1,720,993 research outputs found
Ab initio study on the crystallographic solvation pattern of the cytosine-guanine base pair in DNA
No full textAb initio study of helium dimer
No full textThe interaction of the helium dimer system has been studied by means of a newly developed ab initio approach in the framework of the modern valence bond (VB) theory. The self consistent field for molecular interactions (SCF-MI) wave function is used as reference state for a multistructure VB calculation. The electron correlation contributions are evaluated by a strategy based on the optimization of the virtual space of the VB wave function. The resulting wave function, while very compact, provides results of quality comparable to that of the most accurate calculations realized on this system. The well depth of the interaction potential is found to be 33.7 μEh. The calculated potential is consistent with the existence of a bound vibrational state
Applications of a variational coupled-electron pair approach to the calculation of intermolecular interaction in the framework of the VB theory: Study of the van der Waals complex He-CH4 Source: JOURNAL OF CHEMICAL PHYSICS Volume: 113 Issue: 16 Pages: 6724-6735 Published: OCT 22 2000 Times Cited: 10
No full textA general nonorthogonal coupled electron pair approach for the evaluation of electron correlation contribution is presented in details. The self-consistent field for molecular interactions wave function is used as reference state for a multistructure valence bond (VB) calculation. The central idea of the method is the optimization of the virtual space of the VB wave function by means of a procedure very close to the independent electron pair approach (IEPA) scheme. All the orbitals employed are expanded in the basis set of their fragment so as to exclude the basis set superposition error (BSSE) in a priori fashion. As an example, the application to the study of the van der Waals complex He–CH4 is reported. The equilibrium geometry of the system occurs at a He–C distance of 3.6 Å , with the He atom pointing to the center of one of the faces of the CH4 molecule, with a well depth of 19 cm−1. The potential energy surface of the He–CH4 complex is used to determine the parameters of a potential model which is employed in close-coupling calculations of integral state-to-state cross sections for rotationally inelastic scattering of methane molecules with helium atoms. The predicted values are compared with the available experimental data
Stabilization of the Noncomplementary Guanine-Adenine Base Pairs by Zn(II) Ions. An ab Initio SCF-MI Study
No full textThe structure and energy of the guanine (G)-adenine (A) base pairs in the G(anti)-A(anti) and G(anti)-
A(syn) conformations as well as their complexes with hydrated zinc(II) ions have been investigated by the
self-consistent field for molecular interaction (SCF-MI) ab initio method. The formation of covalent bonds
between the Zn(II) ions and the N7 sites of the guanine bases results in an increase of the binding energy of
the G-A pairs. In addition, the SCF-MI calculations have also been performed for the guanine (G)-guanine
(G)-cytosine (C) (G-GC) base triplet and for its complex with a hydrated Zn(II) ion bound to the N7 site
of the third-strand guanine. The Zn(II) binding causes a significant stabilization of the G-GC triplet. Possible
implications for the conformational changes in alternating d(GA)n sequences are discussed
New basis set superposition error free ab initio MO-VB interaction potential: Molecular-dynamics simulation of water at critical and supercritical conditions
No full textRecently, a controversy has come to light in literature regarding the structure of water in nonambient
conditions. Disagreement is evident between the site–site pair correlation functions of water derived
from neutron diffraction and those obtained by computer simulations which employ effective
pairwise potentials to express the intermolecular interactions. In this paper the SCFMI method
~self-consistent field for molecular interaction! followed by nonorthogonal CI ~configuration
interaction! calculations was used to determine a new water–water interaction potential, which is
BSSE ~basis set superposition error! free in an a priori fashion. Extensive calculations were
performed on water dimer and trimer and a new parametrization of a NCC-like ~Niesar–Corongiu–
Clementi! potential was accomplished. This was employed in the molecular-dynamics simulation of
water. The effect of temperature and density variations was examined. Acceptable agreement
between site–site correlation functions derived from neutron diffraction data and from computer
simulation was reached. In particular, a weakening of the hydrogen bonded structure was observed
on approaching the critical point, which reproduces the experimental behavior. The simulations
were performed using the MOTECC ~modern techniques in computational chemistry! suite of
programs. The present results show the importance of BSSE-free nonorthogonal orbitals in an
accurate description of the intermolecular potential of water
The structure of lithium and potassium cations coordinated by ammonia molecules in the gas phase as revealed by ab initio SCF-MI calculations
No full textThe structure and the energetics of lithium and potassium cations coordinated by ammonia molecules were calculated by an ab initio Hartree Fock study in the absence of basis set superposition error (BSSE). Complexes formed by clusters consisting of up to six ammonia molecules and Li+ or K+ ions were investigated. Gradient geometry optimizations were performed in the framework of the self consistent field for molecular interactions (SCF-MI) theory adopting the recently implemented generalization of the method. The intermolecular interaction energies and PES are a priori corrected for the BSSE. The 6-31+G∗ basis set was used. Ion clusters are stabilized by the metal-N bond and the interaction structure allows the formation of the maximum number of such bonds in the first solvation shell. All the properties studied change in a systematic manner upon addition of a further ammonia molecule. Li+ and K+ complexes exhibit no sign of hydrogen bonding between the molecules of the first coordination shell. However, the Li+ complexes with five and six ammonia molecules show the formation of hydrogen bonds between the coordinated molecules of first and second shells. From the results, it follows that both Li+ and K+ ions form similar complexes with up to four ammonia molecules. However, in the Li+ complexes, the first coordination shell saturates with four molecules and leads to conformations differing from those of the K+ complexes. In accordance with recent neutron diffraction experimental analysis of the structure of Li+ and K+ in NH3 solutions [J. Chem. Phys. 112 (2000) 7147], the lithium is tetrahedrally coordinated while potassium results octahedrally coordinated by solvent molecules. The theoretical results are acceptably consistent with the experiments and can furnish insight into the identification of some features of the clustering process
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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