Full correlation function

To derive a relation between the dielectric constant and the site-site correlation function, we begin by considering a fluid of linear polar molecules. For a dipolar fluid, it is convenient to expand the full correlation function... 

The basic formulation of this problem was given by Van Hove [25] in the form of his space-time correlation functions, G ir, t) and G(r, t). He showed that the scattering functions, as defined above, for a diffusing system are given by the Fourier transformation of these correlation functions in time and space. Incoherent scattering is linked to the self-correlation function, Gs(r, t) which provides a full definition of tracer diffusion while coherent scattering is the double Fourier transform of the full correlation function which is similarly related to chemical or Fick s law diffusion. Formally the equations can be written ... 

The next neighbors occur when a third particle is connected by HB to the two particles 1 and 2. The ideal distance for this configuration is R = VSRh, which in our computation falls close to R = 1.8. The radial distribution function g(R) shows a second peak at this distance (Fig. 2.39). The full correlation function at R = 1.8 is... 

When the solution of the nonlinear Langevin equation was first formulated by Kawasaki, the approximation 2 = was always made. The presence of full correlation functions in phenomenological results for A, as contrasted to zero-order correlation functions in the results of Kawasaki, remained a puzzling point for a few years. Experimental evidence indicated that was a better approximation to 2 than The problem was... 

To measure accurately the (full) correlation function for longer times, the second output channel of one of the discriminators in Fig. 15 is used to simultaneously feed a digital logarithmic correlator. This device covers the time range from microseconds up to thousands of seconds whereby its minimum time resolution strongly depends on the after pulsing characteristics of the phototube. By employing the two... 

While the US procedure could be repeated for every t to yield the full correlation function C(t), a computationally more convenient approach exists [21,175], which allows us to write the rate constant as... 

The integrals are over the full two-dimensional volume F. For the classical contribution to the free energy /3/d([p]) the Ramakrishnan-Yussouff functional has been used in the form recently introduced by Ebner et al. [314] which is known to reproduce accurately the phase diagram of the Lennard-Jones system in three dimensions. In the classical part of the free energy functional, as an input the Ornstein-Zernike direct correlation function for the hard disc fluid is required. For the DFT calculations reported, the accurate and convenient analytic form due to Rosenfeld [315] has been used for this quantity. 

Of course, knowledge of the entire spectrum does provide more information. If the shape of the wings of G (co) is established correctly, then not only the value of tj but also angular momentum correlation function Kj(t) may be determined. Thus, in order to obtain full information from the optical spectra of liquids, it is necessary to use their periphery as well as the central Lorentzian part of the spectrum. In terms of correlation functions this means that the initial non-exponential relaxation, which characterizes the system s behaviour during free rotation, is of no less importance than its long-time exponential behaviour. Therefore, we pay special attention to how dynamic effects may be taken into account in the theory of orientational relaxation. 

If the second term on the right-hand side of the equation is omitted, the latter is transformed into Eq. (2.76). As the omission is possible only for t < tj, Fourier transformation of the reduced equation holds for co-tj 1 only. Consequently, the equality (2.75) is of asymptotic character, and may not be utilized to find full g(co) or its Fourier-transform Kj(t) at any times. When it was nevertheless used in [117], the rotational correlation function turned out to be alternating in sign. The oscillatory behaviour of Kj(t) occured not only in a compressed gas, but also at normal pressure, when Kj(t) should vanish monotonically, if not exponentially. The origin of these non-physical oscillations is easily... 

Figure 25. Electron-transfer rate the electronic coupling strength at T = 500 K for the asymmetric reaction (AG = —3ffl2, oh = 749 cm ). Solid line-present full dimensional results with use of the ZN formulas. Dotted line-full dimensional results obtained from the Bixon-Jortner formula. Filled dotts-effective ID results of the quantum mechanical flux-flux correlation function. Dashed line-effective ID results with use of the ZN formulas. Taken from Ref. [28].

There are several attractive features of such a mesoscopic description. Because the dynamics is simple, it is both easy and efficient to simulate. The equations of motion are easily written and the techniques of nonequilibriun statistical mechanics can be used to derive macroscopic laws and correlation function expressions for the transport properties. Accurate analytical expressions for the transport coefficient can be derived. The mesoscopic description can be combined with full molecular dynamics in order to describe the properties of solute species, such as polymers or colloids, in solution. Because all of the conservation laws are satisfied, hydrodynamic interactions, which play an important role in the dynamical properties of such systems, are automatically taken into account. 

Figure 9.2 displays a division of the mass range into 13 intervals. After multiplication of all the individual normalized correlation values, the composite correlation index is smaller and the method achieves more selectivity than it would if a single correlation function were calculated over the full mass range. A lower limit of 0.0001 was placed on the value for each normalized interval comparison to prevent the method from generating absurdly low values. These... 

The equilibrium interconversion between an ethylene phosphite and a bicyclic spirophosphorane is shown to proceed by the insertion of the phosphite into the labile O-H bond of the hydroxyethyl ester. The mechanism is similar to the insertion of carbenes or nitrenes. Energy relationships of reaction intermediates were studied by MO RHF, MP2(full), MP4SDTQ, and DFT calculations. In most cases, they predicted that hydroxyethyl ethylene phosphates were more stable than the strained spirophosphoranes, which is not supported by the experimental evidence. The best correspondence to experimental data was obtained by DFT calculations with Perdew-Wang correlation functions <2003JST35>. 

The reason why one chose to follow the main liquid-crystalline to gel phase transition in DPPC by monitoring the linewidth of the various or natural abundance resonance is evident when we consider the expressions for the spin-lattice relaxation time (Ti) and the spin-spin relaxation time T2). The first one is given by 1/Ti oc [/i(ft>o) + 72(2ft>o)] where Ji coq) is the Fourier transform of the correlation function at the resonance frequency o>o and is a constant related to internuclear separation. The relaxation rate l/Ti thus reflects motions at coq and 2coq. In contrast, the expression for T2 shows that 1/T2 monitors slow motions IjTi oc. B[/o(0) -I- /i(ft>o) + /2(2u>o)], where /o(0) is the Fourier component of the correlation function at zero frequency. Since the linewidth vi/2 (full-width at half-maximum intensity) is proportional to 1 / T2, the changes of linewidth will reflect changes in the mobility of various carbon atoms in the DPPC bilayer. 

Fig. 4.14 Results on fully protonated PIB by means of NSE [147]. a Time evolution of the self-correlation function at the Q-values indicated and 390 K. Lines are the resulting KWW fit curves (Eq. 4.9). b Momentum transfer dependence of the characteristic time of the KWW functions describing Sseif(Q,t) at 335 K (circles), 365 K (squares) and 390 K (triangles). In the scaling representation (lower part) the 335 K and 390 K data have been shifted to the reference temperature 365 K applying a shift factor corresponding to an activation energy of 0.43 eV. Solid (dotted) lines through the points represent (q-2 power laws. Full...

The concept of numerical convergence is quite separate from the question of whether DFT accurately describes physical reality. The mathematical problem defined by DFT is not identical to the full Schrodinger equation (because we do not know the precise form of the exchange-correlation functional). This means that the exact solution of a DFT problem is not identical to the exact solution of the Schrodinger equation, and it is the latter that we are presumably most interested in. This issue, the physical accuracy of DFT, is of the utmost important, but it is more complicated to fully address than the topic of numerical convergence. The issue of the physical accuracy of DFT calculations is addressed in Chapter 10. 

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