Yuriko Aoki

A theoretical synthesis of polymers by using uniform localization of molecular orbitals: Proposal of an elongation method

The elongation method, a new approach to calculating the electronic structure of polymers efficiently, is proposed as a model for real polymerization reactions following the normal chain reaction processes of initiation, propagation, and termination. The calculations are carried out by repeating the uniform localization of wave functions with inclusion of the interaction between the end group of the cluster and an attacking molecule. As a first step of this treatment, the validity of our method is examined via application to regular and irregular model polymers consisting of hydrogen molecules at the extended Huckel level and comparing the results with those obtained from the conventional variational method. Furthermore, this treatment is performed on the real polymers such as polyethylene and polypropylene stereoisomers, and is proved to be reliable with advantages in computational time and storage. The approach is promising for application to very large systems on which direct variational calculations o...

A new localization scheme for the elongation method

A different localization scheme for the elongation method is developed based on regional molecular orbitals. This scheme is more efficient and more accurate than the previous one especially for covalently bonded systems with strongly delocalized pi electrons. Ab initio test calculations have been performed on three model systems: water chains, polyglycine, and cationic cyanine chains. The dependence on the size of the starting clusters and the effect of the basis set are investigated. Our results are compared with conventional ab initio calculations and it is found in all cases that the error per added unit levels off to a satisfactorily small value as long as the starting cluster is sufficiently large.

Electronic structure of poly(tetrafluoroethylene) studied by UPS, VUV absorption, and band calculations

The electronic structure of poly(tetrafluoroethylene) (PTFE) was studied by UPS, VUV absorption, and ab-initio MO calculations. The UPS spectra give a photoemission threshold energy of 10.6eV, with deeper valence band features consistent with the reported XPS and the oligomer vapour UPS spectra. The UPS spectra are also consistent with the density of states derived from the calculated band structure, which indicates that the uppermost part of the valence band is formed from the C and F 2p orbitals with C-C bonding and C-F antibonding combinations. The VUV absorption spectrum shows an intense peak at 7.7eV, which most probably corresponds to the valence excitation from the top of the valence band to the bottom of the conduction band. With these data, the structure of the occupied and vacant states are deduced.

Assessment of time-dependent density functional schemes for computing the oscillator strengths of benzene, phenol, aniline, and fluorobenzene.

In present study the relevance of using the time-dependent density functional theory (DFT) within the adiabatic approximation for computing oscillator strengths (f) is assessed using different LDA, GGA, and hybrid exchange-correlation (XC) functionals. In particular, we focus on the lowest-energy valence excitations, dominating the UV/visible absorption spectra and originating from benzenelike HOMO(pi)-->LUMO(pi(*)) transitions, of several aromatic molecules: benzene, phenol, aniline, and fluorobenzene. The TDDFT values are compared to both experimental results obtained from gas phase measurements and to results determined using several ab initio schemes: random phase approximation (RPA), configuration interaction single (CIS), and a series of linear response coupled-cluster calculations, CCS, CC2, and CCSD. In particular, the effect of the amount of Hartree-Fock (HF) exchange in the functional is highlighted, whereas a basis set investigation demonstrates the need of including diffuse functions. So, the hybrid XC functionals--and particularly BHandHLYP--provide f values in good agreement with the highly correlated CCSD scheme while these can be strongly underestimated using pure DFT functionals. These results also display systematic behaviors: (i) larger f and squares of the transition dipole moments (mid R:mumid R:(2)) are associated with larger excitation energies (DeltaE); (ii) these relationships present generally a linear character with R>0.9 in least-squares fit procedures; (iii) larger amounts of HF exchange in the XC functional lead to larger f, R:mumid R:(2), as well as DeltaE values; (iv) these increases in f, mid R:mumid R:(2), and DeltaE are related to increased HOMO-LUMO character; and (v) these relationships are, however, not universal since the linear regression parameters (the slopes and the intercepts at the origin) depend on the system under investigation as well as on the nature of the excited state.

Local density of states of aperiodic polymers using the localized orbitals from an ab initio elongation method

An elongation method, which was already proposed to calculate efficiently the electronic states of aperiodic polymers, is developed to provide the local density of states. Elongation method is carried out by repeating the uniform localization of wave functions with inclusion of the interaction between the cluster and an attacking molecule. At first, we develop this method to the ab initio level. Next, we show that the local density of states (LDOS) is obtained efficiently following the elongation step without treating the whole system directly. The validity of our method is examined via application to a hydrogen molecule cluster by comparing the results with those by the conventional ab initio method for the whole system. This approach makes it possible to investigate the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and their density of states on each site of a polymer chain, which will provide a novel approach to examine intra‐ and intermolecular reactivities within and between aperiodic polymers.

Lithium Salt Electride with an Excess Electron Pair—A Class of Nonlinear Optical Molecules for Extraordinary First Hyperpolarizability

