Optimization with constraint to hold atoms on a plane

I want to optimized the structure with constraints.
But I got confused, when reading the manual, What’s the difference between “out-of-plane-bends” and “coplanar bends”.
So as an example, if I want to optimize a benzene and a water with all atoms are in the same plane of the benzene aromatic ring, what should I add in the $opt section(Just like the figure shows)?

Also I don’t understand the description of the “out-of-plane-bends” which looks:
Any suggestions will be appreciated.

I think the out-of-plane definition is ambiguous as it is not clear which atom in the plane defines the apex of the out-of-plane bend.

In any case, I think what you want to do does not require any constraints at all. The optimizer in Q-Chem preserves point-group symmetry, so if you want to optimize a planar structure, just ensure all your atoms are in the plane to begin with. That way you will have at least Cs symmetry which will get preserved during the optimization.

It turns out setting up such an initial structure is not as easy as it should be in IQmol, so I have attached a C2v structure for you to start with. If you want to relax the additional symmetry so that you can obtain a lower Cs energy, simply lengthen one of the O-H bonds to break C2v symmetry.

0  1
      H    0.0000000    2.1490474   -2.0412432
      C    0.0000000    1.2014162   -1.5120289
      C    0.0000000   -0.0000000   -2.2272071
      H    0.0000000   -0.0000000   -3.3120709
      C    0.0000000   -1.2014162   -1.5120289
      H    0.0000000   -2.1490474   -2.0412432
      C   -0.0000000   -1.1777540   -0.0983375
      H   -0.0000000   -2.1379714    0.4107496
      C   -0.0000000    0.0000000    0.6930995
      O   -0.0000000    0.0000000    3.3862223
      C    0.0000000    1.1777540   -0.0983375
      H    0.0000000    2.1379714    0.4107496
      H   -0.0000000    0.8242539    3.8899816
      H   -0.0000000   -0.8242539    3.8899816
      H   -0.0000000    0.0000000    1.7239328

Thank you very much.
I have realized that a good initial structure is also very important for my cases.
Another way to achieve my target, is to construct my molecules on a plane such as you mentioned above, and then using FIX to hold the atoms in a plane like X.

Constrained optimizations are intrinsically harder than unconstrained ones and are best avoided if you can do so. In your case I think using the symmetry to keep everything in a plane is the better approach.