STOCKHOLM (AFP) - One of the three winners of the Nobel Chemistry Prize, Dr Michael Levitt, said on Wednesday he was essentially awarded for work he did when he was 20 years old.

Dr Levitt, speaking from his California home, said he felt like he had a "triple espresso" after being informed that the prize was his for research done decades ago.

"I was doing work before my post doc when I was 20 years old to write a computer programme. I guess I wrote a pretty good programme," Dr Levitt, now 66, chuckled.

He said it was "essentially" this programme, written decades ago, that was being honoured with Wednesday's prize, but added that people continue to benefit to this day.

He said he thought he was dreaming when the call came from Stockholm in the middle of the night.

"I thought I was dreaming. My cell phone never rings. I usually only get text messages or emails. At first I thought it was a wrong number," he said of the call that came just before the official announcement at 2:45 am California time.

Dr Levitt said his first reaction was "disbelief, as it always is. My heart rate jumped".

Dr Levitt, a US-British citizen who now works at Stanford University School of Medicine in California, won the Nobel together with Martin Karplus, a US-Austrian citizen, and Arieh Warshel of the US and Israel.

They were honoured for devising computer models to simulate chemical processes, providing a revolutionary tool for drug designers and engineers.

Dr Levitt said he would be attending the gala Nobel Prize ceremony in December in Stockholm with his family, including his 98-year-old mother - who was "speechless" over his prize - and a two-year-old grandson.