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Perturbation Approach (perturbation + approach)
Selected AbstractsOn the validity of the perturbation approach for the flow inside weakly modulated channelsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2002H. Zhou The equations governing the flow of a viscous fluid in a two-dimensional channel with weakly modulated walls have been solved using a perturbation approach, coupled to a variable-step finite-difference scheme. The solution is assumed to be a superposition of a mean and perturbed field. The perturbation results were compared to similar results from a classical finite-volume approach to quantify the error. The influence of the wall geometry and flow Reynolds number have extensively been investigated. It was found that an explicit relation exists between the critical Reynolds number, at which the wall flow separates, and the dimensionless amplitude and wavelength of the wall modulation. Comparison of the flow shows that the perturbation method requires much less computational effort without sacrificing accuracy. The differences in predicted flow is kept well around the order of the square of the dimensionless amplitude, the order to which the regular perturbation expansion of the flow variables is carried out. Copyright © 2002 John Wiley & Sons, Ltd. [source] On the direct calculation of the free energy of quantization for molecular systems in the condensed phaseJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2009Daan P. Geerke Abstract Using the path integral formalism or the Feynman-Hibbs approach, various expressions for the free energy of quantization for a molecular system in the condensed phase can be derived. These lead to alternative methods to directly compute quantization free energies from molecular dynamics computer simulations, which were investigated with an eye to their practical use. For a test system of liquid neon, two methods are shown to be most efficient for a direct evaluation of the excess free energy of quantization. One of them makes use of path integral simulations in combination with a single-step free energy perturbation approach and was previously reported in the literature. The other method employs a Feynman-Hibbs effective Hamiltonian together with the thermodynamic integration formalism. However, both methods are found to give less accurate results for the excess free energy of quantization than the estimate obtained from explicit path integral calculations on the excess free energy of the neon liquid in the classical and quantum mechanical limit. Suggestions are made to make both methods more accurate. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009 [source] An approximate diatomics in molecules formulation of generalized valence bond theoryJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2008John Cullen Abstract The slow computational speed of the generalized valence bond perfect pairing method (GVB-PP) has been an impediment to its routine use. We have addressed this problem by employing a diatomics in molecules Hamiltonian derived from a second quantization perturbation approach. This results in all three- and four-centered two-electron integrals being dropped from the traditional GVB-PP calculation. For moderate sized molecules, as for example C20 computed with a double zeta + polarization basis, there is on average a fifty-fold decrease in computational times. In this article, we present the theory behind our approach and analyze the accuracy and speed of this approximate GVB-PP method for several cases where density functional methods have produced ambivalent results. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Intruder state avoidance multireference Møller,Plesset perturbation theoryJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2002Henryk A. Witek Abstract A new perturbation approach is proposed that enhances the low-order, perturbative convergence by modifying the zeroth-order Hamiltonian in a manner that enlarges any small-energy denominators that may otherwise appear in the perturbative expansion. This intruder state avoidance (ISA) method can be used in conjunction with any perturbative approach, but is most applicable to cases where small energy denominators arise from orthogonal-space states,so-called intruder states,that should, under normal circumstances, make a negligible contribution to the target state of interests. This ISA method is used with multireference Møller,Plesset (MRMP) perturbation theory on potential energy curves that are otherwise plagued by singularities when treated with (conventional) MRMP; calculation are performed on the 13, state of O2; and the 21,, 31,, 23,, and 33, states of AgH. This approach is also applied to other calculations where MRMP is influenced by intruder states; calculations are performed on the 3,u state of N2, the 3, state of CO, and the 21A, state of formamide. A number of calculations are also performed to illustrate that this approach has little or no effect on MRMP when intruder states are not present in perturbative calculations; vertical excitation energies are computed for the low-lying states of N2, C2, CO, formamide, and benzene; the adiabatic 1A1,3B1 energy separation in CH2, and the spectroscopic parameters of O2 are also calculated. Vertical excitation energies are also performed on the Q and B bands states of free-base, chlorin, and zinc,chlorin porphyrin, where somewhat larger couplings exists, and,as anticipated,a larger deviation is found between MRMP and ISA-MRMP. © 2002 Wiley Periodicals, Inc. J Comput Chem 10: 957,965, 2002 [source] Joint tracking controller for multi-link flexible robot using disturbance observer and parameter adaptation schemeJOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 8 2002Joono Cheong An improved composite controller of singular perturbation approach is designed for controlling a multi-link flexible robot with uncertainties. We adopt the standard form of a singular perturbation approach for modeling. To reduce the coupling effect from flexibility, the bandwidth of a slow subsystem is modulated by considering the fundamental frequency. The disturbance observer provides a means for defining the bandwidth of a slow subsystem as well as compensating disturbances. At the same time, uncertainties in the fast subsystem are updated to enhance the capability for vibration suppression in conjunction with PID (Proportional-integrative derivative) modal feedback. We draw conditions for Lyapunov stability of the modal feedback and adaptive scheme. A numerical simulation will support the validity of our research results. © 2002 Wiley Periodicals, Inc. [source] Stochastic perturbation approach to the wavelet-based analysisNUMERICAL LINEAR ALGEBRA WITH APPLICATIONS, Issue 4 2004M. Kami Abstract The wavelet-based decomposition of random variables and fields is proposed here in the context of application of the stochastic second order perturbation technique. A general methodology is employed for the first two probabilistic moments of a linear algebraic equations system solution, which are obtained instead of a single solution projection in the deterministic case. The perturbation approach application allows determination of the closed formulas for a wavelet decomposition of random fields. Next, these formulas are tested by symbolic projection of some elementary random field. Copyright © 2004 John Wiley & Sons, Ltd. [source] On the fragmentation pathway of the ionized enol of glycine in the gas phaseRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 8 2001T. Marino Density functional and second-order many body perturbation approaches were used to compute the potential energy surface for the fragmentation of the ionized enol of glycine [H2NCH,=,C(OH)2]+· into water and aminoketene radical cation [H2N-HC,=,CO]+·. Two possible pathways were considered. The potential energy surfaces obtained are very similar and both predict the existence of a molecular complex in which the water is coordinated to the aminoketene moiety in two different fashions with a noticeable binding energy. The fragmentation is kinetically controlled by the step in which the molecular complex is formed from the most stable cation enol of glycine. Our quantum-mechanical data confirm the hypothesis that the ylide ion [H3NCHCOOH]+· is an intermediate in the water loss. Copyright © 2001 John Wiley & Sons, Ltd. [source] | |||||||||||||