He was the 20th century's greatest scientist, his name synonymous with genius. But while Albert Einstein's theories are known and lauded the world over, insights into his private life are patchy and largely negative. He has been variously portrayed as a bad father, cruel to his wives and an adulterer.

But that view could now change. Spanning more than 3,500 pages, a newly released set of Einstein's personal correspondence provides new clues into the character of the Nobel Prize-winning scientist. He was open about his love affairs to his wife, lost much of his prize money in bad investments and was a much more devoted father than previously thought.

According to Hanoch Gutfreund of the Hebrew University in Jerusalem, who is chairman of the Albert Einstein Worldwide Exhibition, the new letters shatter myths that the great scientist was always cold towards his family.

"Anybody who wants to write a new biography of Einstein will have an additional resource to take into account. As a result of that, certain chapters in his life will now emerge in a slightly different light than before," said Prof Gutfreund.

Einstein became known as one of the greatest physicists of all time after publishing the theory of special relativity in 1905 and a theory of gravity known as general relativity in 1916. He also made significant contributions to quantum mechanics and cosmology. In 1921, he was awarded the Nobel Prize for physics and has since become most famous for his equation showing the relationship between mass and energy: E=MC2

Einstein was married twice, to Mileva Maric from 1903 until 1919 and to his cousin Elsa from 1919 until her death in 1936. Previously released letters suggested that his first marriage was miserable, and that he cheated on Elsa with his secretary, Betty Neumann. Prof Gutfreund said that though Einstein's marriage to Elsa was best described as one of convenience, he wrote to her constantly, describing, among other things, his experiences touring and lecturing.

"The general concept from everything we knew before was that he was a poor father, that he did not meet his responsibility to his children and that he was quite cruel to his wife," he said.

When he wanted a divorce from his first wife, Einstein gave her the ultimatum that, if she wanted to remain with him and not grant him a divorce, then he expected her to serve him three meals a day in his room but not expect any intimacy in return. "From the documents we have now, a different picture emerges," said Prof Gutfreund. "He does show empathy and compassion."

There is evidence that he diverted part of his winnings from the 1921 Nobel Prize into providing for Mileva and his children. He invested the rest in Europe and America - and lost much of it during the Great Depression.

Einstein was surprisingly candid to Elsa about his extramarital affairs. Between the mid-1920s and his emigration to the US in 1933, there were several women in his life: a Margarete, an Estella, two women called Toni and an Ethel. He shared holidays with them, read books and attended concerts.

In a letter to Elsa, he said women were chasing him, showering him with unwanted attention.

But he was aware of his weaknesses. "He was not capable of long and stable relations with a woman and he actually expressed that in a letter to the son of a friend who died," said Prof Gutfreund. Einstein wrote: "What I admire in your father is that, for his whole life, he stayed with only one woman. This is a project in which I grossly failed, twice."

"If one talks about Einstein in love, his most consistent love from beginning to end was science," said Prof Gutfreund.

Another apparent difficulty for Einstein was his relationship to his schizophrenic son, Eduard. "He refers [in previously known letters] to Eduard as maybe it would have been better if he would not have been born," said Prof Gutfreund. However, in the new letters Einstein writes of his pleasure in receiving poems, pictures and notes from him. Einstein wrote to friends: "The more refined of my sons, the one I considered really of my own nature, was seized by an incurable mental illness."

Einstein was much closer to Elsa's daughter, Margot. He wrote: "I love her [Margot] as much as if she were my own daughter, perhaps even more so, since who knows what kind of brat she would have become [had I fathered her]."

The 1,300 letters, which span from 1912 to Einstein's death in 1955, have been in storage at the Hebrew University in Jerusalem, shielded from the public in accordance with Margot's request that they be locked away for 20 years after her death. Margot died in July 1986.

Though the letters do not concern Einstein's theories, he does mention his weariness at being continually associated with his work. "Soon I'll be fed up with the relativity," he wrote to Elsa. "Even such a thing fades away when one is too involved with it."

Extracts: 'Soon I'll be fed up with the relativity'

Albert Einstein wrote to his wife Elsa almost every day and often to his stepdaughter Margot

To Elsa, from Prague, January 8 1921

My lectures here ... are already behind me. This morning quartet - very beautiful, like old times. The first violin is played by a youth of 80 years! Soon I'll be fed up with the relativity. Even such a thing fades away when one is too involved with it ...

To Margot, from Oxford. May 8 1931

(Members of Einstein's extended family were used to his involvement with two or three women, but had complained about the new additions to his harem.)

This time I'm writing you because you are the most reasonable [member of the family], and the poor mother [Elsa is] already completely meschugge. It is true that M. followed me and her chasing after me is getting out of control. But firstly I could hardly avoid it, and secondly, when I see her, I will tell her that she should vanish immediately ... Out of all the dames I am in fact attached only to Mrs L who is absolutely harmless.

To Elsa

Mrs M definitely acted according to the best Christian-Jewish ethics: 1) one should do what one enjoys and what won't harm anyone else; and 2) one should refrain from doing things one does not take delight in and which annoy another person. Because of 1) she came with me, and because of 2) she didn't tell you a word.

To Elsa from Kiel. June 11 1933

(Elsa managed the financial affairs. From the moment Einstein became famous, she recognised his handwritten manuscripts would be a source of income. This letter was written when Einstein was working on the improvement of the gyroscope compass for the Anschuetz Company. Hermann Anschuetz had provided him with an apartment where he was shielded from the public.)

I don't want to have the Warburgs bothered with my manuscript, and much less Haldane. I don't mind having it sold, but without molesting any prominent people. Thank goodness one cannot sell my skin during my lifetime ... Here there is blessed calm. No one is allowed to ... claim any rights on me. Anschuetz admires me for my abstaining from smoking, and I admire myself, too. In front of my window [are] trees and water, chirping birds. Nothing unexpected occurs, everything quiet and comfortable as if arranged for contemplative musing.