bluuebunny/crossref_metadata_2025_split huggingface dataset:
Core projection effects in atomic frozen‐core calculations: A numerical analysis
Authors: Luaña V. , Pueyo L. 
Published: January 2005

AbstractFrozen‐core calculations on the (He) 2s22p6 – 1S state of the F− ion and the (Ar) 3d44s2 – 5D state of the Cr atom, obtained with differently reduced basis sets and different core‐projection schemes, are presented and compared with the corresponding all‐electron results in order to analyze the behavior of the core projection under these circumstances. Severe truncation of the valence basis set can give frozen‐core results noticeably deviated from the all electron values if the regular projection (projection factors xg = 2.0) is adopted. Much better results are obtained in these cases if a softer projection (xg = 1.0) is used for those core orbitals having high electronic density in regions where the valence basis set has low flexibility. In these circumstances the expectation value of the core projector becomes a nonvanishing correction to the valence energy because the core‐valence orthogonality is incomplete after core projection.
SCF Deformation densities and electrostatic potentials of purines and pyrimidines
Authors: Eisenstein M. 
Published: January 2005

Abstract4‐31G wave functions have been computed for five purines and pyrimidines. The calculated deformation densities have been partitioned into atomic fragments, which were integrated to yield atomic multipole moments. The transferability of atomic fragments between related molecules was verified by constructing model maps for uracil and guanine from appropriate fragments of cytosine and adenine. Model electrostatic potentials calculated from the moments of model atoms are similar to the corresponding 4‐31G potentials. Comparison of 4‐31G and 4‐31G** deformation densities of cytosine provides simple rules for estimating the effects of polarization functions on the atomic multipole moments of most atom types occurring in the purines and pyrimidines. These rules were applied to the other molecules and yielded reasonable approximations for their molecular dipole moments. Substituting CH3 for H has little effect on the deformation density beyond the substitution center.
A comparative study of the bonding in different halides of iridium
Authors: El‐Issa B. D. , Katrib A. , Ghodsian R. , Salsa' B. A. , Addassi S. H. 
Published: January 2005

AbstractX‐ray photoelectron spectroscopy and multiple scattering Xα calculations have been applied to a series of iridium halide complexes in order to corroborate the nature of the bondings inherent in this class of compounds. Our results seem to substantiate contentiously that higher oxidation states of iridium favor the formation of covalent bonds. This conclusion is based on the observation that (1) successive bombardment of the iridium species by Ar ions almost definitely leads to a configuration in which iridium is bound to at most one halide ion, and (2) the theoretical charge per ligand ion approaches systematically a value of {1−} in the limit as the formal oxidation state of iridium approaches {1+}. The theoretical results are further arthenticated by the fact that the experimental ionization energy of the Ir(4f) level in the different iridium halide complexes studied is seen to decrease as a result of exposure to Ar ions.
Quantum phenomena in small semiconductor structures and devices
Authors: Berggren K.‐F. 
Published: January 2005

AbstractThe ability to artificially structure new semiconductor materials on an atomic scale, using advanced crystal growth methods such as molecular beam epitaxy and metal organic chemical vapor deposition, has led recently to the observation of new physical phenomena as well as the creation of entirely new classes of devices based on band gap and wave function engineering. In these lectures an elementary introduction is given to the quantum aspects of these new structures.
Dimensionality dependence of total energy of closed shells in a bare Coulomb field for large atomic number
Authors: March N. H. , Cizek J. 
Published: January 2005

AbstractFor Z particles moving independently in an unscreened Coulomb potential energy −Ze2/r, it is known that the total energy E = −(3/2)1/3Z7/3 in nonrelativistic theory as Z→∞. The generalization of this result to d dimensions in the same asymptotice limit is obtained as

which includes the known result above for the three‐dimensional case.
The mechanism of bacteriorhodopsin functioning. II. Ejection and injection of proton
Authors: Krasnogolovets V. V. , Protsenko N. A. , Tomchuk P. M. 
Published: January 2005

