Shiro Koseki

General atomic and molecular electronic structure system

A description of the ab initio quantum chemistry package GAMESS is presented. Chemical systems containing atoms through radon can be treated with wave functions ranging from the simplest closed‐shell case up to a general MCSCF case, permitting calculations at the necessary level of sophistication. Emphasis is given to novel features of the program. The parallelization strategy used in the RHF, ROHF, UHF, and GVB sections of the program is described, and detailed speecup results are given. Parallel calculations can be run on ordinary workstations as well as dedicated parallel machines.

Relativistic potential energy surfaces of XH2 (X=C, Si, Ge, Sn, and Pb) molecules: Coupling of 1A1 and 3B1 states

Potential energy surfaces of the 1A1 and 3B1 states for XH2 molecules (X=C, Si, Ge, Sn, Pb) are investigated with ab initio full valence multiconfigurational self‐consistent field wave functions, using effective core potentials. Spin–orbit coupling is also calculated to construct relativistic potential energy surfaces. The relativistic potential energy surfaces are compared with the adiabatic nonrelativistic potentials. Simple one dimensional Landau–Zener transition probabilities are calculated at the minimum energy crossing points of XH2 molecules to estimate the intersystem crossing probability.

Theoretical study on silicon‐nitride film growth: Ab initio molecular orbital calculations

The growth mechanism of SiN films in the chemical vapor deposition process was theoretically investigated using ab initio molecular orbital methods. A reactive intermediate SiH2 (or SiCl2) is produced by the thermal decomposition of SiH4 (or SiH2Cl2). The intermediate inserts into the N—H bond of NH3 to form H3Si—NH2 (or HCl2Si—NH2). From the Si atom of the insertion product, H2 (or HCl) is eliminated and a silylene‐like reaction site (—Si—H) is created again. Such processes generate various kinds of SiN particles. On the surface of SiN film, SiH2 (or SiCl2) insertion and H2 (or HCl) elimination seem to occur repeatedly, which leads to the SiN film growth. Thus, the chemical features of SiH2 and SiCl2 may explain the difference of the film‐growth mechanisms between the mixtures of SiH4+NH3 and SiH2Cl2+NH3.

Parallelization of multireference perturbation calculations with GAMESS

The quasi‐degenerate multireference second‐order perturbation theory (MRMP2) routines in the GAMESS suite of program codes have been parallelized using a distributed data interface (DDI). Two typical kinds of molecules were chosen for examination of parallelization speedup using one to eight PCs gathered as a cluster and connected by Fast Ethernet. The first example, in which total energies of several low‐lying electronic states have been obtained for niobium monohydride, give parallelization speedup of 7.15 when eight PCs were used as a cluster. The second example is the ground‐state total energy for a medium sized molecule, 4a,4b,8a,9a‐tetrahydro‐pyridino[1′,2′‐4,3]imidazo‐lidino[1,5‐a]pyridine. When distributed memory is employed, the parallelization speedup improves to 6.84 for the MRMP2 calculations when an eight‐PC cluster is used. These results demonstrate that our efforts to achieve the parallelization of MRMP2 routines have been successful.

Synthesis, electrochemistry and photoexcited-state properties of dinuclear ruthenium complexes bridged by 2,6′-bis(2-pyridyl)-2,2′:6,2″-thiazolo[4,5-d]-benzothiazole

Abstract The new bridging ligand, 2,6′-bis(2-pyridyl)-2,2′:6,2″-thiazolo[4,5-d]-benzothiazole (bptb) and its dinuclear Ru complexes have been synthesized. The combination of the transient absorption (TA) spectra and spectroelectrochemical difference spectra has led to the assignment of the lowest excited state for the dinuclear Ru(II) complexes as a Ru-to-bptb charge transfer state. Although the ab initio molecular calculations of bridging ligands have provided lower HOMO/LUMO energies for bptb compared to those of 2,6-bis(2-pyridyl)benzodiimidazole (dpimbH2), the degree of electronic coupling for the dinuclear bptb complex is smaller than that for the dpimbH2 complex from the analysis of intervalence charge transfer band. Since both dinuclear Ru complexes have a similar coordination environment, the Ru dπ-HOMO Lπ mixing is dominant for the metal-metal interaction through the hole transfer mechanism.

A model for SiNx CVD film growth mechanism by using SiH4 and NH3 source gases

This paper theoretically explores the SiNx CVD film growth mechanism by means of the ab initio molecular orbital method. In a chemical vapor deposition (CVD) reactor, an SiH4 and NH3 gas mixture produces silylenes (X-Si-Y: X and Y are substituents). The insertion of silylene into a surface Si-H or N-H bond is the important part of the CVD film growth mechanism. Following the insertion, H2-elimination reaction occurs from the surface. The film growth mechanism explains Si-H and N-H bonds remaining ih an SiNx film and the deviation from the stoichiometry. Based on the mechanism, it is suggested that a new source gas, SiNH5, instead of SiH4 deposits a stoichiometric SiNx film, and that a transition layer exists between the native oxide and the bulk SiNx film.

Theoretical study on the thermal isomerization reaction between meso and dl cyclomers of 4a,6a,10a,10b-tetrahydropyrido[2,1-c]pyrido[1,2-a]piperazine and its lower homologue

Abstract The reaction mechanism of thermal isomerization from a meso to a dl cyclomer of the titled compounds has been studied theoretically using ab initio RHF, GVB, and MCSCF methods with 6-31G(d) and TZV(d,p) basis sets, followed by the second-order Moeller–Plesset perturbation (MP2) calculations. In 4a,6a,10a,10b-tetrahydropyrido[2,1-c]pyrido[1,2-a]piperazine ( 1 ), it is shown that the energy barrier from a singlet diradical intermediate to the dl cyclomer is smaller than that of the meso cyclomer by 3xa0kcal/mol. Concomitantly, the transition-state structure for the latter case is spatially three times far apart from the intermediate in comparison with that for the former case. This suggests the preference for a formation of the dl cyclomer over the meso cyclomer. On the other hand, it is shown in 4a,4b,8a,9a-tetrahydropyridino[1′,2′-4,3]imidazolidino[1,5-a]pyridine ( 2 ) that no thermal isomerization reaction should take place from the meso to the dl cyclomer. This is because there is essentially no energy barrier from the intermediate to the meso cyclomer but an energy barrier of about 10xa0kcal/mol from the intermediate to the dl cyclomer. Besides, the transition-state structure for the latter case is spatially far apart in comparison with that for the former case. The present findings are compatible with the available experimental information.

Structural properties of 3,10-diazadispiro[5.0.5.3]pentadeca-1,4,8,11-tetraene and its homologues: a generation of an unusually long CC single bond by through-bond interaction

Abstract Full geometry optimizations using the MNDO MO method have been carried out on the title molecule and its homologues. The CC single bond joining the two 1,4-dihydropyridine rings is found to be unusually long, especially for 3,10-diazadispiro-[5.0.5.2]tetradeca-1,4,8,11-tetraene, 1.618 A. A perturbational MO analysis clearly reveals that a through-bond interaction is operative between the two non-conjugated π systems via the CC single bond. The origin of bond elongation common to the molecules is taken as the combined effects due to the through-bond interaction and the inter-ring steric repulsion. The available experimental facts are reasonably accounted for by the theoretical results.