Carlo Petrongolo

SCF Minimal Basis Set Calculations and Exclusive Orbitals for CN−, HCN, N3−, HN3, NCO−, and HNCO

The ground‐state SCF wavefunctions for HCN, HN3, HNCO, and related anions were computed, using the minimal basis set of STOs (best‐atom zetas). The differences in the molecular properties of the anion and the acid are discussed, also using the results of a population analysis. By means of unitary transformations the Boys exclusive orbitals were obtained and an analysis was made of their transferability.

Theoretical investigations on the solvation process

A model to facilitate the computation of the most stable conformer of associated M · H2O (M being a polar molecule) which depends upon the electrostatic interaction energy between the two associated molecules is proposed and tested. SCF electrostatic potentials for the M molecule and a suitable point charge distribution for H2O were employed in the model computations. Energies predicted by the model are found to be in good agreement with those resulting from an ab initio minimal STO basis SCF treatment of some conformations of the H2O dimer.ZusammenfassungEin Modell zur Durchführung der Berechnung des stabilsten Konformeren eines Assoziationskomplexes M · H2O, wobei M ein polares Molekül ist, wird vorgeschlagen und untersucht. Es basiert auf der elektrostatischen Wechselwirkung zwischen beiden Partnern, und zwar wird für das Molekül M der elektrostatische Anteil seines SCF-Potentials und für H2O eine angemessene Punktladungsverteilung zugrunde gelegt. Die resultierenden Energien sind in guter Übereinstimmung mit denen, die sich bei einer ab initio Rechnung mit minimaler STO Basis ergeben.

Nonadiabatic theory of triatomics: General formalism and application to Renner–Teller and conical‐intersection effects

A general theory of nonadiabatic effects in molecular spectra of bound triatomic molecules is presented. Within the nonrelativistic approximation and the expansion method, the formalism is exact and allows a unified study of the various nonadiabatic interactions between two or more electronic states. A full, spinless rovibronic Hamiltonian is used and the rotation, inversion, and nuclear permutation symmetries are considered for defining a rovibronic expansion basis which is symmetry adapted for ABC and AB2 molecules. The Hamiltonian matrix elements on the rotoelectronic‐angular basis are obtained for singlet electronic states, by fully considering both the nuclear and the electronic structure. The equations are then specialized to three‐state Renner–Teller and conical intersection effects in the diabatic representation, and are approximated and compared with previous works. Finally, the expressions of the dipole line strengths are derived and the singularities of the adiabatic representation of conical i...

Minimal‐Basis‐Set LCAO–SCF–MO Calculations for the Ground State of O3, NO2–, NOF, and OF2 Molecules

Ground‐state wavefunctions for the nonlinear triatomic molecules O3,NO2–, NOF, and OF2 are computed. The calculation was performed in two versions both using the minimal set of STOs in the LCAO–SCF–MO approximation and differing only in the choice of the orbital exponents. The results of a population analysis are also presented.

MRD-CI ground state geometry and vertical spectrum of N3

Abstract CI calculations have been carried out for the prediction of the ground state geometry and of the vertical spectrum of N 3 . The first three states are 2 Π g , 4 Π u and 2 Σ + u . The C ∞v correlation diagram for the first dissociation limits is discussed by taking into account possible nonadiabatic pathways.

The protonation of organic molecules: electrostatic versus SCF CNDO calculations for three-membered ring molecules

Abstract A comparison between ab initio and CNDO/2 methods is made both in the electrostatic and in the SCF approximation, in order to obtain a description of the protonation process of organic cycles. From the electrostatic CNDO results it is possible to get a sufficiently reliable picture of the whole protonation process.

Double‐ζ LCAO SCF MO Calculations for NO2 and OF2

The ground‐state wavefunctions for NO2− and OF2 are obtained by using the free‐atom‐ζ double‐ζ STO set in the LCAO SCF MO approximation. Comparisons are made with the corresponding minimal STO set SCF wavefunctions and, for OF2, also with a Gaussian set SCF wavefunction.

Potential surfaces and vibronic coupling for the conical intersection of the Ã2A1 and B̃2B2 states of NH2

Abstract Large-scale ab initio MRD CI calculations are carried out for the A2A1 and B2B2 electronic states of NH2 in the region of their conical intersection. The potential surfaces and the first-derivative bending vibronic coupling are calculated in the bending and antisymmetric-stretching coordinates and are compared with the results of the small-vibration theory of conical intersections.

SCF wavefunction for the ground state of CN− and the change of the correlation energy in some simple protonation processes

Abstract The SCF MO LCAO wavefunction of CN− has been calculated with a double-ζ basis plus 3d polarization functions (Slater type orbitals). A comparison is made between the SCF and the experimental values of the heat of protonation, supplemented with data pertinent to other systems. A discussion on the changes in the correlation energy in these processes is also made.

Configuration-interaction calculations for the ground state of OF2, NO 2 − , CN−: Canonical orbitals and exclusive orbitals

The C.I. method is applied to already published double-ζ SCF wave functions for OF2, NO2−and CN−. All doubly excited configurations are considered. The second order approximation values presented for those cases are compared with the exact energy values in some minor cases. An attempt has been made to improve the rate of convergency of the process by transforming the canonical SCF occupied orbitals into localized ones and the virtual canonical orbitals into a particular type of localized orbitals. These latter show also an intuitively acceptable spatial conformation.ZusammenfassungDie CI Methode wird auf bereits bekannte Doppel-Zeta-SCF-Wellenfuntionen für die Systeme OF2, NO2−und CN− angewandt. Dabei werden alle doppelt angeregten Konfigurationen berücksichtigt. Um die Konvergenz zu verbessern, werden versuchsweise die kanonischen und besetzten SCF-Orbitale in lokalisierte Orbitale und die virtuellen kanonischen Orbitale in einen besonderen Typ von lokalisierten Orbitalen transformiert. Die letzteren zeigen eine besonders geeignete räumliche Anpassung.RésuméPour un calcul de intéraction de configurations, comprenant toutes le configurations à double excitation, on a employé des fonctions donde SCF sur une base doublée. Pour le calcul complet on a essayé des formules dapproximation, quon a pu vérifier avec le calcul exact sur des cas réduits.Pour améliorer la vélocité de convergence du procés on a transformé les orbitales canoniques SCF en orbitales localisées et aprés on a construit, selon un procedé particulier, des orbitales virtuelles localisées qui montrent une conformation dans lespace qui est intuitivement satisfaisante.