Dear Mandal,

Initially, we have to check whether the input specification of reactant, product are correct. If it fails for QST2 then you should guess the correct transition state structure in QST3. If it fails to optimize in QST3, it mean there is error in guessing the transition state structure. So please check whether the initial guess is correct or not. Another way is to optimize the guess structure alone by using the command opt=(ts, calcall, noeigen). Once the geometry is optimized by using the command then you can give the optimized Transition state structure in QST3 method.

Hope it will useful.

Thanks

TS structure varies significantly between gas phase and continuum calculations, especially if continuum has explicit role on reaction coordinates.