  It would be interesting to investigate the accuracy of the results obtained
in the variational method, because it is important for studying hadron spectra.
One can define some criteria to judge the accuracy, or the quality of the trial
function. We employ a simple potential form to check how accurate the
variational results obtained by a single-parameter trial function can be. All
the concerned problems, in particular, the relevant aspects on the application
of hypervirial theorem in variational method for various potential forms, are
discussed in every detail. The qualitative conclusion of the work can be
generalized to much more complicated cases. Our study suggests that the
hypervirial relations can serve as a good and practical criterion for judging
the accuracy of any trial functions.
