Convert ethane to methane
MASS
DH     1.008     

BOND
DH-H1  340.0    1.090

ANGL
DH-H1-DH    35.0      109.50
CT-H1-DH    50.0      109.50

DIHEDRAL
X -CT-H1-X    9    1.40          0.0             3.

NONBON
  DH       1.0000   0.000