Showing papers in "International Journal of Quantum Chemistry in 1981"
TL;DR: For a Coulomb system of particles of charge e, it has been shown that the indirect part of the repulsive Coulomb energy (exchange plus correlation energy) has a lower bound of the form Ce 2/3 ∫ ρ (x) 4/3 dx, where ρ is the single particle charge density.
Abstract: For a Coulomb system of particles of charge e, it has previously been shown that the indirect part of the repulsive Coulomb energy (exchange plus correlation energy) has a lower bound of the form — Ce 2/3 ∫ ρ (x) 4/3 dx, where ρ is the single particle charge density. Here we lower the constant C from the 8.52 previously given to 1.68. We also show that the best possible C is greater than 1.23.
472 citations
270 citations
TL;DR: In this paper, the coupled-cluster approach to obtaining the bond-state wave functions of many-electron systems is extended, with a set of physically reasonable approximations, to admit a multiconfiguration reference state.
Abstract: The coupled-cluster approach to obtaining the bond-state wave functions of many-electron systems is extended, with a set of physically reasonable approximations, to admit a multiconfiguration reference state. This extension permits electronic structure calculations to be performed on correlated closed- or open-shell systems with potentially uniform precision for all molecular geometries. Explicit coupled cluster working equations are derived using a multiconfiguration reference state for the case in which the so-called cluster operator is approximated by its one- and two-particle components. The evaluation of the requisite matrix elements is facilitated by use of the unitary group generators which have recently received wide attention and use in the quantum chemistry community.
142 citations
TL;DR: In this paper, one-electron properties may be evaluated by configuration interaction methods using analytical differentiation of the total energy with respect to an external perturbation parameter, and dipole moments are reported using such a method for CO and H2CO.
Abstract: One-electron properties may be evaluated by configuration interaction methods using analytical differentiation of the total energy with respect to an external perturbation parameter. Dipole moments are reported using such a method for CO and H2CO. Inclusion of single substitutions does not change calculated dipole moments appreciably with this method. The differences between this method and the direct evaluation of an expectation value are discussed.
133 citations
TL;DR: In this article, the effect of using several approximations is considered (i) for representing the isolated molecules (which determine the electrostatic and polarization terms), and (ii) for evaluating the shortrange repulsion term.
Abstract: Some aspects of the use of simplified formulas for the evaluation the interaction energy between two molecules is discussed. This energy is obtained as the sum of four terms: electrostatic, polarization, dispersion, and short-range (exponentially decreasing) repulsion. The effect of using several approximations is considered (i) for representing the charge distributions of the isolated molecules (which determine the electrostatic and polarization terms), and (ii) for evaluating the short-range repulsion term. The various charge distributions considered are semiempirical atomic net charges (Del Re + Pariser-Parr), atomic charges and dipoles (CNDO) and corresponding “effective” atomic charges, and many-centered multipole distributions obtained from ab initio SCF calculations (charges, dipoles, and quadrupoles located on the atoms and the middles of segments joining pairs of atoms, whether chemically bonded or not). As concerns the short-range repulsion, the various procedures considered are a sum of atom-atom terms, a sum of bond-bond terms, and the use of anisotropic van der Waals radii for heteroatoms (oxygen and nitrogen). In all cases, the dispersion energy is obtained as a sum of atom-atom terms (of the R−6 type). These various procedures are checked in the case of the interactions between nucleic acid bases, for two rather different kinds of configurations, namely, hydrogen bonded and stacked. This comparison reveals a rather complicated picture, namely the results got from the various levels of approximation of the molecular charge distribution lead to different degrees of agreement according to different situations, e.g., for guanine-cytosine interactions, qualitative agreement is found between the various methods as concerns the relative order of stacked and hydrogen bonded situations, while agreement is much less satisfactory for adenine-uracile interactions. One of the main conclusions is that a sufficiently sophisticated representation of the molecular charge distribution is required in order to get reliable results for all possible configurations of a complex. Such comparative studies should bring significant help as concerns the development of the reliable simplified procedures for evaluating intermolecular interaction energies.
112 citations
TL;DR: In this paper, a flexible intra-and intermolecular empirical potential function was developed for investigating the geometric structure of large molecular systems. Butler et al. developed a flexible empirical potential for the analysis of molecular systems, which can be used for crystal packing calculations.