AbstractTransmission of vibrational excitation energy conserved in cis‐conformation of retinal to the outlet proton channel is considered from the perspective of quantum theory. A distribution of vibrational excitations in the channel is found; it allowed to calculate the magnitude of the directed drift proton current. The differences between velocities of proton movement in active and passive channels are considered. A transition of retinal from cis‐ to all ‐trans‐ conformation with the subsequent capture of proton by Schiff base out of the inlet channel is described. The lack of proton in this channel, i.e., in the H‐bonded chain, is eliminated at the expense of the capture of a proton out of the cytoplasmic water enviroment. The correspondence between theoretically established states and spectroscopically identified forms of bacteriorhodopsin (inter‐mediates L, M, N, and O) is proposed.
The Hartree–Fock ground state of atomic two‐electron systems and the Wilson–Silverstone 1s orbital
Authors: Hoor Marten J. Ten 
Published: January 2005

AbstractFor the Hartree–Fock ground state of atomic two‐electron systems, the variational function of Wilson and Silverstone, ϕ(r) = (a + kr)−1 exp(‐kr) / (4π)1/2, can be optimized in two complementary ways. For small values of the atomic number Z, all intergrals have been calculated numerically and optimization can be performed accurately. However, as Z increases, loss of significant figures is increasingly detrimental to the optimization process. For sufficiently large values of Z, the integrals may be replaced by asymptotic expansions in terms of (2a)−. As a result of optimization, the parameters and expectation values can be given as expansions in terms of (32Z)−1/2. Both methods yield good results for Z ≈ 25, so that the whole range of Z can be treated accurately. The results have been compared with those derived from other analytical two‐parameter functions. It is found that ϕ(r) is indeed the outstanding two‐parameter function, at least for small and intermediate values of Z.
Fifth‐order constant denominator perturbation therory within a localized bond model: Contributions from single and double excitations
Authors: Cullen J. M. 
Published: January 2005

AbstractA Fifth‐order constant denominator perturbation treatment of all single and double excitations occuring in the third‐order perturbation wave function is presented for the perturbation configuration interaction using localized orbitals (PCILO) method. Contributions from triple and quadruple excitations which decay back to singles and doubles at third order are automatically included in this theory. This method is computationally very fast, with an execution speed proportional to N3, Where N is the number of orbitals present. A [2,1] Padé approximate involving only singles and doubles contributions through to fifth order is shown to be remarkably accurate.
Conformational preferences of modified nucleic acid bases N6‐(Δ2‐isopentenyl) adenine and 2‐methylthio‐N6‐(Δ2‐isopentenyl) adenine by the quantum chemical PCILO calculations
Authors: Tewari Ravindra 
Published: January 2005

AbstractConformational preferences of the hypermodified nucleic acid bases N6‐(Δ2‐isopentenyl) adenine, (i6Ade) and its 2‐methylthio derivative (ms2i6Ade) have been investigated theoretically by the quantum chemical perturbative configuration interaction using localized orbitals (PCILO) method.The predicted most stable conformation of i6Ade and ms2i6Ade are such that the isopentenyl substituent is oriented away from the imidazole moiety of the adenine ring. The atoms N(6), C(10), and C(11) remain coplanar with the adenine ring for both molecules. However, in contrast to the predicted cis orientation of the C(10)C(11) bond with respect to the C(6)N(6) bond in i6Ade, the trans orientation is favored for ms2i6Ade. The plane of the isopentenyl group is rotated by 120° from that of the purine base in i6Ade, whereas rotation by 60° is favored in ms2i6Ade. The favored orientation of the methylthio group with respect to the C(2)N(3) bond is trans; however, the alternative cis arrangement is also quite probable. The conformational implications of the methylthiolation of the isopentenyladenine are brought out in the context of the considerably large range of accessible (β,γ) and χ orientations. The compatible roles of i6Ade and ms2i6Ade in tRNA are thus understood, besides their distinguishing features.
Nonlinear dynamics of a model hydrogen bond
Authors: Besnainou S. 
Published: January 2005

