  We prove dimension formulas for the cotangent spaces $T^{1}$ and $T^{2}$ for
a class of rational surface singularities by calculating a correction term in
the general dimension formulas. We get that it is zero if the dual graph of the
rational surface singularity $X$ does not contain a particular type of
configurations, and this generalizes a result of Theo de Jong stating that the
correction term $c(X)$ is zero for rational determinantal surface
singularities. In particular our result implies that $c(X)$ is zero for
Riemenschneiders quasi-determinantal rational surface singularities, and this
also generalise results for qoutient singularities.
