Higher-order gradients

There are higher multipole polarizabilities tiiat describe higher-order multipole moments induced by non-imifonn fields. For example, the quadnipole polarizability is a fourth-rank tensor C that characterizes the lowest-order quadnipole moment induced by an applied field gradient. There are also mixed polarizabilities such as the third-rank dipole-quadnipole polarizability tensor A that describes the lowest-order response of the dipole moment to a field gradient and of the quadnipole moment to a dipolar field. All polarizabilities of order higher tlian dipole depend on the choice of origin. Experimental values are basically restricted to the dipole polarizability and hyperpolarizability [21, 24 and 21]. Ab initio calculations are an imponant source of both dipole and higher polarizabilities [20] some recent examples include [26, 22] ... 

If temis of higher order than linear in t are neglected, the transverse magnetization evolves in the presence of the first bipolar gradient pulse according to (equation Bl.14.2 and equation B 1.14.61 ... 

The method of Ishida et al [84] includes a minimization in the direction in which the path curves, i.e. along (g/ g -g / gj), where g and g are the gradient at the begiiming and the end of an Euler step. This teclmique, called the stabilized Euler method, perfomis much better than the simple Euler method but may become numerically unstable for very small steps. Several other methods, based on higher-order integrators for differential equations, have been proposed [85, 86]. 

The basic idea of NMA is to expand the potential energy function U(x) in a Taylor series expansion around a point Xq where the gradient of the potential vanishes ([Case 1996]). If third and higher-order derivatives are ignored, the dynamics of the system can be described in terms of the normal mode directions and frequencies Qj and Ui which satisfy ... 

Errors in advection may completely overshadow diffusion. The amplification of random errors with each succeeding step causes numerical instability (or distortion). Higher-order differencing techniques are used to avoid this instability, but they may result in sharp gradients, which may cause negative concentrations to appear in the computations. Many of the numerical instability (distortion) problems can be overcome with a second-moment scheme (9) which advects the moments of the distributions instead of the pollutants alone. Six numerical techniques were investigated (10), including the second-moment scheme three were found that limited numerical distortion the second-moment, the cubic spline, and the chapeau function. 

The first derivative is the gradient g, the second derivative is the force constant (Hessian) H, the third derivative is the anharmonicity K etc. If the Rq geometry is a stationary point (g = 0) the force constant matrix may be used for evaluating harmonic vibrational frequencies and normal coordinates, q, as discussed in Section 13.1. If higher-order terms are included in the expansion, it is possible to determine also anharmonic frequencies and phenomena such as Fermi resonance. 

The advantage of the NR method is that the convergence is second-order near a stationary point. If the function only contains tenns up to second-order, the NR step will go to the stationary point in only one iteration. In general the function contains higher-order terms, but the second-order approximation becomes better and better as the stationary point is approached. Sufficiently close to tire stationary point, the gradient is reduced quadratically. This means tlrat if the gradient norm is reduced by a factor of 10 between two iterations, it will go down by a factor of 100 in the next iteration, and a factor of 10 000 in the next ... 

To determine the optimal parameters, traditional methods, such as conjugate gradient and simplex are often not adequate, because they tend to get trapped in local minima. To overcome this difficulty, higher-order methods, such as the genetic algorithm (GA) can be employed [31,32]. The GA is a general purpose functional minimization procedure that requires as input an evaluation, or test function to express how well a particular laser pulse achieves the target. Tests have shown that several thousand evaluations of the test function may be required to determine the parameters of the optimal fields [17]. This presents no difficulty in the simple, pure-state model discussed above. 

In order to minimize numerical diffusion, Boris and Book [131] formulated the idea of blending a low-order stable differencing scheme with a higher order, potentially unstable, scheme in such a way that steep concentration gradients are maintained as well as possible. The algorithm they proposed consists of the following steps ... 

As shown in this chapter, in the simulation of systems described by partial differential equations, the differential terms involving variations with respect to length are replaeed by their finite-differenced equivalents. These finite-differenced forms of the model equations are shown to evolve as a natural eonsequence of the balance equations, according to the manner of Franks (1967). The approximation of the gradients involved may be improved, if necessary, by using higher order approximations. Forward and end sections can... 

That is, the phase shift depends on the initial position x0, the initial velocity vx0 and the initial acceleration ax0. Higher order terms vanish if the flow field is stationary on the time scale of the NMR experiment (i.e., time-dependent accelerations do not occur in this case). For a gradient pulse of duration t and strength Gx the total phase shift is [see Figure 2.9.4(a)]... 

As an example, consider the case of a uniform mean scalar gradient with pi = p2 = 0.5, r = 0 and cT = [ 1 1 ]T (i.e., Ne = 2). For this case, the scalar variance will grow with time due to the lack of scalar variance dissipation (i.e., r = 0 implies that = 0). Moreover, the higher-order moments (up to 2NC — 1 = 3) should approach the Gaussian values.4 The DQMOM representation for this case yields5... 

Also, the fourth order terms of the gradient expanssion of the kinetic energy have been evaluated [24], leading to more involved expressions. This is one of the problems of the methods based on the gradient expansion the systematic improvement of the results by adding higher orders is not possible because of the asymptotic nature, and... 

As the Fourier coefficients in Eq. (8.17) contain the factor 1 /H2, the high-order structure factors are of decreasing importance in the potential summation. The emphasis on the low-order structure factors is less pronounced for the higher-order electrostatic functions, such as the electric field and the electric field gradient, as summarized in Table 8.1. 

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