  Recent laboratory experiments have measured fusion cross sections at
center-of-mass energies low enough for the effects of atomic and molecular
electrons to be important. To extract the cross section for bare nuclei from
these data (as required for astrophysical applications), it is necessary to
understand these screening effects. We study electron screening effects in the
low-energy collisions of Z=1 nuclei with hydrogen molecules. Our model is based
on a dynamical evolution of the electron wavefunctions within the TDHF scheme,
while the motion of the nuclei is treated classically. We find that at the
currently accessible energies the screening effects depend strongly on the
molecular orientation. The screening is found to be larger for molecular
targets than for atomic targets, due to the reflection symmetry in the latter.
The results agree fairly well with data measured for deuteron collisions on
molecular deuterium and tritium targets.
