Eight isomeric two-dimensional graphane sheets are found in a theoretical study. Four of these nets-two built on chair cyclohexanes, two on boat-are more stable thermodynamically than the isomeric benzene, or polyacetylene. Three-dimensional crystals are built up from the two-dimensional sheets, and their hypothetical behavior under pressure (up to 300 GPa) is explored. While the three-dimensional graphanes remain, as expected, insulating or semiconducting in this pressure range, there is a remarkable inversion in stability of the five crystals studied. Two stacking polytypes that are not the most stable at ambient pressure (one based on an unusual chair cyclohexane net, the other on a boat) are significantly stabilized with increasing pressure relative to stackings of simple chair sheets. The explanation may lie in the balance on intra and intersheet contacts in the extended arrays. Our interest in the field developed from another direction. We have been studying theoretically benzene (C 6 H 6 ) under high pressure. The stoichiometry of the material is, of course, the same as that of graphane. In our calculations we first found that benzene phases under pressure underwent (at a certain pressure) phase transformation to saturated CH structures that were more stable. Among these more stable structures were graphanes. We report here the three-dimensional saturated CH graphane structures, some unique, that emerge from our theoretical work, and their computed behavior under pressure.Results and Discussion CH Structures from Evolutionary Structure Predictions. Once we found some saturated CH structures that were more stable than benzene, we began to look for more of them by evolutionary algorithm structure prediction, using the USPEX method/code (15-17) over the range of 0 to 300 GPa.Five graphane stackings emerged as low in enthalpy at one or another pressure. These multilayer graphane structures are shown in Fig. 1. We call them:• graphane I (space group P-3m1, Z ¼ 2, i.e., 2 CH in the unit cell, -AA-stacking),Note that just as for graphite, diamond, and SiC, there are many potential stacking polytypes built on any component sheet, e.g., -ABC-, -AABB-stacking and so on. These stackings were not explored by us; all of them are likely to be of similar enthalpy at ambient pressure to these five structures.

In the first of a series of four papers on hydrogen under pressure, and its transitions from an initiating molecular state, we begin by defining carefully the problem, and setting the distance scale of interactions of protons and electrons in molecular aggregates of the first of the elements. Following a review of the experimental situation, in particular the phase diagram of hydrogen, in as much as it is known, and the behavior of its vibrons and rotons, we move onto the setting up of a numerical laboratory for probing the underlying physics and chemistry of interactions in hydrogen as the pressure increases. The laboratory consists of the preferred static structures emerging from calculations on the system in the range of 1 atm to 500 GPa, those of Pickard and Needs. The intermolecular (inter-pair) H···H separations naturally decrease with increasing pressure, first rapidly so, then more slowly. The intramolecular (intra-pair) H-H distances vary over a much smaller scale (0.05 Å) as the pressure increases, first decreasing, then increasing, and finally decreasing. We define an equalization function to gauge the approach to equality of the first neighbor and shortest next neighbor H (proton) separations in this numerical laboratory. And we find that metallization is likely to occur before bond equalization.

The role of local geometric and stereo-electronic effects in tuning the preference for different cross-linked adducts between thymine and purinic bases has been analyzed by a computational approach rooted in density functional theory. Our study points out that G--T and T--G tandem lesions are produced according to the same mechanism as A--T and T--A intrastrand adducts, and in both cases purine--T adducts are preferred rather than the opposite sequences. Moreover, use of conceptual DFT tools allows the rationalization of the preferential occurrence of G--T and T--G tandem lesions in place of their A--T and T--A counterparts.