Determination of dielectric dispersion by two methods and application to calculations of van der Waals forces

Values of ε{lunate}(iω), the dielectric permeability on the imaginary frequency axis, have been obtained in the case of liquid water and employed in the calculation of van der Waals forces. Previous work has employed expressions for ε{lunate}(iω) calculated from incomplete spectral data in the uv. In the present work an estimation is obtained of how severe these approximations are in the case of liquid water, where nearly complete spectral data exist, by applying Kramers' formulas. Results are in general agreement with previous calculations which employ a damped harmonic oscillator model. However, for frequencies beyond 10¹⁷ rad/sec, the application of Kramers' formulas indicates that values of ε{lunate}(iω) previously obtained are somewhat low. By increasing the effective absorption frequency for liquid water from the first to somewhat less than the fourth ionization potential, values of ε{lunate}(iω) according to the two methods are in better agreement for large frequencies. The increased effective frequency gives larger values of the van der Waals forces, but it is concluded that, at the most, a 50% increase in their value can be expected.