  We present a new computation of S-wave color-singlet nonrelativistic QCD
matrix elements for the J/psi and the eta_c. We compute the matrix elements of
leading order in the heavy-quark velocity v and the matrix elements of relative
order v^2. Our computation is based on the electromagnetic decay rates of the
J/psi and the eta_c and on a potential model that employs the Cornell
potential. We include relativistic corrections to the electromagnetic decay
rates, resumming a class of corrections to all orders in v, and find that they
significantly increase the values of the matrix elements of leading order in v.
This increase could have important implications for theoretical predictions for
a number of quarkonium decay and production processes. The values that we find
for the matrix elements of relative order v^2 are somewhat smaller than the
values that one obtains from estimates that are based on the velocity-scaling
rules of nonrelativistic QCD.
