Worse fluctuation method for fast Value-at-Risk estimates

We show how one can actually take advantage of the strongly non-Gaussian nature of the fluctuations of financial assets to simplify the calculation of the Value-at-Risk of complex non linear portfolios. The resulting equations are not hard to solve numerically, and should allow fast VaR and $\Delta$VaR estimates of large portfolios, where {\it by construction} the influence of rare events is taken into account reliably. Our method can be seen as a correctly probabilized `scenario' calculation (or `stress-testing').