Full Configuration-Interaction Study on the Tetrahedral Li4 Cluster.

The Li4 cluster low lying electronic states were studied. In particular we investigated the tetrahedral geometry at full CI and coupled cluster level, with basis sets of increasing quality. The (5)A2 electronic state, characterized by having all the valence electrons unpaired, forming a quite stable no-pair bonding state, was studied in greater detail.

Does a Sodium Atom Bind to C60?

A Multi-Reference Configuration-Interaction study of the NaC60 system is presented. It is shown that the experimentally measured dipole moment of this system can be explained by the existence of a charge-transfer state of Na(+)C60(-) nature. Moreover, the present work shows that Configuration-Interaction techniques based on local orbitals permit a Multi-Reference treatment of systems containing several tens of atoms.

Vertical spectrum of the C2H2+ system. An open shell (SC)2-CAS-SDCI study.

The open shell (SC)(2)-CAS-SDCI method along with a basis set of atomic natural orbitals (ANO) has been applied for calculating the main ionization potentials of acetylene, as well as the manifold of excited states of the different symmetries up to 32 eV. In this method, the single and double excitations of a CAS space are generated and the corresponding CI matrix is corrected by means of the (SC)(2) procedure that cancels the size-extensivity error and adds some high order contributions. The mean absolute error for the outer-valence X (2)Pi(u)(1pi(u) (-1)), A (2)Sigma(g) (+)(3sigma(g) (-1)), and B (2)Sigma(u) (+)(2sigma(u) (-1)) states, and the inner-valence C (2)Sigma(g) (+)(2sigma(g) (-1)) state is 0.

Reducing CAS-SDCI space. Using selected spaces in configuration interaction calculations in an efficient way.

A new method is presented, which allows an important reduction of the size of some Configuration Interaction (CI) matrices. Starting from a Complete Active Space (CAS), the numerous configurations that have a small weight in the CAS wave function are eliminated. When excited configurations (e.