Abstract: The development of a flexible intra- and intermolecular empirical potential function is described, which is designed for investigating the geometric structure of large molecular systems. The intramolecular components in the potential consist of harmonic bond stretching and angle bending terms, out-of-plane deformation terms, and torsional terms; intermolecular components include nonbonding, hydrogen bonding, and electrostatic germs. Bond lengths, angles, and torsional angles are predicted to within 2% of experiment, with most cases being within 1%. The suitability of the intermolecular potential was tested by crystal packing calculations; in all cases the results obtained were in excellent agreement with experiment.
85 citations
TL;DR: This paper deals exclusively with the HF (/b X//sup 1/ Sigma /sup +/) molecule and proposes a new method for improving the quality of a wave function.
Abstract: Information theory can be used to evaluate the quality of a wave function by considering its ability to give values for some observables as close as possible to the experimental ones. A new method for improving the quality of a wave function is proposed. This paper deals exclusively with the HF (/b X//sup 1/ Sigma /sup +/) molecule.
80 citations
TL;DR: The symmetry-adapted-cluster (SAC) and SAC-CI theories reported previously have been applied to the study of electron correlations in ground state, singlet and triplet excited states, ionized state, and electron attached state as mentioned in this paper.
Abstract: The symmetry-adapted-cluster (SAC) and SAC–CI theories reported previously have been applied to the study of electron correlations in ground state, singlet and triplet excited states, ionized state, and electron attached state. Formulas for calculations of one-electron properties and transition properties from the SAC and SAC–CI wave functions are given. Calculations were carried out for the ground and Rydberg excited states of water and its positive and negative ions, with the use of the simpler computational scheme than the previous one. The results compare well with experiments.
66 citations
TL;DR: In this article, the exact (nonadiabatic) nuclear and electronic factors of a molecular wave function are expanded in the basis of eigenfunctions of the electronic Hamiltonian according to the Rayleigh-Schrodinger perturbation theory of Born and Huang.
Abstract: The exact (nonadiabatic) nuclear and electronic factors of a molecular wave function are expanded in the basis of eigenfunctions of the electronic Hamiltonian according to the Rayleigh–Schrodinger perturbation theory of Born and Huang. Thus it is shown that, with rare exceptions, the exact nuclear factor (a marginal amplitude) is a nodeless function. The nodes in vibrationally excited nuclear wave functions within the Born–Oppenheimer approximation become node-avoiding minima in the exact nuclear wave function. Corresponding to each node-avoiding minimum in the nuclear wave function the exact (nonadiabatic) effective potential for the nuclear motion has a spiky barrier superimposed upon the Born–Oppenheimer (adiabatic) eigenenergy of the electronic Hamiltonian. These barriers are the result of nonadiabatic coupling between electronic states, which is strongest in the vicinity of the nodes in the Born–Oppenheimer-approximation nuclear (vibrational) wave function.
63 citations
TL;DR: In this paper, a procedure for calculating the molecular electrostatic potentials on surface envelopes surrounding macromolecules is presented, and a new representation of potential is employed in studying B-DNA double helices.
Abstract: A procedure for calculating the molecular electrostatic potentials on surface envelopes surrounding macromolecules is presented. This new representation of potential is employed in studying B-DNA double helices and, from the results, deductions are drawn on the interaction specificities of B-DNA with electrophiles.
59 citations
TL;DR: An ab initio LCAO SCF MO calculation was performed on planar Fe-porphine with a double zeta basis set consisting of 300 CGTO's in this paper.
Abstract: An ab initioLCAO SCF MO calculation was performed on planar Fe-porphine with a double zeta basis set consisting of 300 CGTO's. SCF wave functions of several states of Fe-porphine and its cation were obtained. The net charge of Fe is in the range of 1.39 to 1.53. The highest occupied orbital is ascertained to be a pure porphine π-MO, 1a1u. The calculated ionization potentials of the two highest occupied orbitals, 1a1u and 5a2u are 5.98 and 6.43 eV, respectively. They are in good agreement with experiments. The role of the porphine macrocycle on the oxidation of Fe is discussed in terms of gross atomic populations and with contour maps of the density difference.
TL;DR: In this article, a method for calculating the steric accessibilities of atoms within macromolecules for various probes is presented and an application is made to the accessibility of atoms in B-DNA.
