Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281, 9000 Gent, Belgium. Mail

Lundi, 15 octobre 2012, 14 h 00, Bibliothèque du LCT, tour 12-13, 4e étage

By including one electron or two electron constraints as potentials in the molecular Hamiltonian, one can formulate a constrained Hartree-Fock theory, directing the solutions towards wave functions that yield specific desired values for some observables or desired values for chemical concepts such as atomic populations or bond orders.

The corresponding Euler-Lagrange equations lead to a modified Fock matrix, where the contribution from the constraints only depends on the overlap matrix, when using the Mulliken or Hirshfeld atoms-in-molecules method or the overlap matrix and the density matrix in a way similar to regular Hartree-Fock theory.

Imposing a set of atomic population constraints in the Mulliken sense, reveals that the energy shows a quadratic dependence on the fixed charges, thereby linking it closely to the electronegativity equalization method. This behavior provides a procedure to obtain the atomic electronegativity and hardness parameters in the electronegativity equalization method.

Going further, one can impose constraints on bond orders in such a way that a specific resonance structure can be imposed. Results will be shown for benzene, showing the impressive flexibility of the molecule to lead to a very low energy wave function, away from the RHF solution, but fulfilling the contraints of a Kekulé or Dewar structure. Finally, by providing a framework for