Q:

insert word between lines

I have pdb (protein data base) file which has thousands of lines. 
REMARK   1                     PDB file generated by ptraj (set  1000)
ATOM      1  O22 DDM     1       2.800   4.419  20.868  0.00  0.00
ATOM      2  H22 DDM     1       3.427   4.096  20.216  0.00  0.00
ATOM      3  C22 DDM     1       3.351   5.588  21.698  0.00  0.00
ATOM      4  H42 DDM     1       3.456   5.274  22.736  0.00  0.00
ATOM      5  C23 DDM     1       2.530   6.846  21.639  0.00  0.00
ATOM      6  H43 DDM     1       2.347   7.159  20.611  0.00  0.00
ATOM      7  O23 DDM     1       1.313   6.498  22.334  0.00  0.00
ATOM      8  H23 DDM     1       0.903   5.837  21.771  0.00  0.00
ATOM      9  C24 DDM     1       3.073   8.109  22.266  0.00  0.00
ATOM     10  H44 DDM     1       3.139   7.837  23.319  0.00  0.00
ATOM     11  O24 DDM     1       2.218   9.278  22.007  0.00  0.00
ATOM     12  H24 DDM     1       1.278   9.184  22.179  0.00  0.00
ATOM     13  C25 DDM     1       4.494   8.317  21.764  0.00  0.00
ATOM     14  H45 DDM     1       4.391   8.452  20.687  0.00  0.00

'
I want to insert word "TER" every 81 lines in that file whcih contains more than 20,000 lines but ignoring the first line since it is a comment.
I browse through internet, seems SED can do it. But i am lost.
Can anyone guide?
Thanks in advance.

A:

Try this:
sed -i -e '1~81 i\TER' file