A new lithium salt electride with an excess electron pair is designed, for the first time, by means of doping two sodium atoms into the lithium salt of pyridazine. For this series of electride molecules, the structures with all real frequencies and the static first hyperpolarizability (beta 0) are obtained at the second-order Møller-Plesset theory (MP2). Pyridazine H 4C 4N 2 becomes the lithium salt of pyridazine Li-H 3C 4N 2 as one H atom is substituted by Li. The lithium salt effect on hyperpolarizability is observed as the beta 0 value is increased by about 170 times from 5 to 859 au. For the electride effect, an electride H 4C 4N 2...Na 2 formed by doping two Na atoms into pyridazine, the beta 0 value is increased by about 3000 times from 5 to 1.5 x 10 (4) au. Furthermore, combining these two effects, that is, lithium salt effect and electride effect, more significant increase in beta 0 is expected. A new lithium salt electride Li-H 3C 4N 2...Na 2 is thus designed by doping two Na atoms into Li-H 3C 4N 2. It is found that the new lithium salt electride, Li-H 3C 4N 2...Na 2, has a very large beta 0 value (1.412 x 10 (6) au). The beta 0 value is 2.8 x 10 (5) times larger than that of H 4C 4N 2, 1644 times larger than that of Li-H 3C 4N 2, and still 93 times larger than that of the electride H 4C 4N 2...Na 2. This extraordinary beta 0 value is a new record and comes from its small transition energy and large difference in the dipole moments between the ground state and the excited state. The frequency-dependent beta is also obtained, and it shows almost the same trends as H 4C 4N 2 << Li-H 3C 4N 2 << H 4C 4N 2...Na 2 << Li-H 3C 4N 2...Na 2. This work proposes a new idea to design potential candidate molecules with high-performance NLO properties.

An elongation method for large systems toward bio-systems.

The elongation method, proposed in the early 1990s, originally for theoretical synthesis of aperiodic polymers, has been reviewed. The details of derivation of the localization scheme adopted by the elongation method are described along with the elongation processes. The reliability and efficiency of the elongation method have been proven by applying it to various models of bio-systems, such as gramicidin A, collagen, DNA, etc. By means of orbital shift, the elongation method has been successfully applied to delocalized π-conjugated systems. The so-called orbital shift works in such a way that during the elongation process, some strongly delocalized frozen orbitals are assigned as active orbitals and joined with the interaction of the attacking monomer. By this treatment, it has been demonstrated that the total energies and non-linear optical properties determined by the elongation method are more accurate even for bio-systems and delocalized systems like fused porphyrin wires. The elongation method has been further developed for treating any three-dimensional (3D) systems and its applicability is confirmed by applying it to entangled insulin models whose terminal is capped by both neutral and zwitterionic sequences.

The nitrogen edge‐doped effect on the static first hyperpolarizability of the supershort single‐walled carbon nanotube

The nitrogen edge‐doped effect on the structure, dipole moment, and first hyperpolarizability of the supershort single‐walled carbon nanotube (5, 0) has been studied systematically. For the nitrogen edge‐doped effect on the structure, the mean diameter on the nitrogen‐doped side (Du) decreases as the number of doped‐nitrogen (n) increases (4.044 (1) > 3.991 (2) > 3.941 (3) > 3.891 (4) > 3.844 Å (5)). Significantly, the nitrogen edge‐doped effects on the dipole moment and first hyperpolarizability are revealed for the first time and these new effects are dramatic for the supershort single‐walled carbon nanotube (5, 0). Among the β0 values of these seven nitrogen‐doped structures, the largest β0 (3155 au) is larger by almost 450 times than the very small β0 (7 au) of undoped structure (D5h). For nitrogen‐doped structures, the order of the β0 values is 3155 (1) > 2677 (2A) ≈ 2817 (2B) > 1465 (3A) ≈ 1458 (3B) > 670 (4) > 254 au (5), which shows two interesting relationships between the β0 value and nitrogen‐doped number: (1) the smaller the nitrogen‐doped number, the larger the β0 value. (2) The structures with the same number of doped‐nitrogen have almost the same β0 values (1465 for 3A and 1458 au for 3B). As for the frequency‐dependent β (−ω; ω, 0) and β (−2ω; ω, ω), the dependence on the nitrogen‐doped number (n) is similar to the case of static β0. For β (−2ω; ω, ω) values at ω = 0.005 au are 3220 (1) > 2720 (2A) ≈ 2862 (2B) > 1480 (3A) ≈ 1477 (3B) > 676 (4) > 256 au (5). In addition, the important monotonic dependences of the β value on the Du and electronic spatial extent 〈R2〉 are also observed. The new knowledge of influence the β value will be beneficial to design high‐performance nonlinear optical (NLO) materials.

An efficient cluster elongation method in density functional theory and its application to poly‐hydrogen‐bonding molecules

The elongation method to synthesize the electronic states of polymers is developed at the level of the density functional theory using the linear combination of Gaussian‐type orbitals local spin density method. In this treatment, the interactions between the localized molecular orbitals of a cluster and the canonical molecular orbitals of an attacking monomer are successively included, where the Kohn–Sham equation is self‐consistently solved instead of the Hartree–Fock equation in the conventional ab initio method. In the process of the cluster extension, an efficient treatment is implemented to calculate the matrix elements of Coulomb integral and exchange‐correlation potential. The reliability and the efficiency of this method are examined via applications to hydrogen molecule cluster, linear water cluster (H2O)n and formamide cluster (CHONHH2)n. It was shown that the present method saves significantly the computational time and disk storage in the large cluster calculations, and provides good agreement...

Efficiency and accuracy of the elongation method as applied to the electronic structures of large systems.

Current state of development of the elongation method originally proposed by Imamura is presented. Recent progress in methodology, including geometry optimization and employment of the fast multiple method, is highlighted. The accuracy and efficiency of the elongation method as compared to exact canonical Hartree–Fock and Kohn–Sham approaches are discussed. Potential applications are illustrated by wide range of calculations for model systems. The elongation calculations are demonstrated to be much more efficient compared to the conventional ones with high accuracy maintained. The elongation CPU time is shown by the model calculations as linear or sub‐linear scaling for quasi‐one‐dimensional systems. Future work of development into post‐Hartree–Fock methodologies are pointed out.