Daubechies wavelets are a powerful systematic basis set for electronic structure calculations because they are orthogonal and localized both in real and Fourier space. We describe in detail how this basis set can be used to obtain a highly efficient and accurate method for density functional electronic structure calculations. An implementation of this method is available in the ABINIT free software package. This code shows high systematic convergence properties, very good performances and an excellent efficiency for parallel calculations.

Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp(3) bonds and it provides an explanation to several features in experimental x-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that Z-carbon is a transparent wide band-gap semiconductor with a hardness comparable to diamond.

Using the minima hopping global geometry optimization method on the density functional potential energy surface we show that the energy landscape of boron clusters is glass like. Larger boron clusters have many structures which are lower in energy than the cages. This is in contrast to carbon and boron nitride systems which can be clearly identified as structure seekers. The differences in the potential energy landscape explain why carbon and boron nitride systems are found in nature whereas pure boron fullerenes have not been found. We thus present a methodology which can make predictions on the feasibility of the synthesis of new nano structures. The experimental synthesis of fullerenes is a very difficult task. The carbon fullerene structures were therefore theoretically predicted [1] long before they could be produced in the lab [2]. Many more hollow and enhodedrally doped fullerene structures made out of elements different from carbon have also been proposed since then theoretically [3] in searches of other possible building blocks for nano-sciences. It is however surprising that since the experimental discovery of the carbon fullerenes some 25 years ago no other fullerenes have been synthesized. So the question is whether experimentalists have just not yet found a way to synthesize these theoretically predicted fullerenes, or whether they do not exist at all in nature. We have recently shown [4] that all the theoretically proposed endohedral Si 20 fullerenes are meta-stable and can thus most likely not be found in nature. In this letter we investigate in detail boron clusters. Following the B 80 fullerene structure proposed by Szwacki et al. [5] various other fullerene [6] and stuffed fullerene structures [7] were proposed. Subsequently it was however shown for B 80 that there exist non-fullerene structures [8] which are lower in energy. We will contrast the characteristics of the potential energy landscape (PES) of these boron clusters with those of systems found in nature, namely carbon and boron nitride fullerenes and find that there are important differences.To explore the energy landscape of the boron, carbon and boron nitride clusters we do global geometry optimizations on the density functional potential energy surface with the minima hopping algorithm [9]. This algorithm can render the global minimum configuration as well as many other low energy meta-stable structures. All the density functional calculations are done with the BigDFT electronic structure code [10] which uses a systematic wavelet basis together with pseudopotentials [11] and the standard LDA [11] and PBE [12] exchange correlation functionals.We start out by analyzing the B 16 N 16 cluster which was found to be short lived in experiments [13]. In this system structural rigidity is imposed by a strong preference for sp2 hybridization [14] as well as by the requirement that bonds are only formed between atoms of different type. This leads to a small configurational density of states. As shown in Fig. 1 there exists a fairly large energ...

60439By adding a non-linear core correction to the well established Dual Space Gaussian type pseudopotentials for the chemical elements up to the third period, we construct improved pseudopotentials for the Perdew Burke Ernzerhof (PBE) [J. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)] functional and demonstrate that they exhibit excellent accuracy. Our benchmarks for the G2-1 test set show average atomization energy errors of only half a kcal/mol. The pseudopotentials also remain highly reliable for high pressure phases of crystalline solids. When supplemented by empirical dispersion corrections [S. Grimme, J. Comput. Chem. 27, 1787 (2006); S. Grimme, J. Antony, S. Ehrlich, and H. Krieg, J. Chem. Phys. 132, 154104 (2010)] the average error in the interaction energy between molecules is also about half a kcal/mol. The accuracy that can be obtained by these pseudopotentials in combination with a systematic basis set is well superior to the accuracy that can be obtained by commonly used medium size Gaussian basis sets in all-electron calculations.

State-of-the art experimental techniques such as scanning tunneling microscopy have great difficulties in extracting detailed structural information about molecules adsorbed on surfaces. By combining atomic force microscopy and Kelvin probe force microscopy with ab initio calculations, we demonstrate that we can obtain a wealth of detailed structural information about the molecule itself and its environment. Studying an FFPB molecule on a gold surface, we are able to determine its exact location on the surface, the nature of its bonding properties with neighboring molecules that lead to the growth of one-dimensional strips, and the internal torsions and bendings of the molecule.