Self-consistent field calculations using two-body density functionals for correlation energy component: II. Small molecules

The role of topology in organic molecules: origin and comparison of the radical character in linear and cyclic oligoacenes and related oligomers

We discuss the nature of electron-correlation effects in carbon nanorings and nanobelts by a combined approach based on FT-DFT and RAS-SF methods.

Highly polar bonds and the meaning of covalency and ionicity--structure and bonding of alkali metal hydride oligomers

The hydrogen-alkali metal bond is simple and archetypal, and thus an ideal model for studying the nature of highly polar element-metal bonds. Thus, we have theoretically explored the alkali metal hydride monomers, HM, and (distorted) cubic tetramers, (HM)4, with M = Li, Na, K, and Rb, using density functional theory (DFT) at the BP86/TZ2P level. Our objective is to determine how the structure and thermochemistry (e.g., H-M bond lengths and strengths, oligomerization energies, etc.) of alkali metal hydrides depend on the metal atom, and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The H-M bond becomes longer and weaker, both in the monomers and tetramers, if one descends the periodic table from Li to Rb. Quantitative bonding analyses show that this trend is not determined by decreasing electrostatic attraction but, primarily, by the weakening in orbital interactions. The latter become less stabilizing along Li-Rb because the bond overlap between the singly occupied molecular orbitals (SOMOs) of H* and M* radicals decreases as the metal ns atomic orbital (AO) becomes larger and more diffuse. Thus, the H-M bond behaves as a text-book electron-pair bond and, in that respect, it is covalent, despite a high polarity. For the lithium and sodium hydride tetramers, the H4 tetrahedron is larger than and surrounds the M4 cluster (i.e., H-H > M-M). Interestingly, this is no longer the case in the potassium and rubidium hydride tetramers, in which the H4 tetrahedron is smaller than and inside the M4 cluster (i.e., H-H < M-M).

Merging multiconfigurational wavefunctions and correlation functionals to predict magnetic coupling constants

We study the performance of different approaches that combine multiconfigurational wavefunctions with correlation functionals for the calculation of magnetic coupling constants of several materials and molecules. The systems under study include four antiferromagnetic materials: NiO, KNiF(3), K(2)NiF(4) and La(2)CuO(4); two biradicals: alpha-4-Dehydrotoluene and 1,1',5,5'-Tetramethyl-6,6'-dioxo-3,3'-biverdazyl; two molecular complexes: [Cu(2)Cl(6)](-2) and Copper(II) acetate monohidrate; and the prototypical H-He-H system. On average, the best results are obtained with a recently proposed method [Phys. Rev. A 75, 012503 (2007)] that estimates the correlation energy of density functionals from a pair of alternative spin densities built from the natural orbitals and occupation numbers of the multiconfigurational wavefunction.