Atsushi Ikawa

Orbital Optimization in the Resonating Hartree-Fock Approximation and Its Application to the One Dimensional Hubbard Model

A tractable direct optimization algorithm is developed to optimize orbitals in the Slater determinants (S-dets) in a resonating Hartree-Fock (Res HF) wave function. We reduce the variation space using the orbitals to put the first order energy variation in the steepest descent direction. The orbitals in the next iteration are determined so as to minimize the energy functional including up to the second order variation. This algorithm is applied to the one dimensional Hubbard model of half-filling. The optimized S-dets much deviate from the trial S-dets prepared from the HF calculations. The Res HF ground state generated with a few S-dets explains from 99.9 to 95.0% of the ground state correlation energy in all the correlation regimes. We have spin correlation functions with the correct short and long range behaviors and the lowest triplet and singlet spin excitations with correct dispersions, suggesting that the optimization of orbitals incorporates long range spin fluctuations and their mode-mode couplings.

Electronic and Lattice Structures of Isolated Se Chains and Defects in Them.II

Electronic and lattice structures of terminal and internal defects in a Se chain are systematically studied with the semi-empirical model developed previously. Structures of the defects are as follows: negative ion 1 C 1 - , a little altered helix; neutral radical 2 C 1 0 , and positive singlet ion 1 C 1 + , planer structures with four atoms; positive triplet ion 3 C 1 + , an acute triangle; n-type polaron 2 P - , locally relaxed helix; p-type polaron 2 P + and self-trapped triplet exciton 3 Ex, planer zig-zag structures with five atoms; intimate valence alternation pair (IVAP), a ramification of a Se atom. The IVAP is of C 3 + -C 1 - type. Interactions of a Se chain with Li, H and O are studied as a model of the effects of the wall on a Se chain enclosed in a mordenite channel (M-Se). A Se chain makes unusual charge transfer complexes with Li. Hydrogen binds to an internal Se making it C 3 + type or blocks a terminal Se. Based on these calculations we propose a model for photo-induced events in M-Se.

Electronic and lattice structures of isolated Se chains and defects in them. I : A semi-empirical model and properties of regular Se helix

We make a semi-empirical model for chains of Se based on the INDO and MINDO type models. It includes some other elements to mimic Se chains enclosed in channels of mordenite crystals (M-Se). With this model we calculate the electronic and lattice structures of isolated Se chains under the unrestricted Hartree-Fock approximation. The model gives a helical structure and a band gap of a long Se chain consistent with data of M-Se. The band gap and width in finite chains have chain length dependences. The calculated rotational potential shows that the helical structure is considerably soft against fluctuations of dihedral angles. The rotational potential has another minimum to make a kink in the helix. The kink has no gap level. A long regular planer zigzag chain is unstable and remains to be semiconducting. The stabilization mechanism of the helical structure is elucidated.

The SDW-CDW Phase Transition in the One Dimensional Extended Hubbard Model

We study the SDW-CDW phase transition in the one dimensional extended Hubbard model at a half-filling. A scaling analysis using the resonating Hartree-Fock method with the numbers of electrons from 8 to 20 strongly suggests that the ground state is many-fold degenerate( 1 A 1 + , 1 B 1 + , 3 A 2 - , and 3 B 2 - states) in the SDW region while it is doubly degenerate( 1 A 1 + and 1 B 2 - states) in the CDW region. Only the 1 A 1 + state makes a continuous phase transition from the SDW state to the CDW one. The 1 B 1 + , 3 A 2 - , and 3 B 2 - states cross the 1 B 2 - state around the phase boundary. We show that the continuous phase transition in the 1 A 1 + state is brought about by defects connecting the SDW-CDW domains called halfons. The correlation structures of these states are shown to clarify the origin of the degeneracies of the ground states.

