  We study how uncertainties in the rate coefficients of chemical reactions in
the RATE06 database affect abundances and column densities of key molecules in
protoplanetary disks. We randomly varied the gas-phase reaction rates within
their uncertainty limits and calculated the time-dependent abundances and
column densities using a gas-grain chemical model and a flaring steady-state
disk model. We find that key species can be separated into two distinct groups
according to the sensitivity of their column densities to the rate
uncertainties. The first group includes CO, C$^+$, H$_3^+$, H$_2$O, NH$_3$,
N$_2$H$^+$, and HCNH$^+$. For these species, the column densities are not very
sensitive to the rate uncertainties but the abundances in specific regions are.
The second group includes CS, CO$_2$, HCO$^+$, H$_2$CO, C$_2$H, CN, HCN, HNC
and other, more complex species, for which high abundances and abundance
uncertainties co-exist in the same disk region, leading to larger scatters in
the column densities. However, even for complex and heavy molecules, the
dispersion in their column densities is not more than a factor of ~4. We
perform a sensitivity analysis of the computed abundances to rate uncertainties
and identify those reactions with the most problematic rate coefficients. We
conclude that the rate coefficients of about a hundred of chemical reactions
need to be determined more accurately in order to greatly improve the
reliability of modern astrochemical models. This improvement should be an
ultimate goal for future laboratory studies and theoretical investigations.
