available optical data in a wide range of frequencies. The authors were directed to calculation of the van der Waals force at rather small separations $d\sim
1\;nm$. The important imaginary frequencies where $\varepsilon_1(i\zeta)$ has to be known with the best possible precision are around $\zeta\sim\omega_c=c/2d$. For $d\sim 1\; nm$ important frequencies are $\zeta\sim 100\; eV$. To have $\varepsilon_1(i\zeta)$ in this frequency range one has to integrate in the dispersion relation (\[K-K\]) (but for $\varepsilon_1$) at $\omega\gtrsim 100\; eV$. For these high frequencies the directly measured quantity is ${\rm Im}\left(1/\varepsilon(\omega)\right)$. The real part of $1/\varepsilon(\omega)$ must be restored with the Kramers-Kronig relation. The complete procedure for calculation of $\varepsilon(i\zeta)$ is rather complex [@Dag00]. In our case, for $d\gtrsim 10\;nm$ this procedure can be significantly simplified.

Segelstein [@Seg81] compiled the data for the absorption coefficient of water in very wide range of wavelengths from $10\;
nm$ to $1\; m$. These data can be used instead of $\varepsilon_1^{\prime\prime}(\omega)$ to calculate $\varepsilon_1(i\zeta)$. The absorption coefficient $\mu(\omega)$ is related with the imaginary part of the complex refractive index $\tilde{n}(\omega)=n(\omega)+ik(\omega)$ by the relation $\mu(\omega)=2\omega k(\omega)/c$. Between $n(\omega)$ and $k(\omega)$ exists similar dispersion relation as between $\varepsilon_1^{\prime}(\omega)$ and $\varepsilon_1^{\prime\prime}(\omega)$: $$\label{refr}
    \tilde{n}(i\zeta)=1+\frac{2}{\pi}\int\limits_0^{\infty}d\omega
    \frac{\omega k(\omega)}{\omega^2+\zeta^2}.$$ If we know $\tilde{n}(i\zeta)$ 