Resonating Hartree-Fock Studies on Electron Correlation and Bond Alternation in the One Dimensional Half Filled Hubbard Model

Effects of the on-site electron-electron Coulomb interaction U on bond alternation in the one dimensional half filled Hubbard model are studied by the resonating Hartree-Fock method. Two kinds of SDW neutral solitons S 0 A and S 0 B exist in a bond alternated lattice which are centered at a long bond and a short bond, respectively. S 0 A enhances but S 0 B suppresses the alternating bond order (ABO) due to bond alternation. Quantum fluctuations in the ground state involve more S 0 A s than S 0 B s because the former has lower energy than the latter with increasing bond alternation so that they enhance the ABO in the true ground state and consequently the bond alternation. The spin and singlet pair correlation functions show that electrons tend to make singlet pairs on nearest neighbor short bonds even from small U s and small bond alternations.

Geometry Dependence of the Optical Absorption Spectra of a Se Chain.

Optical absorption spectra of a regular Se chain are calculated with an INDO type model under the Hartree-Fock and single excitation configuration interaction approximations. The calculated spectra can explain the observed ones in isolated Se chains in mordenite channels (M-Se). If the chain is compressed along the chain axis, its dihedral angles become smaller and the lowest singlet excitation shifts toward lower energies. The red shift explain the observed ones in M-Se under pressure. It is suggested that the observed red shifts of the LP (lone pair) →σ * absorption peaks in liquid S and Se with the increase of temperature in semiconducting regions are due to the increase of bond angles in the chains.

Resonating Hartree-Fock Studies on Electronic Structures of Excited States in the One Dimensional Bond Alternated Hubbard Model

Electronic structures of low lying excited states in the 1D bond alternated Hubbard model are investigated using the resonating Hartree-Fock method. In homopolar states with the wave vector k =0, main units of quantum fluctuations (QFs) are two kinds of SDW neutral solitons whose centers are on a long bond and a short bond, respectively, and magnon fragments. QFs in ionic states contain charged defects such as SDW charged solitons and polarons as well as homopolar ones. The charge, spin and singlet pair correlation functions of the excited states show different correlation structures due to different QFs. The QFs are not spatially scaled but quantum motions of defects give constant correlation energy per site. The lowest singlet and triplet collective excitations have dispersions with gaps. Their QFs are k -dependent and scaled density wave like oscillations. A part of the QFs looks like unscaled SDW defects.

Design of Donor Oligomers to Produce Parallel Spins Upon Charge Transfer

Abstract We design covalently bonded dimers of donor molecules to give parallel spin coupling owing to the dynamic spin polarization of linking bonds if the donors become cation radicals upon charge transfer. We show the triplet nature of the ground state of the dications of the dimers by UHF calculations in the PPP model.

Electronic and lattice structures in potassium doped stage-1 polyacetylene

Summary form only given. Interchain transfers (ICT) via dopants and interchain Coulomb (ICC) and dopants Coulomb (DC) potentials in a Thomas Fermi approximation are calculated in a K doped stage-1 (CH)/sub x/.. Owing to the large sizes of the valence orbitals of K, dopants give a dense net of ICTs . We made Hatree Fock calculations to see their effects on the electronic and lattice structures of stage-1 (CH)/sub x/ using a resonable intrachain Hamiltonian. The ICTs little affect intrachain states but much change the orbital energy spectrum though only a little affect it near the Fermi level. The DC converts the BOW charged soliton lattice into a CDW bipolaron lattice. The ICC enlarges the gap and makes the phases of the bipolaron lattices ordered. The calculation with these three interactions resulted in highly K doped (CH)/sub x/ with a gap and lattice distortions similar to those of charged soliton lattices. Electron correlation may be important to get a metallic (CH)/ssub x/. Effects of incommensurability will also be disscussed.

Ionic state of meta- triphenyl- dicarbene

Abstract The electronic structure of an ionized high spin molecule, meta-triphenyl-dicarbene, is studied with a semi-empirical model for π and n electrons using the unrestricted Hartree-Fock (UHF) and resonating HF (Res HF) approximations. The calculated spin multiplicity of the ground state is quartet and the injected hole or electron is in the π system, which is consistent with the experiments. In the UHF approximation, it makes a SDW polaron localized around a carbene site. The Res HF approximation shows that a quantum motion of the SDW polaron between the two carbene sites and quantum fluctuations of its electronic structure more stabilizes the quartet state.