Hi Oleg

Thanks for your suggestion.I tried as you suggested with opt(maxstep=3) but still after optimization, it is changing to enol form.(seen in Gaussview). [check attached files]

Strangely, it worked well when HF theory was used (all basis sets except 3-21g).

But the problem is when I use DFT theory.

Thank you.

Are you sure that keto form is stable (minimum). What I mean is do you expect a barrier between the keto and enol form? If there is no barrier then you will always end up with an enol form (which has a minimum). In order to confirm this point  you can run a potential energy scan along the O-H distance. In Gaussian you can do this with relaxed scan option (opt=modredundant).