Fix bond length

GROMACS version: 5.1.5.
GROMACS modification: Yes/No
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I want to ask about fixing the bond length in Gromacs. Specifically, I want to use TraPPE for united atom polypropylene.
First of all, I read that I can fix the bonds in the mdp file by choosing “constraints=all-bonds” and “constraint-algorithm=LINCS”. Is that correct? Is something about LINCS parameters that I have to take into consideration?
Moreover, what should I write for bonded interactions in the top file? For example, should I choose a harmonic potential? Does it play a role?
Finally, is there a way to fix the bond length from the top file? One way I was thinking is to increase a lot the force constant in the harmonic equation of bonded interactions. Is that reasonable or will it have a totally different effect than the LINCS algorithm?
Any answer would be appreciated. Thank you in advance.

That is correct.

Bonded parameters are irrelevant when the bonds are constrained.

You can explicitly define a constraint length in the topology simply by supplying the value. Normally these are read from the force field b0 value.

This will not fix the bond lengths, and will potentially break the assumptions of the force field.

Thank you a lot for your answer. It is totally clear to me.