eps is the threshold (in atomic units) that is decisive whether the image of an atom generated
by a symmetry operation and a second atom in the vicinity of this image are considered equivalent.
This allows to obtain symmetric structure from almost-symmetric inputs (e.g. you run some optimization in
C1 and now you suspect that the actual symmetry of the minimum is higher).
Values larger than 1.0 a.u. too often lead to unusable results. I suggest to inspect the coordinates by hand and
to check which atom breaks the assumed symmetry and to either rectify by hand or to accept that symmetry is
lower than expected.
Hope this helps...
Cheers,
Andreas