AbstractThe dynamics of a model triatomic hydrogen bond are analyzed through classical mechanics. The approximate separability of the equations of motion induces the existence of an adiabatic invariant for the intramolecular mode motion. This mode is governed by a Mathieu equation, a feature already present for the antisymmetric mode in symmetric triatomics. The corresponding spectrum readily obtained as the Fourier transform of a classical trajectory shows that the fundamental frequency is shifted to lower values owing to the anharmonicity of the potential. We observe also a substructure of combination lines generated by a nonlinear resonance with the intermolecular mode. These properties are consistent with experimental observation. In a four‐atoms model, the lines are split when intramolecular Fermi resonance occurs. When the intermolecular mode becomes chaotic there is no vibrational heating of the intramolecular neighboring mode which tends to behave like a local isolated oscillator.
A computational analysis of ortho‐lithiation in aromatic systems
Authors: Jayasuriya Keerthi , Iyer Sury 
Published: January 2005

AbstractWe have analyzed the ability of a tertiary amide group to activate an ortho position on aromatic system toward an electrophilic attack using an ab initio SCF molecular orbital method. The molecular electrostatic potential was used to analyze the electronic nature of the ortho‐lithiated intermediate complex and possible sites for electrophilic attack.
Interaction energy in small N‐particle systems and the permutation group
Authors: Holas A. , Działak H. , Olszewski S. 
Published: January 2005

AbstractThe distribution of energy levels of an N‐identical‐particle system among the irreducible representations of the permutation group is examined for a “spin‐free” problem. There are calculated the sums of powers of the eigenenergies obtained from the original N!‐fold degenerate energy level when the interaction between the particles is switched on. The first‐order perturbation theory is applied. Some results of the present approach are shown to be equivalent to the results of the Weyl's theory of the permutation group. The distribution of energy levels is found to be symmetric with respect to the average energy level belonging to a self‐conjugate representation. For a special case of the Coulomb interaction the wave function of the highest level is fully symmetric, whereas that of the lowest level is fully antisymmetric with respect to the interchange of the coordinates of any two particles. The average energy and the dispersion of the levels belonging to a given representation are calculated for some N.
Core/valence partition and relativistic effects in effective potentials for transition metals
Authors: Pacios L. Fernandez , Calzada P. Gomez 
Published: January 2005

AbstractTwo different versions, relativistic and nonrelativistic, of the “shape‐consistent” effective potential (EP) formalism are analyzed in ab initio calculations for transition metals. The influence of alternative core/valence partitions on the reliability of the procedure is discussed. The accuracy of EP results for transition metals is shown to depend on adequate choices of the valence subspace and proper inclusion of relativistic effects.
Intershell nl4f electron correlation effects
Authors: Jankowski K. , Malinowski P. , Sokołowski A. 
Published: January 2005

AbstractThe pair correlation energies for some nl4f pairs of the ground state of the Yb atom are calculated for the first time. The partial wave (PW) increments to the second‐order pair energies are generated using numerical first‐order radial pair functions obtained as the solution of two‐dimensional differential equations. The analysis of the PWs contributions shows the dominant role of the df, fg, and gh PWs for the 4d4f pair, of the pf and dg PWs for the 4p4f and 5p4f pairs, and of the sf and pg PWs for the 4s4f, 5s4f, and 6s4f pairs. A discussion of the similarities and differences of the structure of the correlation energy found in this paper with those calculated earlier for smaller atoms is given.
Natural energy orbitals and the one‐particle Green's function
Authors: Holleboom L. J. , Snijders J. G. , Baerends E. J. 
Published: January 2005

AbstractIt is shown how the properties of the one‐particle Green's function lead naturally to the definition of the so‐called natural energy orbitals. These orbitals allow the fully correlated total energy of a system to be written in Hartree–Fock‐like fashion and might therefore provide a bridge between sophisticated correlated wave functions and approximate theories of chemical structure and reactivity based on a Hartree–Fock‐like energy expression. Moreover these orbitals form the basis for a self‐consistent scheme to calculate the one‐particle Green's function. The relation between these natural energy orbitals and the extended Koopmans' theorem is considered. Finally it is shown that the exactness of the lowest extended Koopmans' ionization potential implies the linear independence of the corresponding Dyson orbital from all other Dyson orbitals.
Estimation of minima in 1A1 states of the ozone molecule
Authors: Moscardó Federico , Andarias Ramón , San‐Fabián Emilio 
Published: January 2005

