  With recent developments in simulating nonadiabatic systems to high accuracy,
it has become possible to determine how much energy is attributed to nuclear
quantum effects beyond zero-point energy. In this work we calculate the
non-relativistic ground-state energies of atomic and molecular systems without
the Born-Oppenheimer approximation. For this purpose we utilize the fixed-node
diffusion Monte Carlo method, in which the nodes depend on both the electronic
and ionic positions. We report ground-state energies for all systems studied,
ionization energies for the first-row atoms and atomization energies for the
first-row hydrides. We find the ionization energies of the atoms to be nearly
independent of the Born-Oppenheimer approximation, within the accuracy of our
results. The atomization energies of molecular systems, however, show small
effects of the nonadiabatic coupling between electrons and nuclei.
