Clusters computer code

The computational demands for Hollander s simulations (in 2000) were typically of the order of 1 CPU week on a PentiumIII/500 MHz at 400 MB of memory per run. The computer code was fully parallelized and was run on a 12 CPU Beowulf cluster. 

Using particle partitioning, particle codes exhibit excellent scalability on distributed computing platforms (i.e., cluster computers). However, with complex chemistry, care must be taken when implementing chemical lookup tables to avoid scale-up bottlenecks. 

One might, of course, try to resolve the failures of the standard singlereference CC approaches at larger intemuclear separations in a bmte-force manner by including the triply excited, quadruply excited, pentuply excited, etc. clusters in a completely iterative fashion (a new programming technique developed by Kdllay and Suij 74) allows one to write efficient computer codes for CC methods with clusters of any rank). Unfortunately, the resulting... 

One-electron cluster calculations are carried out based on the local density functional approach using the DV-Xtt computation code developed by Adachi et al. (5). The exchange-correlation energy is taken into account using the Slater s XCl potential (6). In the present work, a is set at 0.7, which is found to be the most appropriate value in many cases (7). The MOs are obtained as linear combinations of atomic orbitals (LCAO). The most remarkable feature of our program is that the atomic orbitals (AOs) are numerically calculated in each iteration and optimized for the chemical environment. The details of this program have been discribed in ref. (5)... 

Full-scale treatments of correlation effects, found to be necessary to repair some of the known deficiencies of the HF model wavefunction, generally are done by Configuration Interaction theory [l),(5j. With highly developed computer codes it has been found possible to include more than 10s — 10 determinants, either explicitly or implicitly in the wavefunction expansion. Unfortunately, procedures for selecting the most important terms in the Cl expansion have proved to be a source of difficulty, despite successes of Coupled Cluster methods and related schemes (6],[7]. 

Since organometallic clusters are well defined molecular objects of finite size (although perhaps a bit large), standard molecular quantum mechanical methods like ab initio or density functional techniques are the ideal tools for the description of their electronic structures. Unfortunately, the dimensions of these molecules , combined with the fact that they contain transition metal atoms, places this class of compounds well beyond the range of practical ab initio calculations. Only recently, due to improvements in quantum mechanical computer codes and to the extremely rapid increase in computing power, has it become posable to successfully tackle the electronic structure problem of ligated dusters on a quantitative or semiquantitative basis. So far, most of the theoretical analyses have been based on approximate computational schemes, topological considerations, or even simple empirical rules. 

Summarizing, benchmark calculations were calculated for many different configurations of LWR-type fuel clusters and neutron absorbers. It is possible to very exactly model these systems in the computer code... 

Benchmark calculations performed on both fuel types utilizing the KENO IV (Ref. 3) computer code with cross, sections generated by EGGNIT (Ref. 4) and THERMOS (Ref. 5), have on the average been about 1% conservative in terms of keff, regardless of the neutron absorber placed between clusters. 

In this section, we discuss the results of the benchmark ECCSD calculations for the STO-3G [146] model of N2, used by Head-Gordon et al in their QCCD calculations [71,89], and the minimum basis set (MBS) S4 model introduced in Refs. [143-145]. As in our earlier work [78], all ECCSD, LECCSD, BECCSD, and QECCSD calculations reported here were performed using the pilot computer codes, in which the relevant cluster amplitudes, and determined by minimizing the... 

The impurity-state orbitals for the Cl crdculation were obtained by the spin-restricted LDA cluster calculation using the ordinary SCAT computation code. Although the Slater s Xa potential with a = 0.7 was used as the exchange-correlation potential, it has been confirmed that the results of the spin-restricted calculation is almost identical with those by the VWN potential. ... 

With respect to standard molecular-cluster techniques, this approach has some attractive features explicit reference is made to the HF LCAO periodic solution for the unperturbed (or perfect) host crystal. In particular, the self-embedding-consistent condition is satisfied, that is, in the absence of defects, the electronic structure in the cluster region coincides with that of the perfect host crystal there is no need to saturate dangling bonds the geometric constraints and the Madelung field of the environment are automatically included. With respect to the supercell technique, this approach does not present the problem of interaction between defects in different supercells, allows a more flexible definition of the cluster subspace, and permits the study of charged defects. The perturbed-cluster approach is implemented in the computer code EMBEDOl [703] and applied in the calculations of the point defects both in the bulk crystal, [704] and on the surface [705]. The difficulties of this approach are connected with the lattice-relaxation calculations. 

At first, the series has a new editor. I, Michael Springborg, am a professor of physical and theoretical chemistry at the University of Saarland in Saarbriicken, Germany, and have research interests on the border between chemistry and physics. The major parts of the research activities of my group concentrate on development and application of theoretical methods and accompanying computer codes for the calculation of properties of materials. Of particular interest to us are structural and electronic properties of systems that are larger than small molecules, but smaller than macroscopic solids. These systems include clusters and colloids, polymers and chain compounds, and surfaces without and with adsorbants. Also fundamental issues like the theoretical treatment of extended systems exposed to electromagnetic fields as well as foundations of density-functional theory are of interest to us. 

The band structures have been calculated by a computer code Solid 2000. The code is based on the Hartree-Fock SCF method for infinite 3D-periodic cyclic cluster [55] with the quasi-relativistic INDO Flamiltonian [56]. Based on the results... 

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