Convergence of the Virial Series

An approach based on the virial expansion suffers from the difficulty of evaluating higher coefficients for highly asymmetric particles and from the non-convergence of the virial series at the concentrations required for formation of a stable nematic phase Lattice methods therefore take precedence over the virial expansion as a basis for quantitative treatment of the liquid crystalline state. 

The great advantage of Mayer s theory is that it is formally exact within the radius of convergence of the virial series and it predicts the properties characteristic of all charge types without the need to... 

For low density (large Vm), the series (2.30) is expected to achieve useful accurary with only a few terms. Higher densities within the domain of convergence require additional terms to achieve a desired accuracy. For some densities, the virial series may not converge at all. 

Before comparing theory and experiment let us discuss the convergence of the semiclassical expansion of the dielectric second virial coefficient. In Table 1-15 the classical dielectric virial coefficient the first and second quantum corrections, and the full quantum result are reported. An inspection of this table shows that the quantum effects are small for temperatures larger than 100 K, and /it(/) can be approximated by the classical expression with an error smaller than 2.5%. At lower temperatures the dielectric virial coefficient of 4He starts to deviate from the classical value. Still, for T > 50 K the quantum effects can be efficiently accounted for by the sum of the first and second quantum corrections. Indeed, for T = 50, 75, and 100 K the series (7) + lli 1 (7) + (7) reproduces the exact results with errors... 

The two forms of the virial expansion given by Eqs. (3.10) and (3.11) are infinite series. For engineering purposes their use is practical only where convergence is very rapid, that is, where no more than two or three terms are required to yield reasonably close approximations to the values of the series. This is realized for gases and vapors at low to moderate pressures. 

If the basic set xpk is chosen complete, the virial theorem will be automatically fulfilled and no scaling is necessary. In such a case, the wave function under consideration may certainly be expressed in the form of Eq. III. 18, but, if the basis is chosen without particular reference to the physical conditions of the problem, the series of determinants may be extremely slowly convergent with a corresponding difficulty in interpreting the results. It therefore seems tempting to ask whether there exists any basic set of spin orbitals. which leads to a most "rapid convergency in the expansion, Eq. III. 18, of the wave function for a specific state (Slater 1951). 

We first consider the case of a two-component solution (biopolymer + solvent) over a moderately low range of biopolymer concentrations, i.e., C < 20 % wt/wt. The quantities pm x in the equations for the chemical potentials of solvent and biopolymer may be expressed as a power series in the biopolymer concentration, with some restriction on the required number of terms, depending on the steepness of the series convergence and the desired accuracy of the calculations (Prigogine and Defay, 1954). This approach is based on simplified equations for the chemical potentials of both components as a virial series in biopolymer concentration, as developed by Ogston (1962) at the level of approximation of just pairwise molecular interactions ... 

The virial expansion of Equation (4B-4) is inappropriate as the concentration increases since the series no longer converges at sufficiently high solute volume fractions. In this case,... 

Using eqs. (l)-(9), along with empirical pure-electrolyte parameters 3 ), 3 > 3 and and binary mixture parameters 0, one can reproduce experimental activity-coefficient data typically to a few percent and in all cases to + 20%. Of course, as noted above, the most accurate work on complex, concentrated mixtures requires that one include further mixing parameters and also for calculations at temperatures other than 25°C, include the temperature dependencies of the parameters. However, for FGD applications, a more important point is that Pitzer1s formulation appears to be a convergent series. The third virial coefficients... 

Examination of the terms to O(k ) in the SL expansion for the free energy show that the convergence is extremely slow for a RPM 2-2 electrolyte in aqueous solution at room temperature. Nevertheless, the series can be summed using a Pade approximant similar to that for dipolar fluids which gives results that are comparable in accuracy to the MS approximation as shown in figure A2.3.19(a). However, unlike the DHLL + i 2 approximation, neither of these approximations produces the negative deviations in the osmotic and activity coefficients from the DHLL observed for higher valence electrolytes at low concentrations. This can be traced to the absence of the complete renormalized second virial coefficient in these theories it is present... 

The last restriction in this equation is needed to satisfy the conditions of the theorem which allows the topological reduction. In this series, there is one term with no yo 8f bonds and only one term with one yo 8f bond. This latter term has only two field points and just one bond. There are an infinite number of graphs with two yo 8f bonds, and even the series for these terms looks like a low-density virial series which might not be expected to be convergent or meaningful at high densities. 

Unless the convergence limit of equation (82) is known and the requisite coefficients determinable, some approximation must be made for the infinite series. Ross and Morrison have taken the reduced virial eoefficients, D2o/r, E2olr and higher terms (where r is the Lennard-Jones parameter for the distance of maximum attraction), to be equal to 2, a value supported by the studies of Ree and Hoover. The K used by Ross and Morrison (Krm) may be written as Krm = AJKnit is not clear what advantages this local isotherm offers over other approximate equations derived for a two-dimensional gas, in particular the simpler Hill-de Boer equation. 

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