  By relating the functional averages of a generic scalar operator in
simulations with Open (O) and Periodic (P) boundary conditions (BCs)
respectively for $SU(3)$ lattice gauge theory, we show that the scalar glueball
mass and the glueball to vacuum matrix element can be extracted very
efficiently from the former. Numerical results are compared with those
extracted from the two point function of the time slice energy density (both
PBC and OBC). The scaling properties of the mass and the matrix element are
studied with the help of Wilson (gradient) flow.
