Argon topology???

ar.top

[ defaults ]
; nbfunc        comb-rule       gen-pairs       fudgeLJ fudgeQQ
  1             1               no              1.0     1.0

[ atomtypes ]
Ar  39.948    0.0   A     0.00622127     9.69576e-06

[ moleculetype ]
; molname       nrexcl
Arg              2

[ atoms ]
; id    at type res nr  residu name     at name  cg nr   charge
1       Ar       1       Arg              Ar        1       0

[ system ]
; Name
Argon

[ molecules ]
; Compound        #mols
Arg               3