima of the induction interaction.

Long-range coefficients for molecular interactions. II

Abstract General formulae for the coefficients Cn shaping the long-range behaviour of electrostatic, induction and dispersion interactions between molecular systems, are given in terms of the spherical tensor theory in its real form. The coefficients depend on a function P, describing in a compact way the relative orientation of the partners, and the permanent and induced moments of the individual molecules. Use of a hypergeneralized London formula allows the molecular dispersion coefficients to be expressed in terms of static multipole polarizabilities and their related excitation energies. Explicit formulae are presented for the first few coefficients of the linear water dimer (H2O2)2. The transformation properties of the real spherical tensors under rotation and translation of the origin of the body-fixed reference frame are given in appendices, together with tables containing the coefficients needed in the expansion of the electric potential due to a non-uniform external field as well as the explicit relations connecting real spherical and cartesian forms for molecular moments and polarizabilities up to l = 3.

Two-centre Coulomb integrals over STOs from analytical evaluation of k-integrals by contour integration in the complex plane

Abstract The k-dependent multipole expansion of rab−1 based on a Fourier transform allows for a compact analytical evaluation in Roothaan closed form of the general two-centre Coulomb integrals over STOs using the finite representation of spherical Bessel functions followed by contour integration in the complex plane.

A model for the elementary prediction of the angular geometries of van der Waals dimers

Abstract A simple electrostatic model based on the characterization of a molecule by its first two observable electric moments is proposed for van der Waals dimers of polar molecules. The relative stability of different angular geometries of the complexes is deduced from a pictorial analysis of the first few electrostatic interactions between these moments. The predictions resulting for 67 angular geometries of 35 van der Waals dimers of small molecules are in agreement with experiment.

On the angular shape of van der Waals dimers of small polar molecules

Abstract Ab initio calculations on twenty van der Waals dimers of small polar molecules at the experimentally observed intermolecular separation R, using Hartree-Fock molecular moments, show that the minima in the electrostatic interaction expanded up to R−6 converges to angular structures which are close to those observed experimentally for such complexes.

Linear pseudostate calculation of the partial-wave components of second-order energy for the ground state of atomic two-electron systems in hydrogenic perturbation theory

Abstract The linear pseudo-spectrum giving exact polarizabilities and dispersion coefficients for H atoms is used as such in the hydrogenic perturbation-theory calculation of the partial-wave expansion of the second-order energy for the ground state of the He sequence. The results for the first six partial waves are highly accurate.

Electrostatic interactions and the shape of van der Waals dimers

Abstract In this paper we analyze the angular dependence of the expanded electrostatic interaction in seven van der Waals complexes made by dipolar molecules (HF, H 2 O, NH 3 ) giving H-bonding and homonuclear diatomics (H 2 , N 2 ) having a permanent quadrupole moment in their ground state. It is found that including terms up to R −5 the absolute minima of the expanded electrostatic interaction evaluated at the experimental intermolecular separation converge to a reasonably accurate estimate of the angular shape actually observed for such van der Waals dimers.

Understanding van der Waals bonding

Abstract The bonding in van der Waals complexes is examined in the light of the physical nature of the components of the intermolecular energy. Attraction due to electrostatic, dispersion and induction contributions offsets the Pauli repulsion due to the exchange overlap of the closed-shell electron-charge distributions. Spherical tensor expansion of the intermolecular potential in the long range allows the attractive energy components to be related to the electric properties of the molecules, whose relative values explain the main features of the bonding in the van der Waals region. General formulae are derived for the dispersion coefficients of two linear molecules, showing explicitly the anisotropy of the interaction.

Long-range dispersion coefficients for like centrosymmetric linear molecules and an application to H2H2

Abstract The real spherical tensor theory of long-range intermolecular coefficients developed in previous papers is applied to derive explicit formulae for the first three dispersion coefficients for like centrosymmetric linear molecules. The expansion of angledependent coefficients in associated Legendre polynomials allows one to identify the isotropic and anisotropic components of the dispersion interaction in terms of London dispersion constants, the treatment of higher coefficients being simplified by the coupling of the elementary (1, 1′)-polarizations to resultant angular momenta LA and LB onto each molecule. The contributions from all coupling schemes are given explicitly for C6, C8, c10, and numerical results are presented for H2H2 using two-term reduced spectra values from the Kaiserlautern group.