1. Current Research and Principal Research Interests
Our principal interests focus on the development and application of quantum chemical theories which are applicable to electronic structures of various types molecules in both the ground and excited states, aiming the theories feasible for sizable molecules and electronically excited states of organic high spin molecules as models for moleculebased magnetic materials. Our approach is based on the SACCI method.
Nowadays the sophisticated
ab initio calculations are becoming more and more useful tools for researches
in the broad areas of chemistry, physics and related sciences. However,
the quantitative description of the electronic structure for openshell
systems has long been a challenging subject in quantum chemistry. The long
expansion of the wave function is generally required for such systems if
the conventional configuration interaction (CI) method is used. This is
due to the complicated correlation effects among electrons in openshell
systems. On the other hand, for closedshell systems, it is well known
that the socalled dynamical correlation effects can be treated with good
accuracy by the coupled cluster (exponential) expansion with single and
double excitations from the HartreeFock (HF) electronic configuration.
The symmetryadapted cluster (SAC) expansion is one of the coupled cluster
type theories. In 1978, Nakatsuji proposed an attractive approach to the
electronic structure theory for openshell systems, called symmetryadaptedcluster
configurationinteraction (SACCI). In the SACCI method, the wave function
is represented by a linear combination of excited functions from correlated
one described by the SAC expansion instead of the uncorrelated HF configuration.
Bypassing the explicit consideration of manyelectron excitations from
the HF configuration usually required for openshell systems, the SACCI
method remarkably reduces the degrees of freedom to describe the correlated
wave functions for openshell systems including excited states, so that
computations of their complicated wave functions become feasible by invoking
accessible computational resources.
Our research activities can be categorized as follows:
(1) Application of the SACCI method to spectroscopic studies.
The SACCI method has been implemented up to septet states and has been applied to various states including the ground, excited, ionized, and electronattached states (see Ref. below). We applied the SACCI method to electronic spectra of pigments of industrial importance, phthalocyanines (tetrabenzotetraazaporphyrins), before the recent calculations by time dependent density functional theory (TDDFT) were reported for the molecules. The electronic structures of this class of pigments in the excited states have been studied by some groups in terms of semi empirical methods but there were few ab initio calculations for the excited states. The SACCI method can treat such large molecules as phthalocyanines when used with the socalled perturbation selection technique. The assignments of absorption bands up to 4.4eV were made based on the theoretical spectra by the SACCI method. The electronic structure of the B band in UV region calculated by us is in good agreement with that proposed by MCD experimentalists.
We apply the SACCI method to the absorption and emission spectroscopic analysis for triplettriplet or quintetquintet transitions occurring in typical highspin hydrocarbons such as diphenylenemethylene or mphenylenebis(phenylmethylene).
(2) Development of analytical gradients for the SAC/SACCI method: Aiming to predict general molecular properties of the ground, excited, ionized, electron attached and high spin states.
It is required to evaluate energy derivatives for predicting general molecular properties, for example, electric dipole moments, geometries, chemical shifts, etc. For the closedshell systems, the rapid developments of the analytical gradient method have enabled us to perform geometry optimizations as routine works. However, the stable and semi stable geometries of molecules with high spin multiplicities or in the excited states are still hard to compute. We have formulated analytical expressions of the first derivatives of energies and have developed computer programs to calculate the analytical energy derivatives for the SACCI method. The programs have been used to calculate equilibrium geometries and adiabatic (00) excitation energies of the ground and excited states of some small molecules with singlet to quartet spin multiplicities.
(3) Development of an efficient localized MO based theory for very large systems.
Expanding applicability of the theory to larger systems is one of the most important subjects in modern quantum chemistry. The shortrange nature of the correlation among electrons is useful to make the theory computationally less demanding without loosing substantial accuracy. This can be realized through the localization of molecular orbitals and dropping less important nonlocally excited functions from the SAC and SACCI expansions. We also developed programs for calculating the energy gradients more economically than those based on the conventional delocalized MO formulation.
(4) Methodological establishment of ab initio calculations for magnetic properties of organic openshell systems.
Prediction for fine structure tensors of spin multiplet states from molecular systems has been a longstanding issue in chemistry since Higuchi's pioneering work. Correlating three spin problems have challenged theoreticians. Recently, experimentally determined fine structure constants from high spin states of highly symmetric molecular systems have appeared, attracting general attention. Spin correlation functions have rarely been used for calculating fine structure tensors so far. In this project, we develop methods in ab initio level for calculating fine structure tensors by invoking spin correlation functions and spincoupled wave functions for high spin hydrocarbons and other high spin molecules of chemical importance, combining the SACCI method with the spin correlation functional analysis. We predict the fine structure tensors of the electronic excited states from various types of high spin molecules, serving for experimental challenges for the detection of the excited states.
2. Selected Publications
1. "SACCI study of the excited states of free base tetrazaporphin", K. Toyota, J. Hasegawa, and H. Nakatsuji, Chem. Phys. Lett., 250, 437442 (1996).
2. "Excited states of free base phthalocyanine studied by the SACCI method", K. Toyota, J. Hasegawa, and H. Nakatsuji, J. Phys. Chem. A, 101, 446451 (1997).
3. "Analytical energy gradients of the excited, ionized, and electronattached states calculated by the SACCI generalR method", M. Ishida, K. Toyota, M. Ehara, and H. Nakatsuji, Chem. Phys. Lett., 347, 493498 (2001).
4. "Analytical energy gradient of highspin multiplet state calculated by the SACCI method", M. Ishida, K. Toyota, M. Ehara, and H. Nakatsuji, Chem. Phys. Lett., 350, 351358 (2001).
5. "Elimination of singularities in molecular orbital derivatives: minimum orbitaldeformation (MOD) method", K. Toyota, M. Ehara, H. Nakatsuji, Chem. Phys. Lett., 356, 16 (2002).
6. "SACCI generalR method: Theory and applications to the multielectron processes", M. Ehara, M. Ishida, K. Toyota, and H. Nakatsuji (Ed. by K. D. Sen, "Reviews in modern quantum chemistry A celebration of the contributions of Robert G Parr", pp. 293319), World Scientific, Singapore, in press.
