Molecular two-centre integrals between 2 pπ and 3 pπ atomic orbitals

The evaluation of two-centre Coulomb-penetration and exchange-penetration integrals is discussed. All possible penetration integrals between 2pπ and 3pπ atomic orbitals exponent α are reduced analytically and tabulated for α = 1·00 (0·25) 10·00.     

Stable domination and weight

We develop the theory of domination by stable types and stable weight in an arbitrary theory.     

A determination of the electronegativity parameter in molecular orbital calculations on heteronuclear molecules containing nitrogen

Using the semi-empirical LCAO molecular orbital method, the electronegativity parameter δ for nitrogen has been chosen to give reasonable fit between computed and ‘correct’ bond orders for a series of heterocyclic molecules. A δ-value of about 0·9–1·0 has been found for nitrogen in six-membered rings, and about 1·8–2·0 for pyrrole.     

Hydrogen bonds and van der waals forces in ice at ambient and high pressures

The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high-pressure phases of ice. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that transition pressures obtained from density-functional theory exchange-correlation functionals which neglect vdW forces are greatly overestimated.