Abstract: A method for calculating the “static” steric accessibilities of atoms within macromolecules for various probes is presented and an application is made to the accessibility of atoms within B-DNA. Changes in accessibility in passing from the individual subunits of the nucleic acid to a single helical strand and then to a double helix are discussed. The influence of various base sequences is also investigated. The present methodology and results are compared to earlier attempts at the calculation of steric accessibilities and their significance for the interpretation of chemical reactivity is substantiated by an illustrative example.
TL;DR: In this paper, a general formula for the energy is derived and it shows a striking difference with the usual formulas through an "extra" term, which constitutes a generalization of previous ones for the virial theorem when there exist nonusual boundary conditions.
Abstract: Diagonal hypervirial relations are applied to enclosed one-dimensional systems when wave functions obey Dirichlet boundary conditions. A general formula for the energy is derived and it shows a striking difference with the usual formulas through an “extra” term. It constitutes a generalization of previous ones for the virial theorem when there exist nonusual boundary conditions. The values obtained are compared with other approximate methods. Several extensions are proposed.
TL;DR: In this article, the infrared spectra in the NH, OH, and CO stretching regions are reviewed for various methylated and halogenated 2(4)-oxopyrimidines and uracils, as well as other nucleic acid derivatives, in the vapor phase and in low-temperature matrices.
Abstract: Infrared spectra in the NH, OH, and CO stretching regions are reviewed for various methylated and halogenated 2(4)-oxopyrimidines and uracils, as well as other nucleic acid derivatives, in the vapor phase and in low-temperature matrices. The 2-oxopyrimidines are predominantly in the enol form both in the vapor phase and in low-temperature matrices. By contrast, the 4-oxopyrimidines exhibit comparable populations of the keto and enol forms, with KT ≈ 1–2, and a difference in chemical binding energy between the two forms in the gaseous phase of the order of 1–2 kcal/mol. The observed tautomeric equilibria in the gas phase point to the need for a drastic revision of interpretations of theoretical methods, and simultaneously provide the appropriate quantitative data necessary for testing the results of quantum mechanical calculations. In sharp contrast to other heterocyclic systems, several of the bases found in natural nucleic acids were found to exist predominantly in the keto or amino forms, as in the solution phase. In particular, uracils exist predominantly in the keto form. This has made possible, for this class of compounds, to evaluate the heats of sublimation and vaporization, and to relate these data to hydrogen bonding and stacking interactions in the condensed phases. Examples are presented of base analogs which do exhibit appreciable tautomerism in solution. Some biological implications of the foregoing are presented in relation to the types of heterocyclic bases found in natural nucleic acids, and to concepts of spontaneous and induced mutagenesis.
TL;DR: In this paper, the symmetry basis vectors which represent in box form the Hamiltonian operator were used to simplify the resolution of the two-rotor equation of the acetone molecule.
Abstract: The group theory for nonrigid molecules is used for studying the internal dynamics of the two equivalent C3v rotor “bent” molecules. Special emphasis is given to the deduction of the symmetry basis vectors which represent in box form the Hamiltonian operator. It is shown that these basis vectors may be advantageously employed in order to simplify the resolution of the two-rotor equation. The procedure is applied to the acetone molecule. It is found that the lowest solutions are clustered into groups of four. The four lowest levels are related to vibrational states, the upper 64 to vibro–rotational states, in which the rotors are rotating in a restricted manner. Only few states show some cogwheel effect. Internal rotation contributions to the principal thermodynamic parameters of acetone are also computed.
TL;DR: In this paper, a variational method is used to estimate the many-electron integrals by Monte Carlo using the fermion Metropolis sampling algorithm, where the square of a correlated antisymmetric wave function is sampled.
Abstract: Results of research directed towrad the development of compact and accurate correlation functions for many-electron systems are reported. The computational tool used is the variational method in which the many-electron integrals are calculated by Monte Carlo using the fermion Metropolis sampling algorithm. That is, a many-fermion system is simulated by sampling the square of a correlated antisymmetric wave function. The principal advantage of the method is that interelectronic distance r/sub ij/ may be included directly in the wave function without adding significant computational complexity. In addition, other quantities of physical and theoretical interest such as electron correlation functions and representations of Coulomb and Fermi holes are very easily obtained. Preliminary results are reported for He, H/sub 2/, and Li/sub 2/.
