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At least 505 records · Page 28Linked to original sources
PubMed2006

Computational method for the quantum Hamilton-Jacobi equation: bound states in one dimension.

An accurate computational method for the one-dimensional quantum Hamilton-Jacobi equation is presented. The Mobius propagation scheme, which can accurately pass through singularities, is used to numerically integrate the quantum Hamilton-Jacobi equation for the quantum momentum function. Bound state wave functions are then synthesized from the phase integral using the antithetic cancellation technique. Through this procedure, not only the quantum momentum functions but also the wave functions are accurately obtained. This computational approach is demonstrated through two solvable examples: the harmonic oscillator and the Morse potential. The excellent agreement between the computational and the exact analytical results shows that the method proposed here may be useful for solving similar quantum mechanical problems.

Journal Article↗
PubMed2005

[Prediction of photolysis half-lives of PCDD /Fs adsorbed on spruce needles optimized by genetic algorithm].

Adopting quantum chemical parameters of PCDD/Fs computed with quantum chemical PM3 algorithm, quantitative structure-property relationship (QSPR) model, which could predict photolysis half-life (t1/2) of PCDD/Fs adsorbed to spruce [Picea abies (L.) Karst.] needle surfaces, is established using genetic algorithm (GA) algorithm. It is considered that the main factors affecting lg t1/2 of PCDD/Fs are the energy of the highest occupied molecular orbital (EHOMO), ELUMO - EHOMO and average molecular polarizability (alpha). The lg t1/2 values increase with the increasing of EHOMO and a. The relationship between the lg t1/2 values and ELUMO - EHOMO is a parabolic curve. The lg t1/2 values increase with the increasing of ELUMO - EHOMO when ELUMO - EHOMO > or = 7.847 and decrease with the increasing of when ELUMO - EHOMO < or = 7.847.

Algorithms↗
PubMed2003

Determination of the two-dimensional detective quantum efficiency of a computed radiography system.

Based on a recently described method for determining the two-dimensional presampling modulation transfer function (MTF), the aperture mask method, a method for determining the two-dimensional detective quantum efficiency (DQE) of a digital radiographic system was developed. The method was applied to a new computed radiography (CR) system and comparisons with one-dimensional determinations of the presampling MTF and the DQE were performed. The aperture mask method was shown to agree with the conventional tilted slit method for determining the presampling MTF along the axes. For the particular CR system studied, the mean of one-dimensional determinations of the DQE in orthogonal directions led to a representative measure of the average DQE behavior of the system up to the Nyquist frequency along the axes, but a deviation was observed above this frequency. In conclusion, the method developed for determining the two-dimensional DQE can be used to determine the imaging properties of a digital radiographic detector system over almost the entire frequency domain, the exception being the lowest frequencies (< or = 0.1 mm(-1)) at which the validity and the reliability of the method are low.

Algorithms↗
PubMed2006

A simple procedure for calculating condensed phase heat of formation of nitroaromatic energetic materials.

A simple procedure has been introduced for calculating solid and liquid heats of formation of nitroaromatic energetic compounds. This model assumes that the heat of formation of a nitroaromatic compound of composition C(a)H(b)N(c)O(d) can be expressed as a new correlation which depends on elemental composition and various structural and special functional group parameters. Condensed phase heats of formation predicted using the method described herein and complicated quantum mechanical computations [B.M. Rice, J. Hare, Thermochem. Acta 384 (2002) 377] have a root mean square (rms) deviation of 5.9 and 11.1 kcal/mol for 19 well-known organic nitroaromatic compounds. Predicted heats of formation for 29 polycyclic nitroaromatic energetic compounds have a rms deviation from experiment of 10.6 kcal/mol. The results show that the present method gives comparable prediction respect to the other methods such as complex quantum mechanical computation.

Algorithms↗
PubMed1993

Agonistic activity and charge driven interaction potentiality of PAF derivatives.

Taking into account the contribution of the C2 substituent to the interaction ability of PAF derivatives, an investigation has been undertaken on: i) the role of the presence of two unitary charges of opposite sign in the molecule; ii) the role of the spacing of the same charged moieties. For this purpose, the theoretical analysis of model compounds for newly synthesized and already known compounds has been performed by means of computer-aided modelling, molecular mechanics and semiempirical quantum chemical computational methods. The combination of the results with the available pharmacological data allows a discussion about the stereoelectronic features required for agonist interaction at the PAF receptor.

Animals↗
PubMed1993

Molecular mechanical parameters (MM2 force field) for the N(sp3)-O(sp3) bond.

Based on results of MP2/6-31G* ab initio calculations an MM2 molecular mechanical parameter set has been developed for molecules containing N(sp3)-O(sp3) single bonds, existing parameters concerning the other bonds being retained. The new parameter set was tested for small organic compounds. A simple, generally applicable multilinear regression algorithm has been used and a program written to complement an existing force field (e.g. MM2) with such parameters extracted from quantum chemical computations.

Algorithms↗
PubMed2005

Electrical addressing of confined quantum systems for quasiclassical computation and finite state logic machines.

