It is also crucial to understand the nature of any deal that might emerge.

Whatever nuclear work or research Iran may have conducted in the past, it cannot simple "unlearn" any knowledge it may have acquired.
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Iran nuclear deal

    20 January - First day of implementation of interim nuclear deal. IAEA begins verifying Iranian compliance; P5+1 and EU suspend sanctions
    18 February - Iran and P5+1 begin talks in Vienna to find a "comprehensive solution"
    19 April - Completion of dilution of Iran's 20% enriched uranium stockpile in hexafluoride form
    19 July - Expiry of six-month interim period for finding comprehensive solution. If not renewed by mutual consent, P5+1 may increase sanctions
    November - Iran and P5+1 aim to "conclude negotiating and commence implementing" the second step of any comprehensive solution

    Iran nuclear deal: Key points

Thus, it is not going to be possible to arrive at a deal where it can be said for certain that Iran will never be able to develop a nuclear weapon.

So for the major UN players, a successful deal will be one where robust answers can be provided to two key questions:

    Can the international community detect with a very high level of certainty any effort by Iran to seek a nuclear weapon in the future?
    And crucially, how far away would Iran then be from having a nuclear device? Bluntly, how long would it take for Iran to obtain a nuclear weapon?

Everything depends upon a sufficient degree of transparency in Iran's nuclear programme and a sufficient degree of warning before Iran "gets the bomb".

Diplomats close to the talks say that there are areas where the two sides are closer than might be expected and others where they are a long way apart.