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The isotropic hyperfine coupling constants (HFCCs) are calculated for H2O+·, CH2O+·, CH3O·, CH3NH·, CH3CH2·, and CH3OCH+·3, using the method of SAC (symmetry adapted cluster expansion)-CI. After examining various basis sets, we found that the double-zeta quality basis sets of Dunning are the best among the sets examined. The calculated values agree fairly well with the experimental values. We point out that the conventional configuration selection based on the energy criterion has an inherent limit for the calculation of HFCCs. The influence of molecular vibrations on the HFCCs has been semiquantitatively examined for CH2O+·, CH3O·, CH3NH·, and CH3CH·2. [ABSTRACT FROM AUTHOR]

We propose a model for chemisorptions and surface reactions in which ‘‘adcluster’’ (admolecule+cluster) is dipped onto the electron ‘‘bath’’ of a solid metal and an equilibrium is established for the electron and/or spin transfer between them. The equilibrium condition is described with the use of the chemical potentials of the adcluster and the solid surface. Since the adcluster is a partial system, the number of the transferred electrons n is not necessarily an integer. Some typical behaviors of the energy E(n) of the adcluster are explained. A molecular orbital model is proposed to calculate the energy E(n) and the electronic structure of the adcluster. The electrostatic energy due to a charge polarization is also important though it is insensitive to the chemical structure of the adcluster. Sample application is given for the Pd–O2 system. [ABSTRACT FROM AUTHOR]

We consider several generalizations of the exponential ansatz in a rather formal way, giving several new wave functions which we call exponentially generated (EG) wave functions. There are three distinct ways of the exponential-type generations of the wave functions, two of which are new. They are named ESAC (extended symmetry-adapted-cluster) wave function and exponentially generated CI (EGCI) wave function. The ESAC wave function is a simple extension of the SAC wave function and is applicable even when the Hartree–Fock reference configuration is not dominant. The EGCI wave function is a CI wave function constructed in the spirit of the cluster expansion theory. Formally, it has the merits of both the CI theory and the cluster expansion theory; for example, the upper bound nature, size consistency, and the applicability to quasidegenerate states and excited states. We then introduce several new wave functions by a multiple and mixed use of the exponential-type operators. We call such a class of wave functions multiexponentially generated (MEG) wave functions. There are many possibilities for the MEG wave functions, and the MR-SAC wave function proposed previously is one of them. When the system involves several classes of electron correlations, the MEG wave function permits an optimal (physically and practically) use of the exponential-type operators for the distinct classes of electron correlations. We described the method of solution of the EG and MEG wave functions and examined size consistency and some other properties. [ABSTRACT FROM AUTHOR]

Multireference cluster expansion theory, called MR–SAC (multireference symmetry-adapted-cluster) theory, is presented. This theory is exact and unique, and yet does not include noncommutative algebra operator without imposing the completeness of the multireference space. The ansatz is simple enough for a general use in the study of potential energy surfaces of the ground and excited states of molecules. We have explained a recommended choice of the multireference operators on the basis of the analysis of an origin of the breakdown of the single reference theory. The method of solution of the MR–SAC theory is shown. The theory is given for the ground and excited states of closed-shell molecules, doublet and triplet states, and molecules with other symmetries. Test calculations are given for the ground and excited states of the CO molecule at the equilibrium and elongated distances. [ABSTRACT FROM AUTHOR]

The symmetry adapted cluster (SAC) and SAC-configuration interaction (SAC-CI) theories are applied to the calculations of the ground and excited states of MnO-4. With the use of three different active spaces, we examine the convergence of the results. Electron correlations work to relax charge polarizations of the Mn–O bonds in the ground state. The experimental spectrum of MnO-4 is well reproduced by the present calculations. All of the observed peaks are assigned to the electronic allowed transitions to the 1T2 excited states. They are characterized as the excitations from the nonbonding orbitals of oxygen to the antibonding orbitals between the metal and oxygens. For reasonable descriptions of the ground and excited states of MnO-4, electron correlations should be considered with the use of large enough active space for expressing the relaxations of valence electrons. In this sense, all of the previous theoretical assignments of the spectra are unreliable, since they are not the results of calculations including sufficient amount of electron correlations. [ABSTRACT FROM AUTHOR]

An ab initio model of scanning tunneling microscopy (STM) is presented in the framework of the perturbation theory of Bardeen. The tip and sample are represented by the cluster model, and the first-order wave functions are calculated by ab initio Hartree–Fock (HF), configuration interaction (CI), and symmetry adapted cluster (SAC)/SAC-CI methods. This model was applied to simple tip–sample systems such as Pd2–Ag2, Pd2–C6H6, Pd2–(C6H6)2, and Li2–Li2. The calculated STM images were related to the HOMO, LUMO, etc. orbital symmetries of the tip–sample systems. The contribution of the Rydberg orbital was examined at different tip–sample distances. Electron correlations were found to modify the behavior of the tunneling transition probability of the Li2–Li2 system vs that calculated by the HF method. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The symmetry adapted cluster (SAC) and SAC-configuration interaction (SAC-CI) theories are applied to the calculations of the ground and excited states of the chromate ion CrO2-4. Electron correlations are very large for this molecule and work to relax the charge polarization of the Cr–O bonds in the ground state. The experimental spectrum of CrO2-4 is well reproduced by the present calculations, which is the first ab initio study of the excited states including electron correlations. All of the observed peaks are assigned to the dipole allowed transitions to the 1T2 excited states. Furthermore, many kinds of forbidden transitions are calculated in the lower energy region. Both allowed and forbidden transitions are characterized as the electron-transfer excitations from oxygen to metal. In comparison with the previous theoretical studies, the present SAC-CI results are in good agreement with experiment and give reliable assignments of the spectrum. We also compare the electronic structures and spectra of CrO2-4, MoO2-4, MnO-4, RuO4, and OsO4, which have been studied by the SAC and SAC-CI methods. [ABSTRACT FROM AUTHOR]

Quasi-relativistic ab initio calculations are performed for (super 199)Hg nuclear magnetic shielding constants and chemical shifts in a series, which included spin free relativistic (SFR) term and magnetic interaction (RMI) term. It is important to increase in the flexibility of the innermost and outermost orbitals of the basis set.