  We calculate pair potential curves for interacting Rydberg atoms in a
constant electric field and use them to determine the effective $C_3$
dipole-dipole and $C_6$ van der Waals coefficients. We compare the $C_3$ and
$C_6$ with experiments where the angle of a polarizing electric field is varied
with respect to the axis of a quasi-1-dimensional trap at ultracold
temperatures. The dipoles produced via polarization of the atoms have an
angular dependent dipole-dipole interaction. We focus on the interaction
potential of two rubidium Rydberg atoms in $50S_{1/2}$ states in the blockade
regime. For internuclear distances close to the blockade radius, $R_{bl}
\approx 4 - 6\,\mu$m, molecular calculations are in much better agreement with
experimental results than those based on the properties of single atoms and
independent calculations of $C_3$ and $C_6$ which were used to analyze the
original experiment. We find that the calculated $C_6$ coefficient is within
$8\%$ of the experimental value while the $C_3$ coefficient is within $20\%$ of
the experimental value.
