The Spin-Partitioned Total-Position Spread Tensor: An Application To Diatomic Molecules.

The spin partition (SP) of the total-position spread (TPS) tensor is applied to the case of a few light diatomic molecules at full configuration interaction (FCI) level. It appears that the SP-TPS tensor gives informations that are complementary with respect to the corresponding spin-summed (SS) quantity. The spin-summed total position-spread tensor (SS-TPS, Λ) is defined as the second moment cumulant of the total position operator, and the SP-TPS is its partition in equal (Λαα+ββ) and different spin (Λαβ+βα) contributions.

The spin-partitioned total position-spread tensor: An application to Heisenberg spin chains.

The spin partition of the Total Position-Spread (TPS) tensor has been performed for one-dimensional Heisenberg chains with open boundary conditions. Both the cases of a ferromagnetic (high-spin) and an anti-ferromagnetic (low-spin) ground-state have been considered. In the case of a low-spin ground-state, the use of alternating magnetic couplings allowed to investigate the effect of spin-pairing.

The total position-spread tensor: spin partition.

The Total Position Spread (TPS) tensor, defined as the second moment cumulant of the position operator, is a key quantity to describe the mobility of electrons in a molecule or an extended system. In the present investigation, the partition of the TPS tensor according to spin variables is derived and discussed. It is shown that, while the spin-summed TPS gives information on charge mobility, the spin-partitioned TPS tensor becomes a powerful tool that provides information about spin fluctuations.

Partly saturated polyacene structures: a theoretical study.

Planar molecular edifices obtained by joining polyacene fragments (polyacene stripes) are investigated at tight-binding (i.e., with a Hückel Hamiltonian) and ab initio level.

Beryllium dimer: a bond based on non-dynamical correlation.

The bond nature in beryllium dimer has been theoretically investigated using high-level ab initio methods. A series of ANO basis sets of increasing quality, going from sp to spdf ghi contractions, has been employed, combined with HF, CAS-SCF, CISD, and MRCI calculations with several different active spaces. The quality of these calculations has been checked by comparing the results with valence Full-CI calculations, performed with the same basis sets.

The Total Position Spread in mixed-valence compounds: A study on the H4+ model system.

The behavior of the Total Position Spread (TPS) tensor, which is the second moment cumulant of the total position operator, is investigated in the case of a mixed-valence model system. The system consists of two H2 molecules placed at a distance D. If D is larger than about 4 bohr, the singly ionized system shows a mixed-valence character.

Behavior of the Position-Spread Tensor in Diatomic Systems.

The behavior of the Position-Spread Tensor (Λ) in a series of light diatomic molecules (either neutral or negative ions) is investigated at a Full Configuration Interaction level. This tensor, which is the second moment cumulant of the total position operator, is invariant with respect to molecular translations, while its trace is also rotationally invariant. Moreover, the tensor is additive in the case of noninteracting subsystems and can be seen as an intrinsic property of a molecule.

A theoretical study of closed polyacene structures.

The electronic structure of planar molecular edifices obtained by joining polyacene fragments (polyacene stripes) is investigated at tight-binding and ab initio levels. It is shown that the presence of 60° angles in the molecular skeleton induces the formation of singly-occupied molecular orbitals, whose combination gives rise to quasi-degenerate electronic states. The ab initio investigation requires therefore the use of CAS-SCF and MR-PT approaches.