[ moleculetype ]
; name  nrexcl
ethane          1

[ atoms ]
;   nr    type   resnr  residu    atom    cgnr  charge
     1     CH3       1     ETH     ET1       1   0.000
     2     CH3       1     ETH     ET2       2   0.000

[ bonds ]
;  ai    aj  funct
    1     2      5

[ constraints ]
; i j type lenght
1 2 1 0.2345