L. Pastor-Abia

Magnetic molecules created by hydrogenation of Polycyclic Aromatic Hydrocarbons

Present routes to produce magnetic organic-based materials adopt a common strategy: the use of magnetic species (atoms, polyradicals, etc.) as building blocks. We explore an alternative approach which consists of selective hydrogenation of Polycyclic Aromatic Hydrocarbons. Self-Consistent-Field (SCF) (Hartree‐Fock and DFT) and multi-configurational (CISD and MCSCF) calculations on coronene and corannulene, both hexahydrogenated, show that the formation of stable high spin species is possible. The spin of the ground states is discussed in terms of the Hund rule and Lieb’s theorem for bipartite lattices (alternant hydrocarbons in this case). This proposal opens a new door to magnetism in the organic world.

New correlation energy functionals with explicit dependence on the number of electrons

Based upon the idea of effective number of electrons, we develop simple but accurate correlation energy functionals to be used for density functional theory calculations. We derive both a spin-independent and a spin-dependent functional. The spin-dependent one, used in conjunction with Becke’s exchange functional [A. D. Becke, Phys. Rev. A 38, 3098 (1988)], yields excellent results for thermochemistry calculations, giving an average absolute error of 2.9 kcal/mol for a test set comprised of the enthalpies of formation of the 148 molecules in the extended G2 set [L. A. Curtiss, K. Raghavachari, P. C. Redfern, and J. A. Pople, J. Chem. Phys. 106, 1063 (1997); L. A. Curtiss, P. C. Redfern, K. Raghavachari, and J. A. Pople, ibid. 109, 42 (1998)] plus the total energies of the atoms H through Ar. We also discuss the problem of fractional occupation number, and we show that the corresponding principle of integer preference can be fulfilled by the procedure that we propose to build correlation energy functionals.

Correlation energy functionals dependent on an effective number of electrons: Charged species and equilibrium geometries

Recently proposed spin-dependent and spin-independent correlation energy functionals [Perez-Jimenez et al., J. Chem. Phys. 116, 10571 (2002)] based on an effective number of electrons N are extended to deal with charged systems. By introducing the concept of an effective atomic number Z analogous to N, the spin-dependent functional in combination with Beckes exchange [Becke, Phys. Rev. A 38, 3098 (1988)] yields a mean absolute error (MAE) of 5.4 kcal/mol for the 88 ionization potentials and 58 electron affinities included in the extended G2 set, and a MAE of 4.1 kcal/mol for the 312 data comprising the above plus the 148 enthalpies of formation of the extended G2 set and the 18 total energies of the neutral atoms H through Ar. Geometry optimizations performed on the 53 molecules of the G2-1 test set with the above combination of exchange and correlation functionals yield MAEs of 0.017 A and 1.5 degrees for the 68 bond lengths and 29 angles analyzed as compared with the experimental estimates.

Learning to Play in a Stylized (Chinos) Game: Some Preliminary Results

In a previous paper, we studied the rich strategic and information‐revelation considerations arising in simple guessing game which is very popular in some European countries: the so‐called Chinos game. Here, we complement that analysis with a preliminary study of the learning dynamics induced when players no longer can assume that others play according to fixed (possibly equilibrium) rules but, instead, have to use past evidence to understand or interpret their opponent’s behavior. We show that if all players are flexible learners, they will eventually succeed in playing according to the unique Nash equilibrium of the game. This outcome, however, is also found to be rather fragile: if only one player (say, a “noise agent”) is unable or unwilling to learn, this wrecks havoc in the overall population dynamics. In that case, this dynamics never settles and a complex cyclical pattern typically ensues. The implications of this conclusion for the study of information‐revelation mechanisms in the real world (e.g...