AbstractCalculations have been made at the minima of the X1A1 ground state of the ozone molecule. The equilibrium geometries have been obtained by means of CID calculations. The criterion adopted for the choice of configurations gives realistic results. CIPSI calculations at the two minima lead to an estimated gap of 0.92 eV between them.Our results agree with the analysis of previous theoretical works on the relative stability of open and cyclic zone structures, showing that the D3h minimum is stable relative to the ground state dissociation limit.
Convergence radii of the perturbation expansions for the ground‐state energies of finite Hubbard models
Authors: Hashimoto Katsufumi , Čížek Jiří , Paldus Josef 
Published: January 2005

AbstractThe ground state energies of finite Hubbard molecules are calculated by numerically solving the Lieb–Wu equations for a complex Hubbard repulsion parameter U. From the positions of the singular points located in the complex plane, the radii of convergence of the perturbation expansions for the ground state energies are determined.
Modified all‐valence INDO/spd method for ground and excited state properties of isolated molecules and molecular complexes
Authors: Lipiński Jóazef 
Published: January 2005

AbstractA new semiempirical all‐valence method, GRINDOL (Ghost and Rydberg INDO), based on the INDO approximation, is described. Linderberg–Seamans relation (extended to the d and Rydberg orbitals) for the resonance integrals and a new semitheoretical expression for the core‐core repulsion term and energy correction including basis‐set superposition error (intermolecular as well as intramolecular) has been applied. The proposed method enables calculation of ground and excited state properties. The ground state results (including intermolecular interactions) as well as the spectral properties are in reasonable agreement with relevant experimental (or ab initio) studies for isolated molecules, molecular complexes, and transition metal compounds. The method contains only one adjustable parameter, all two‐center integrals and terms are only basis‐set dependent. The one‐center integrals are evaluated from the respective atomic terms.
An indexing scheme for spin‐free configurations of electrons
Authors: Dinesha K. V. , Rettrup S. , Sarma C. R. 
Published: January 2005

AbstractA simple procedure has been created for indexing the spin‐free configurations of an N‐electron system in spin state S. This indexing has been linked to a procedure for generating the matrix representations of the generators of the unitary group U(n) to enable a direct configuration study to be undertaken.
Structure of consistent ground state: Re—derivation of RPA vacuum and inclusion of higher RPA effects
Authors: Toyoda Kaoru 
Published: January 2005

AbstractHere is presented a method to determine the consistent ground state (CGS) which satisfies the so‐called killer condition for the excitation operator. This method may be called an extended application of the procedure employed by Weiner and Goscinski in deriving the random phase approximation (RPA) vacuum.The RPA vacuum is derived by solving the recurrence formula of the configuration coefficients of a multiconfigurational state vector. The role of boson approximation to the primitive p‐h excitation operator is also investigated and by using the present formalism the cluster‐expansion‐type CGS is derived as the RPA vacuum under the boson approximation.Inclusion of the effects of a higher RPA in the CGS leads to the simultaneous equations of the configuration coefficients of the CGS. In including the effect of the second RPA, only the symmetry‐broken CGS can exist. When the third RPA effect is involved instead of the second RPA, there can be a symmetry‐adapted CGS, in which the picture of electron pairs acquired in the standard RPA vacuum is modified. Thus the exact CGS vectors are analytically obtained in the case of simple model systems of two or four electrons.
Correlation between the conformation of nucleoside antibiotics and their biological activity
Authors: Saran Anil 
Published: January 2005

