Pseudoatom types

Similar results were obtained for the VC-B and MoC-B systems. Only a small decrease in the B2p-M(nd) states overlap was observed with an increase of the atomic number of the metal. Transformation of the electron states of the isolated B impurity depending on the band structure of the matrix (TiC and TiN) shows that B LDOSs are again determined by the relative energies of the impurity and crystal states. The increase of the interval between the states (from TiC-B to TiN-B) leads to an enhancement of the impurity state localisation, which in the case of TiO-B results in a clear pseudoatomic type for the LDOS of the impurity. 

For a general formulation of the Zintl-Klemm concept, consider an intermetallic AmX phase, where A is the more electropositive element, t3 pically an alkali or an alkaline earth metal. Both A and X, viewed as individual atoms, are assumed to follow the octet rule leading to transfer of electrons from A to X, i.e., A AF, X —> X , so that mp = nq. The anionic unit X arising from this electron transfer is considered to be a pseudoatom, which exhibits a structural chemistry closely related to that of the isoelectronic elements [11]. Since bonding also is possible in the cationic units, the numbers of electrons involved in A-A and X-X bonds of various types (caa and exx> respectively) as well as the number of electrons e not involved in localized bonds can be generated from the numbers of valence electrons on A and X, namely and ex, respectively, by the following equations of balance ... 

Because electron density is a local property, electron density studies of the peptide-like molecules show that the nonspherical part of the deformation density (i.e the P]m parameters of Eq. 8) remain essentially the same for a given atom in the same environment (the peptide residue, a phenyl ring, a methyl group...) [29], The same observation was made for porphyrin ligands [30] and by Brock, Dunitz, and Hirshfeld [37] for naphthalene and anthracene type molecules. All these observations suggest that the multipole parameters are highly transferrable from one atom to a chemically similar atom in different molecules and crystals. A key question is is it possible to determine for each chemical type of a given atom a small set of pseudoatom multipole parameters, and can such parameters be used to calculate electrostatic properties of new molecules To answer this question [29], two accurate but low resolution X-ray data sets (sin 0/Xmax = 0.65 A-1) were... 

Another approach to treating the boundary between covalently bonded QM and MM systems is the connection atom method,119 120 in which rather than a link atom, a monovalent pseudoatom is used. This connection atom is parameterized to give the correct behavior of the partitioned covalent bond. The connection atoms interact with the other QM atoms as a (specifically parameterized) QM atom, and with the other MM atoms as a standard carbon atom. This avoids the problem of a supplementary atom in the system, as the connection atom and the classical frontier atom are unified. However, the need to reparameterize for each type of covalent bond at a given level of quantum chemical theory is a laborious task.121 The connection atom method has been implemented for semiempirical molecular orbital (AMI and PM3)119 and density functional theory120 levels of theory. Tests carried out by Antes and Thiel to validate the connection atom method at the semiempirical level suggested that the connection atom approach is more accurate than the standard link atom approach.119... 

The side chain dihedral angle Xi is important for high-resolution definition of protein structures as it determines the angle at which each amino acid side chain branches out from the protein backbone. Moreover, in combination with certain types of NOEs, it can allow stereospecific assignment of prochiral /3-methylene protons, which improves the precision of NMR structures by obviating the need to include pseudoatom distance corrections.140-142... 

Initially, this tight-binding scheme has been designed to model materials with tetrahedral local order. Porezag et aL [76] have proposed a TB scheme based on the DPT theory (DFTB). In this approach, the pseudoatomic orbitals Slater-type orbitals and optimized to be solutions of the self-consistent modified atomic Kohn-Sham equations ... 

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