Pseudoatomic orbitals

The Siesta (Spanish Initiative for Electronic Simulations with Thousands of Atoms) method [383,384,400] achieves linear scaling by the exphcit use of locahzed Wannier-like functions and numerical pseudoatomic orbitals confined by a spherical infinite-potential wall [401]. As the restriction of the Siesta method we mention the difficulty of the all-electron calculations and use of only LDA/GGA exchange-correlation functionals. 

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 ... 

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... 

In the course of X-ray ED determination, experimental structure factor amplitudes are LS projected onto the superposition of density units. These units can be products of usual orbital basis fimctions [49, 50] (density-matrix fitting), or nucleus-centered density functions (pseudoatoms) [51]. 

In LDA calculations, pseudopotentials (or effective core potentials) are almost always used to increase the efficiency of calculations, even for calculations involving hydrogen. This allows smoother wavefunctions, which in turn reduces the number of basis functions. It has been found that transferability (the ability of a pseudoatom to mimic a full-core atom) is governed by norm conservation [39], and pseudopotentials are constructed so that the pseudo-orbitals match the full-core orbitals outside the core. 

This paper is purported to outline how the processes of hole segregation and self-organization can be modeled on the basis of the recently proposed string approach [18-20] involving the concept of pseudoatoms with quantized hole orbitals of rank tj. The condensation of pseudoatoms into pancakes with 1/8 initiates formation of bosonic stripes (BS) classified by the discrete width w =ija, where a -0.385 nm denotes the mean parameter of CuO2 layers. 

Fig. 6. Orbital energies of valence s orbitals for group 11 and 12 metals. DHF and HF refer to Dirac-Hartree Fock and Hartree-Fock results, respectively p denotes calculations for the pseudoatoms where the effect of the 4f shell (Au, Hg) and 5f shell (,nE, ujE) has been omitted. Data taken from Bagus et al. (1975) and Seth et...

The Hamiltonian integrals and the overlap integrals are calculated with atom-centered localized atomic orbitals under a two-center approximation. These atom-centered orbitals are constructed by solving modified KS equations of spherical pseudoatoms with confinement potentials ... 

Figure 8 Orbital energies of the valence ns electrons (n = 4 to 7). HF-ps and DF-ps indicate nonrelativistic and relativistic calculations for the pseudoatoms respectively, Note that the error in Ref. 100 has been corrected...

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