Stefano Polezzo

The electronic structure of sulphur compounds

LCAO SCF molecular orbitals and energies of H2S are computed using a minimal basis set augmented by sulphur 3d orbitals and with optimized 1s H and 3d orbital exponents. The dissociation energy, the electric dipole moment, the two independent field-gradient components at the sulphur nucleus and some transition energies are calculated and compared with the experimental quantities.

Direct minimization of the energy functional in the LCAO-MO density matrix formalism: V. Application to the separated electron pair theory

The general multiconfiguration self-consistent-field method is presented along the density matrix formalism. The proposed optimization procedure for orbitals makes use of an orthogonal transformation in the space spanned by the fixed basis set. Acting on the unconstrained parameters of the transformation a direct minimization of the energy expression is performed using a gradient approach. A similar method may also be applied to the optimization of the expansion coefficients. The method works not only for the ground state of a given system, but also for any excited state, yielding an upper bound to the true energy of the considered state.

Direct optimization of orbitals for multiconfiguration self-consistent field theory by unitary transformations

In this paper the orbital variations which make stationary the MCSCF energy are generated by unitary transformations worked out directly from the null-gradient condition. A number of iterative procedures are given, compared with other recent proposals and tested numerically on LiH for SCF and geminal theory. A satisfactory convergence rate is found, particularly for the MCSCF case.

Generalized self-consistent valence bond method for ground and excited potential energy surfaces

A new method is presented for computing ground and excited states potential energy surfaces, within the context of the valence bond (VB) theory, retaining the possibility of describing electronic molecular changes in terms of valence bond structure concepts. The separated electron pair (SEP) theory in the rank two geminal approximation is reformulated and compounded with the straightforward VB formalism: the set of non-orthogonal orbitals generated by the SEP optimization is used to construct VB structures, which allow a compact and correct description of the wavefunction. The procedure is applied to the study of the ground and excited states of LiH of Σ and Π symmetry (A 1Σ+, C 1Σ+, a 3Σ, b 3Π, B 1Π). All the qualitative features of these states are reproduced, including the peculiarities of the A 1Σ+ state. Each state is described by one to four structures, from small values of the internuclear distance up to dissociation: a clear interpretation of the wavefunction is therefore provided.

Direct minimization of the energy functional in LCAO-MO density matrix formalism

A general theory is presented for the optimization of the coefficients of orbitals and configuration interaction expansion in the case of multiconfiguration wavefunctions containing all single excitations. The orbital coefficients are optimized by suitable orthogonal transformations of the atomic basis; the Cl coefficients are determined solving the usual secular problem. The energy minimization is performed directly by a gradient approach. The method works both for ground and excited states and no convergence difficulties are met. Computational examples are given for H2O and H2S molecules.

Pseudopotential calculations on hydrogen bonded systems : H2O, CH3OH and HCOOH dimers

The pseudopotential method is applied to the electronic structure determination of H2O, CH3OH and HCOOH monomers and dimers. The quantities computed are the dimerization energies, optimum geometries, charge distributions, dipole moments with their derivatives and force constants. All results compare well with those determined by all-electron calculations.

Valence-only model potential calculations on copper hydride molecule

Different sets of one-electron functions obtained according to the strong-orthogonal geminal theory (GEM) [1], the Generalized Molecular Orbital (GMO) method [2] and the exchange maximization between virtual and occupied orbitals (EVO) [3], are tested as basis for CI calculations. The efficiency of the three procedures is discussed investigating the electronic structure of the CuH molecule using an effective-core potential. The values computed for the bond length, the dissociation energy and the vibrational frequency of the ground electronic state are compared with the experimental ones. The charge distribution is examined to estimate the contribution of the d electrons to the Cu-H bond. Comparisons are made with the results obtained by other theoretical works in which the copper atom is treated as a one valence electron atom.

An ab initio calculation of the dissociation C2H2(1Σ+) → C2H(2Σ+) + H(2S)

The collinear dissociation of acetylene to C2H and H is studied by a generalized self-consistent procedure. The dissociation energy, the C-H force constant and stretching frequency are computed.

Valence-only molecular calculations with a non-empirical pseudopotential method

Two model potentials are proposed for effective core potentials which enable the molecular electronic structure to be described in terms of valence-only electrons. These pseudopotential operators consist of a coulomb local potential and two sets of projection operators simulating the non-local potential and preventing the collapse of the valence orbitals into the inner cores; they are angular momentum dependent. The parameters entering in their definition are determined on purely theoretical grounds from neutral atom SCF calculations. These parameters turn out to be completely independent of the valence description. The two models are presented for atoms up to chlorine and tested on some hydride and non-hydride molecules using a minimal basis set of Slater orbitals and computing molecular energy levels, minimum geometries, dipole moments and force constants. The results are compared with those of the corresponding all-electron calculations, the agreement obtained being to a few per cent or better for all ...

Computer generation of symmetry-adapted molecular orbitals

Abstract A practical method is proposed by which a computer is used to generate molecular symmetry-adapted orbitals. Input requires only the nuclear coordinates The procedure has been extensively tested on several mono and polynuclear complexes of various geometries. It has proven to be efficient and economical