We discuss the ability of a number of standard and non-standard computational techniques to reproduce dispersion forces, using examples from the literature as well as some new examples. We conclude that there are still some cases where standard methods are not so far successful. There are some promising directions under study, however. Manuscript received: 15 March 2001 Final version: 26 October 2001.

ABSTRACT:We discuss dispersion forces, beginning with toy models that illustrate the limitations of various standard approaches. For metallic cohesion of very thin layers, we show that because the local density approximation (LDA) misses distant dispersion interactions, it also makes significant errors in the maximum cohesive force, a short-ranged property. Furthermore, perturbative methods fail for such large planar systems, and CI methods are impractical. For large planar and linear systems in the well-separated limit we show that insulating and metallic systems can exhibit very different dispersion forces, pairwise summation of atomic R Ϫ6 terms failing for the metallic cases. This could have implications for the interaction between nanotubes and between graphene planes: these planes are zero-gap insulators at large separation and weak metals at graphitic equilibrium. Graphitic cohesion and intercalation are fundamental to a hydrogen economy and to various nanotechnologies, yet our arguments strongly suggest that all standard methods are inadequate for these phenomena. We argue that nonlocal RPA-like correlation energy formulae contain all the required "seamless" physics of long-and short-ranged interaction, as needed for graphitic and other soft-matter systems. Indeed full calculations of this type are currently being attempted for graphite, and appear to be very delicate. We discuss recent efforts to approximate these calculations, and propose a new scheme.

Protonated 4-bromophenol and 4-bromoanisole produced by methane chemical ionization are found to easily be dehalogenated upon high (8 keV) or low (20-30 eV) energy collisional activation giving essentially phenol and anisole radical cations, respectively. Under similar conditions, protonated unsubstituted anisole is also readily demethylated generating the phenol ion but not cyclohexadienone ions. Other nonconventional isomers of ionized phenol are only detected by MS/MS/MS experiments performed on [M-CO] •+ ions from salicylaldehyde. (U)B3LYP/6-311++G(d,p) and CASPT2/6-31G(d,p) calculations indicate the higher stability of the phenol radical cation with respect to the other six-membered-ring isomers. The least energy demanding fragmentation, namely, the decarbonylation, is shown to involve the intermediacy of six-membered ketones, open-chain ketenes, and five-membered cyclopentadiene isomeric ions. The rate determining step corresponds to the enol-keto interconversion with an energy barrier of about 276 kJ/mol relative to the phenol ion, which is markedly smaller than that required for hydrogen atom loss, deprotonation, or CO loss from an open-chain form. This suggests a crucial role played by the solvent in the readiness of the deprotonation of phenol ions in nonpolar media. The adiabatic ionization energy of phenol is evaluated as IE a (C 6 H 5 O) ) 8.35 ( 0.2 eV (exptl: 8.49 eV), and the proton affinity of the phenoxy radical is evaluated as PA(C 6 H 5 O) ) 863 ( 10 kJ/mol (exptl: 860 kJ/mol), PA(phenol) ) 826 ( 10 kJ/mol (exptl: 818 kJ/mol), and PA(anisole) ) 848 ( 10 kJ/mol.