Metal clusters are really a challenge both for SCF-like methods and for the researcher.
Are you sure that the multiplicity is 1? Low-spin states are rare in metal clusters. Probably, you should start with the highest possible spin. Ni atom alone has at least 3 unpaired electrons, and each Ag atom has 1 unpaired electron. One cannot even guess how all these spins are paired in the cluster. Probably, 40 unpaired electrons (37 from Ag and 3 from Ni, mult=41) will be a good start. After converging the highest-spin state, you may use these orbitals as a start for lower-spin states. Alternatively, try mult=5 (3 unpaired electrons from Ni and 1 unpaired electron from Ag cluster, supposing other spins of Ag cluster are paired).
Well, MULT=41 is a little bit too strong, because silver is not ferromagnetic, and such a state will be rather unfavorable... but MULT=5 is OK. Another possibility is MULT=3 (1 electron from Ni and the only unpaired electron of Ag are now paired, and two last electrons from Ni are still unpaired). Try both MULT=5 and MULT=3.
In addition, guess=huckel is not a good start for metal clusters, try guess=hcore instead.
Finally, I see that you're trying to reproduce the surface of Ag by such a cluster. This is not good at all. Periodic models are more adequate for these purposes.
[ This message was edited on Sat Oct 8 '11 at 3:53am by the author ]