TL;DR: The importance of the gramicidin channel, its structural elements, and the importance of peptide libration mechanism are reviewed in this paper, and the need to extend analyses to include an understanding of multiple conformational states is outlined, and results from approximate conformational energy calculations are presented showing that there are numerous low-energy conformations for the side chains of gramigidin.
Abstract: The significance of the gramicidin channel, its structural elements, and the importance of the peptide libration mechanism are reviewed. Elemental ionic processes of the single-channel currents are considered, which indicate the possibility of two- and three-site models. Data are presented which demonstrate the absence of interactions required for the three-site model, and the resulting free-energy profile for the two-site model is given for the most probable conducting state. The need to extend analyses to include an understanding of multiple conformational states is outlined, and results from approximate conformational energy calculations are presented showing that there are numerous low-energy conformations for the side chains of gramicidin. Specifically the interactions between L·Trp9 and L·Trp15 side chains and between D·Leu4…D·Leu10 are analyzed to show the existence of several energy minima separated by appreciable barriers. These findings provide the basis for understanding facilitated entry of the second ion into the channel and the occurrence of lipid-dependent dispersion of conductance states. Finally perspectives of the molecular mechanism of anesthetics are discussed relevant to the gramicidin channel mean lifetime and single-channel currents.
TL;DR: In this paper, various computational procedures for treating the problem of lattice summations in ab initio crystal orbital calculations are briefly reviewed and the cutoff problem is discussed and several numerical examples for demonstrating the fast convergence of the total energy per unit cell with respect to the inclusion of further neighbors' interactions, in case reasonable cutoff procedures are applied.
Abstract: The various computational procedures for treating the problem of lattice summations in ab initio crystal orbital calculations are briefly reviewed and the cutoff problem is discussed. Several numerical examples are presented for demonstrating the fast convergence of the total energy per unit cell with respect to the inclusion of further neighbors' interactions, in case reasonable cutoff procedures are applied. The importance of electrostatic considerations for the interpretation of long range effects is stressed.
TL;DR: In this article, a comparison of electrostatic charge and energy distribution on the basis of the CNDO/2 method for six forms of prostaglandins (PGF2α, PGF1β, PGE1, PEG2, PGB1, and PGA1) is reported.
Abstract: The paper reports comparison of electrostatic charge and energy distribution on the basis of the CNDO/2 method for six forms of prostaglandins–PGF2α, PGF1β, PGE1, PGE2, PGB1, and PGA1–having diverse physiological action. The isopotential mapping done in three dimension showed that the lower value of electrostatic potential and proximity of the two low energy regions around O9 and O11 in PGE2 and PGF2α is probably responsible for their higher abortificient activity. We also compare here the variation of the long- and short-range interaction between ring–chain and chain–chain portion of different forms and compared them with the variation in their action.
TL;DR: In this article, the authors extended previous results on the hypervirial analysis of confined systems and proposed an efficient alternative way to solve the Mathieu equation for the periodic potentials.
Abstract: Previous results on the hypervirial analysis of confined systems are extended. Periodic potentials are discussed and an efficient alternative way to solve the Mathieu equation is proposed.
TL;DR: In this article, the C-matrix single-center expansion method with use of computer algebra was used to automatically generate formulas for all two-center Coulomb integrals over Slater-type orbitals.
Abstract: Formulas can be automatically generated for all two-center Coulomb integrals over Slater-type orbitals by means of the “C-matrix” single-center expansion method with use of “computer algebra.” The formula coefficients may be stored in two-dimensional arrays.
TL;DR: In this paper, new theoretical developments based upon hypervirial analysis of enclosed systems are given. And the results which were given by the authors in two previous communications are generalized. But these results are based on the assumption that systems obey unsymmetrical boundary conditions.
Abstract: New theoretical developments based upon hypervirial analysis of enclosed systems are given. Systems which obey unsymmetrical boundary conditions are analyzed. Several results which were given by the authors in two previous communications are generalized.
TL;DR: Based on in vitro culture studies, it has been proposed that the photochemical generation of superoxide in the intraocular chambers provides a significant cataractous insult and these deleterious reactions could be attenuated greatly by physiological levels of ascorbic acid.
