I have optimized a structure which is slightly bend (bond angle ~168°). Another structure with a linear bond angle (180°) was also optimized (constrained optimization) which turned out to be first order saddle point. In constraint geometry optimization one bond angle and two dihedral angles were frozen. I would like to do a IRC scan for the geometry (without imposing constraints) along the frozen parameters. Suggest a way to do the same. Any help would be greatly appreciated.