Conduction spectroscopy measures the current I through a nanosystem as a function of the voltage V between two electrodes. The differential conductance, dI/dV, has peaks that can be assigned to resonance conditions with different electronic levels of the system. Between these increments, the current has roughly constant plateaus. We discuss how measurements of the current vs. voltage can be used to perform Boolean operations and hence construct finite state logic machines and combinational circuits. The inputs to the device are the source-drain voltage, including its sign, and a gate voltage applied in a manner analogous to optical Stark spectroscopy. As simple examples, we describe a two-state set-reset machine (a machine whose output depends on the input and also on its present state) and a full adder circuit (a circuit that requires three inputs and provides two outputs).

Journal Article↗
PubMedDate not supplied

The Cerec 3--a quantum leap for computer-aided restorations: initial clinical results.

The Cerec 3 system simplifies and accelerates the fabrication of ceramic inlays, onlays, veneers, and quarter, half, and complete crowns for anterior and posterior teeth. Cerec 3 software simplifies occlusal and functional registration. Proper occlusion is established accurately and quickly; manual adjustment is reduced to a minimum. The separate grinding device, working true to morphologic detail and with fine surface quality, is connected to the optical unit by radio control. Equipped with a laser scanner, it can also be used for indirect application through a standard personal computer. The Cerec 3 system is network and multimedia ready and, in combination with an intraoral color videocamera or a digital radiography unit, can be used for patient education and for user training. The Cerec 3 system thus is a diagnostic, restorative, training, and documentation center for the dental practice.

Adult↗
PubMed2004

e-Quantum chemistry free resources.

Among computational chemistry methods, quantum mechanics calculates geometries and electronic structures with accuracy especially for systems with electronic delocalization. The use of a multiconfigurational approach is able to treat highly degenerated states such as those occurring at the transition state in some chemical reactions. Moreover, an accurate description of potential energy surfaces can be obtained with the evaluation of the dynamic electron correlation effects by this approach. Molecular properties range from simple dipole moments, vibrational frequencies or IR intensities to frequency dependent hyperpolarizabilities. Quantum chemical calculations are thus an attractive source of molecular descriptors which can be used in QSAR/QSPR studies and which can express all electronic and geometric properties of molecules. A survey and a comparison of the performance of free e-resources for semi-empirical and ab initio calculations is provided.

Data Collection↗
PubMed2004

Fully quantum mechanical energy optimization for protein-ligand structure.

We present a quantum mechanical approach to study protein-ligand binding structure with application to a Adipocyte lipid-binding protein complexed with Propanoic Acid. The present approach employs a recently develop molecular fractionation with a conjugate caps (MFCC) method to compute protein-ligand interaction energy and performs energy optimization using the quasi-Newton method. The MFCC method enables us to compute fully quantum mechanical ab initio protein-ligand interaction energy and its gradients that are used in energy minimization. This quantum optimization approach is applied to study the Adipocyte lipid-binding protein complexed with Propanoic Acid system, a complex system consisting of a 2057-atom protein and a 10-atom ligand. The MFCC calculation is carried out at the Hartree-Fock level with a 3-21G basis set. The quantum optimized structure of this complex is in good agreement with the experimental crystal structure. The quantum energy calculation is implemented in a parallel program that dramatically speeds up the MFCC calculation for the protein-ligand system. Similarly good agreement between MFCC optimized structure and the experimental structure is also obtained for the streptavidin-biotin complex. Due to heavy computational cost, the quantum energy minimization is carried out in a six-dimensional space that corresponds to the rigid-body protein-ligand interaction.

Carrier Proteins↗
PubMed1995

Molecular dynamics simulations of alcohol dehydrogenase with a four- or five-coordinate catalytic zinc ion.

A detailed parameterization is presented of a zinc ion with one histidine and two cysteinate ligands, together with one or two water, hydroxide, aldehyde, alcohol, or alkoxide ligands. The parameterization is tailored for the active site of alcohol dehydrogenase and is obtained entirely from quantum chemical computations. The force-field reproduces excellently the geometry of quantum chemically optimized zinc complexes as well as the crystallographic geometry of the active site of alcohol dehydrogenase and small organic structures. The parameterization is used in molecular dynamics simulations and molecular mechanical energy minimizations of alcohol dehydrogenase with a four- or five-coordinate catalytic zinc ion. The active-site zinc ion seems to prefer four-coordination over five-coordination by at least 36 kJ/mol. The only stable binding site of a fifth ligand at the active-site zinc ion is opposite to the normal substrate site, in a narrow cavity behind the zinc ion. Only molecules of the size of water or smaller may occupy this site. There are large fluctuations in the geometry of the zinc coordination sphere. A four-coordinate water molecule alternates frequently (every 7 ps) between the substrate site and the fifth binding site and even two five-coordinate water molecules may interchange ligation sites without prior dissociation. Ligand exchange at the zinc ion probably proceeds by a dissociative mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Alcohol Dehydrogenase↗