  The description of elastic, nonadhesive contacts between solids with
self-affine surface roughness seems to necessitate knowledge of a large number
of parameters. However, few parameters suffice to determine many important
interfacial properties as we show by combining dimensional analysis with
numerical simulations. This insight is used to deduce the pressure dependence
of the relative contact area and the mean interfacial separation $\Delta
\bar{u}$ and to present the results in a compact form. Given a proper unit
choice for pressure $p$, i.e., effective modulus $E^*$ times the
root-mean-square gradient $\bar{g}$, the relative contact area mainly depends
on $p$ but barely on the Hurst exponent $H$ even at large $p$. When using the
root-mean-square height $\bar{h}$ as unit of length, $\Delta \bar{u}$
additionally depends on the ratio of the height spectrum cutoffs at short and
long wavelengths. In the fractal limit, where that ratio is zero, solely the
roughness at short wavelengths is relevant for $\Delta \bar{u}$. This limit,
however, should not be relevant for practical applications. Our work contains a
brief summary of the employed numerical method Green's function molecular
dynamics including an illustration of how to systematically overcome numerical
shortcomings through appropriate finite-size, fractal, and discretization
corrections. Additionally, we outline the derivation of Persson theory in
dimensionless units. Persson theory compares well to the numerical reference
data.