AbstractNucleoside antibiotics, which result as a consequence of minor modifications in pyrimidine and purine nucleosides, exhibit a wide variety of antiviral, antibacterial, antitumor, and cancerostatic properties. The conformational properties of a number of these antibiotics have been investigated by using the quantummechanical PCILO method, and the results indicate that the nucleoside antibiotics and their parent nucleosides have very similar conformational preferences. This similarity is strikingly marked in the situations which prevail in an aqueous medium. As a result, these antibiotics easily get incorporated in growing chains of RNA and DNA by mimicking their parent nucleosides and then bring about the inhibition of protein, RNA, or DNA syntheses. The experimental observations corroborate these deductions, and thus a correlation has been obtained between the conformation and the biological activity of nucleoside antibiotics; it is the striking conformational similarity between the nucleoside antibiotics and their parent nucleosides which gives rise to their biological activity. The PCILO investigations carried out on two 3‐deazapurine nucleosides demonstrate that the converse of the above correlation also holds true.
The change in the dressed potential of a polyatomic molecule in intense photon fields: Simple rules based on the nuclear charge‐mass ratio
Authors: Saito Nakaya , Hall George G. 
Published: January 2005

AbstractAn explicit expression for the dressed potential of a polyatomic molecule, in the adiabatic approximation, is derived. This expression clearly shows the importance of the nuclear charge‐mass ratio (NCMR) for the change of potential due to photon fields. It is found from a simple calculation that the 1H atom is the only atom having an abnormal NCMR value; all other atoms have similar, or the same, values. This means that only those molecules containing a 1H atom should be strongly affected by fields. On the basis of this new physical insight, we postulate two rules, which enable us to classify molecules, with respect to their response to intense photon fields, into three classes: high‐sensitive, low‐sensitive, and insensitive molecules. Qualitative verification is also given by using water isotopes.
Theoretical study of some heterocyclic amines with applications to the chemistry of 9‐amino‐1,2,3,4‐tetrahydroacridine
Authors: Pop Emil , Brewster Marcus E. , Kaminski J. J. , Bodor Nicholas 
Published: January 2005

Abstract9‐Amino‐1,2,3,4‐tetrahydroacridine (THA), a potent cholinesterase inhibitor, was recently used in the treatment of Alzheimer's disease. On attempting to prepare a dihydropyridine ⇆ pyridinium salt‐based redox chemical delivery system (CDS) to enhance brain delivery of THA, several of the practical synthetic challenges were examined by using a theoretical MO approach. The structures, reactivities and stability of THA, derivatives of THA and a model compound, 4‐aminopyridine, a simple dibasic heterocyclic amine, were studied in the framework of the AM‐1 approximation. The study included the possible protonated forms of THA and 4‐aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4‐aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4‐aminopyridine, consistent with experimental results. Electron delocalization is responsible for the remarkable stability of these molecules and for the observed lack of reactivity of the amino group, both in the basic and protonated forms. The site of N‐alkylation of the 9‐nicotinamide derivative of THA (an intermediate in the synthesis of THA‐CDS) is controlled by electronic, thermodynamic, and steric factors.
New concepts in bonded functions theory. III. Orthogonalization problem
Authors: Gołȩlbiewski A. , Brocławik E. , Witko M. 
Published: January 2005

AbstractOne of possible approaches to the CI method is based on Boys bonded functions which can be generated in a systematic way forming an independent set of high internal symmetry. The main disadvantage of bonded functions is their nonorthogonality. In this paper a scheme is proposed for passing to orthogonalized set of bonded functions together with the appropriate algorithm for the transformation of the energy matrix H. The orthogonalization matrices are shown to reflect high symmetry of the canonical set of bonded functions, and in what follows they can be defined by short vectors. Moreover, the orthogonalization transformation can be handled in a blockwise manner.
Products of class operators of the symmetric group
Authors: Katriel Jacob 
Published: January 2005

AbstractA combinatorial derivation of the product of the class of three cycles, [(1)N−3(3)]N with an arbitrary class operator of the symmetric group SN is presented. The form of this result suggests a conjecture concerning the expression of the general class operator product in terms of a relatively small number of reduced class coefficients. The conjecture is applied to the determination of the products of [(1)N−4(4)]N, [(1)N−4(2)2]N, and [(1)N−5(5)]N with arbitrary class operators. General expressions for the reduced class coefficients of the simplest type are obtained.