A recently suggested procedure for the systematic optimization of gradient-corrected exchange-correlation functionals [A. D. Becke, J. Chem. Phys. 107, 8554 (1997)] has been applied to the extended G2 test set [L. A. Curtiss et al., J. Chem. Phys. 106, 1063 (1997)], which consists of the standard heats of formation of 148 molecules. The limit of reproduction of the experimental data in this test set is found to be 1.78 kcal/mol mean absolute error, with a maximum of 8.89 kcal/mol error for the ozone molecule. This compares rather well with previous results for G2 theory itself (1.58 and 8.2 kcal/mol, respectively). We show that fair stability can be obtained by our optimization procedure.

No abstract

We examine and compare two previously introduced functions of the one-particle density matrix that are suitable to represent its off-diagonal structure in a condensed form and that have illustrative connections to the nature of the chemical bond. One of them, the Localized-Orbital Locator (LOL) [J. Molec. Struct. (THEOCHEM) 527, 51 (2000)], is based only on the noninteracting kinetic-energy density τ and the charge density ρ at a point, and gives an intuitive measure of the relative speed of electrons in its vicinity. Alternatively, LOL focuses on regions that are dominated by single localized orbitals. The other one, the Parity Function P [J. Chem. Phys. 105, 11134 (1996)], is a section through the Wigner phase-space function at zero momentum, and contains information about the phase of the interference of atomiclike orbital contributions from bound centers. In this paper, we discuss the way in which these functions condense information in the density matrix, and illustrate on a variety of examples of unusual chemical bonds how they can help to understand the nature of “covalence.”

Four novel blue/green luminescent compounds, Zn(2-py-in)2(THF) (1), BPh2(2-py-in) (2), Be(2-py-in)2 (3), and BPh2(2-py-aza) (4), where 2-py-in = 2-(2-pyridyl)indole and 2-py-aza = 2-(2-pyridyl)-7-azaindole, have been synthesized and fully characterized. The 2-py-in ligand and 2-py-aza ligand in the new compounds are chelated to the central atom. Compounds 2−4 are air stable and readily sublimable, with a melting point above 250 °C. In the solid state, compounds 1−4 have an emission maximum at λ 488, 516, 490, and 476 nm, respectively. The structures of compounds 2 and 4 are similar. The blue shift of emission energy displayed by compound 4, in comparison to that of 2, is attributed to the presence of an extra nitrogen atom in the 2-py-aza ligand as confirmed by ab initio calculations on compounds 2 and 4. Electroluminescent devices of compounds 3 and 4 were fabricated by using N,N ‘-di-1-naphthyl-N,N ‘-diphenylbenzidine (NPB) as the hole transporting layer, Alq3 (q = 8-hydroxyquinolato) as the electron transporting layer, and compound 3 or 4 as the light emitting layer. At 20 mA/cm2 the EL device of 3 has an external efficiency of 1.06 cd/A while the EL device of 4 has an external efficiency of 2.34 cd/A, demonstrating that compounds 3 and 4 are efficient and promising emitters in electroluminescent devices.