Abstract: Based on in vitro culture studies, it has been proposed that the photochemical generation of superoxide in the intraocular chambers provides a significant cataractous insult. Rat lens when maintained in medium generating excessive O is damaged as indexed by deterioration of the cation pump. If O levels are low (ambient levels), peroxidative degradation of lipids has been observed. These reactions are intuitively deleterious to tissue physiology and may contribute to the manner through which light may be toxic to lens in the long run, giving rise to senile cataracts. It was interesting to note that these deleterious reactions could be attenuated greatly by physiological levels of ascorbic acid. The significance of these in vitro studies to the in vitro situatiion as offered by the diurnal habitat of mankind remains yet to be known, but studies simulating such situation in experimental animal models are in progress.
TL;DR: In this article, the conditions for instability of solutions of Hartree-Fock and projected Hartree Fock equations are derived in a form involving finite real symmetric matrices, also expressed in terms of the Fock-Dirac density matrix, both at the spin-orbital and at the orbital level.
Abstract: The conditions for instability of solutions of Hartree–Fock and projected Hartree–Fock equations are derived in a form involving finite real symmetric matrices. These conditions are also expressed in terms of the Fock–Dirac density matrix, both at the spin–orbital and at the orbital level. The particular variations which give rise to the so-called singlet and triplet instabilities are described.
TL;DR: In this paper, stable state conductivity and dielectric measurements for bovine serum albumin and lysozyme as a function of hydration and NaCl content are reported for the range of 10−5 Hz to 33 GHz.
Abstract: Steady-state conductivity and dielectric measurements in the range of 10−5 Hz to 33 GHz are reported for bovine serum albumin and lysozyme as a function of hydration and NaCl content. The electrical properties are understandable in terms of the existence of charge carriers whose hopping-type transport process is directly influenced by the nature of the protein-water interaction. These mobile charges are considered to be protons originating from ionizable carboxylic groups.
TL;DR: In this article, local coordinate systems are introduced into open sets of topological spaces, defined for the nuclear configuration space R of polyatomic nuclear systems, and the open sets are directly related to common chemical concepts, such as chemical structure, reaction mechanism, and stability of reaction paths.
Abstract: Local coordinate systems are introduced into open sets of topological spaces, defined for the nuclear configuration space R of polyatomic nuclear systems. The open sets are directly related to common chemical concepts, such as chemical structure, reaction mechanism, and stability of reaction paths. A topological model of molecules and chemical reactions is a generalization of the geometrical model, and is particularly suitable to account for the nonrigid nature of chemical structures. The coordinate neighborhoods defined on R and on its submanifolds represent the extent and natural limits for partial analyses of multidimensional potential surfaces.
TL;DR: A hierachy of criteria is suggested that offers a systematic approach to the vertex discrimination and eventually establishes in most cases the graph center as a single vertex, a single bond (edge), or a single group of equivalent vertices.
Abstract: While the concept of the graph center is unambiguous (and quite old) in the case of acyclic graphs, an attempt has been made recently to extend the concept to polycyclic structures using the distance matrix of a graph as the basis. In this work we continue exploring such generalizations considering in addition to the distance matrix, self-avoiding walks or paths as graph invariants of potential interest for discriminating distinctive vertex environments in a graph of polycyclic structures. A hierachy of criteria is suggested that offers a systematic approach to the vertex discrimination and eventually establishes in most cases the graph center as a single vertex, a single bond (edge), or a single group of equivalent vertices. Some applications and the significance of the concept of the graph center are presented.
TL;DR: In this paper, a quantum chemical model for predicting the optimal molecular environment exerting the highest catalytic activity on transition complex has been proposed and applied for the CO2 hydration reaction at the nonempirical LCAO MO SCF level.
Abstract: The quantum chemical model for predicting the optimal molecular environment exerting the highest catalytic activity on transition complex has been proposed. It was applied for the CO2 hydration reaction at the nonempirical LCAO MO SCF level. The possible use of the resulting optimal charge distribution of environment for design of new synthetic catalysts and explanation of the carbonic anhydrase action was discussed. In addition, a simplified approach based on the difference electrostatic molecular potentials evaluated within the many center multipole expansion is presented. It may enable us to perform corresponding estimations even by means of a programmable calculator.