Even if you do not want the job, write a thank you letter respectfully withdrawing your application.
This is not another cover letter, but instead a concise way to remind the employer why you’re the right choice for the job. While you can include both a thank you and a sales pitch in your letter, keep it short.
This thank-you letter is also the perfect opportunity to discuss anything of importance that your interviewer neglected to ask or that you neglected to answer as thoroughly or as well as you would have liked. In other words, restate why you want the job, what your qualifications are, how you might make significant contributions, and so on. You may also view your thank-you letter as a follow-up 'sales' letter. On the other hand, if you bonded with the interviewer over a specific topic, you can mention it in your note as well, as a way to remind them of who you are. If the interviewer brought up any questions about your qualifications, address them here. Note the interviewer’s issues and concerns.