Is it correct to sort the Qur'an according to its subjects?

This is a matter of ijtihad. If it is done in good faith, for a better understanding and for the researched thing to be found, those who are competent in this subject can make ijtihad. Because there is no falsification on the existing Qur'an. It will only be seen as a tafsir arranged according to its subjects. However, our preference would be that any application on its current structure would be a very risky business and should not be done.