I know how you feel.
You're a great candidate. The right knowledge, skills and abilities to do the job. You like the opportunity - the company, the people - everything. It's a great fit.
Then something funny happens on the way to a perfect match. You felt liked you bombed the last round of interviews, the final conversations before you expected an offer to come from the company in question. As you arrived home, you had that sinking feeling in your stomach. You blew it.
So you naturally start thinking about what you can do to make it right. Then you have a great idea:
"I'm going to drop them a thank you note, and apologize for not being my best in the interview process. I'll acknowledge it and explain it to them. It's better to get in front of this to maximize my potential of getting this job."
Except it's not. You're your own worst critic. It's never in your best interest to hit a company with a follow up note explaining poor performance in an interview. Consider the following:
1. You're probably harder on yourself then they are. Odds are that some of the people who interviewed you still consider you viable.
2. Detailed notes explaining poor interview performance don't really help you, but they can always hurt you. From the company - "I thought there was something, and now I'm sure of it - let's move on."
3. You have no idea who you are being compared against. Even if you blew it, you might be the most attractive candidate.
4. The longer the note, the less attractive you look as a candidate. Thank you note became 3 big paragraphs with a lot of explaining nebulous things? Check please...
Did you blow the interview? By your standards, yes. But let's talk about them (the company) - what did they think?
You don't know.
But if you tell them you didn't do well, odds are they're going to be happy to agree with you.
Send a graceful thank you note expressing optimism for the future. Maybe a witty observation if you have a low risk one available. Get in and out with a length that you could send in 3 twitter posts.
And leave the analysis of your interview performance to another day.
--Signed, Uncle KD