As Ralf said, when you place binaries into %{_libdir} you have to
deal
with different paths on different systems.
On x86:
/usr/lib/gromacs-2/bin/wheel
On x86_64:
/usr/lib64/gromacs-2/bin/wheel
I don't see any problem with using %{_libdir} for this: it should be straight forward
to script into the environment modules and it would even allow multilib if for some
obscure user that were desirable.
Jens