Natural Orbitals (natural + orbital)

Distribution by Scientific Domains

Selected Abstracts

Electron invariants and excited state structural analysis for electronic transitions within CIS, RPA, and TDDFT models

A. V. Luzanov
Abstract We revisit the interpretative scheme (Luzanov et al., Theor Exp Chem 1974, 10, 354) of singly excited configuration interaction (CIS) model given earlier at semiempirical level. Detailed computations and spectral (natural orbital) treatment of the CIS density matrices of various types are presented. The corresponding hole-particle densities and related excitation localization indices are described. All the quantities are extended to the excited states calculated in the random phase approximation and closely related time-dependent density functional theory (TDDFT). The localization indices and charge transfer numbers which are invoked to describe interfragment interactions provide a basis for our scheme which is referred to as the excited state structural analysis for electronic transitions. The proposed analysis is exemplified by various moderate and large-size conjugated molecules treated within ab initio TDDFT and the Parizer,Parr,Pople approximation. Finally, we propose a possible generalization to the electronic transitions between CIS-like states followed by applications to singlet organic biradicals treated within the ,-electron spin-flip CIS. 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

Spin-optimized resonating Hartree-Fock configuration interaction

Ryo Takeda
Abstract The resonating Hartree-Fock Configuration Interaction (Res HF-CI) method is an efficient tool to investigate complicated strongly correlated systems such as ion-radical systems. In this method, we explore several spin-unrestricted Hartree-Fock (UHF) solutions that are energetically low-lying. However it is difficult to choose the symmetry-broken references appropriately as the site increases. In this study, we present the spin-optimized procedure, which is based on the Lwdin spin-projection method, for the Res HF-CI theory, denoted as SO Res-HF CI. We apply this SO Res-HF CI method to depict the potential curves of typical polyradical systems and compared the computational results using complete-active-space (CAS) CI based on UHF natural orbital (UNO), spin-projected UHF, and the previous version of Res HF-CI. We discuss the relation between computational results and the electronic configurations that are important to cover the electron correlation effects for each system. Further, we apply SO Res-HF CI method for the simple organic radical. In addition, we extend this scheme to the GHF case, and show that the use of GHF as a seed of SO Res-HF CI is desirable for the spin-frustrated systems. 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source]

Theory of chemical bonds in metalloenzymes III: Full geometry optimization and vibration analysis of ferredoxin-type [2Fe,2S] cluster

Mitsuo Shoji
Abstract The nature of chemical bonds in a ferredoxin-type [2Fe,2S] cluster has been investigated on the basis of natural orbitals and several bond indices developed in Parts I and II of this study. The broken-symmetry hybrid density functional theory (BS-HDFT) with spin projection approach has been applied to elucidate the natural orbitals and occupation numbers for a model compound [Fe2S2(SCH3)4] (1), which is used to calculate the indices. The molecular structure, vibration frequencies, electronic structures, and magnetic properties in both oxidized and reduced forms of 1 have been calculated and compared with the experimental values. The optimized molecular structures after approximate spin projection have been in good agreement with experimental data. The structure changes upon one-electron reduction have been slight (<0.1 ) and only limited around one side of the Fe atom. Raman and infrared (IR) spectra have been calculated, and their vibration modes have been assigned using the bridging 34S isotope substitution. Their magnetic properties have been examined in terms of spin Hamiltonians that contain exchange interactions and double exchange interactions. The BS-HDFT methods have provided the magnetic parameters; i.e., effective exchange integral (J) values and valence delocalization (B) values, which agree with the experimental results. It is found that large charge transfer (CT) from the bridging sulfur to the iron atoms has led to the strong antiferromagnetic interactions between iron atoms. 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source]

Thinking inside the box: Novel linear scaling algorithm for Coulomb potential evaluation

David C. Thompson
Abstract Beginning with the Poisson equation, and expanding the electronic potential in terms of sine functions, the natural orbitals for describing the particle-in-a-box problem, we find that simple analytic forms can be found for the evaluation of the Coulomb energy for both the interacting and non-interacting system of N -electrons in a box. This method is reminiscent of fast-Fourier transform and scales linearly. To improve the usefulness of this result, we generalize the idea by considering a molecular system, embedded in a box, within which we determine the electrostatic potential, in the same manner as that described for our model systems. Within this general formalism, we consider both periodic and aperiodic recipes with specific application to systems described using Gaussian orbitals; although in principle the method is seen to be completely general. 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source]

Relativistic correlating basis sets for actinide atoms from 90Th to 103Lr

Takeshi Noro
Abstract For 14 actinide atoms from 90Th to 103Lr, contracted Gaussian-type function sets are developed for the description of correlations of the 5f, 6d, and 7s electrons. Basis sets for the 6d orbitals are also prepared, since the orbitals are important in molecular environments despite their vacancy in the ground state of some actinides. A segmented contraction scheme is employed for the compactness and efficiency. Contraction coefficients and exponents are so determined as to minimize the deviation from accurate natural orbitals of the lowest term arising from the 5fn,16d17s2 configuration. The spin-free relativistic effects are considered through the third-order Douglas-Kroll approximation. To test the present correlating sets, all-electron calculations are performed on the ground state of 90ThO molecule. The calculated spectroscopic constants are in excellent agreement with experimental values. 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source]

The Low-Lying Excited States of 2,2,-Bithiophene: A Theoretical Analysis

CHEMPHYSCHEM, Issue 12 2003
Mercedes Rubio Dr.
Abstract The low-energy regions of the singlet,singlet, singlet,triplet, and triplet,triplet electronic spectra of 2,2,-bithiophene are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 11Bu and 21Bu, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 23Ag and 23Bu, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states. [source]