Interesting but scary scenario to speculate on. Many people/organizations/countries have for sure tried to make the kind of model you ask for. Take this with a grain of salt, I felt inspired to write something ironic.
My guess: If it is a small and limited scale nuclear war (if there can be such a thing) every equity except defense contractors/military industry will fall through the floor in the short term. If the war is small in scale, but proceeds for some time, the leading/winning side will not take as much of a hit as the losing side. In fact if one side is winning, equities there will probably rise (see below). At the start of World War 2, Japan increased production on all fronts, but you can guess how much of value was destroyed/spent at the end of the war. The best time to buy would probably be after the war, since prices pretty much can’t get lower than that. In addition to that, there will probably be a great rebuilding after a war, with a lot of relief and stimulus flowing into the country.
NB: Judging by the events of Brexit, the election, and recent ICBM launches — the markets may drop by a percent or so, only to regain it in the late afternoon.
Another thing that complicates this is that: In a war, governments may freeze the stock market, and heavily manipulate their currency, bonds, inflation etc. (they will want to print money to fund the war and freeze money so that it stays inside the country and avoid panic).
Historical examples: http://www.investmentoffice.com/io/Investment_Thoughts/Markets_in_History/Stock_Markets_during_the_second_World_War.php
If it’s an actual global nuclear war: You better have a pen-and-paper model ready, on how to make the gold bars, medicine, ammunition and food rations in your basement last during for the duration of the nuclear winter. The only stop loss is your own or your family’s potential death. While the equity market almost ceases to exist, SpaceX will now compromise 98% of the SP500 index (Swedish-based Fingerprint Cards being the remaining 2 %, since their shareholders refused to sell), with great margins on tickets to Mars. The step from here is to get in early on Marsian-based equities, since they will be in an embryo-stage before growing exponentially in value. For backtesting, look up BRIC indices. Since the memory of Earth’s demise will be too hard too bear, a company like Apple will have to rename themselves to some kind of vegetable that grows on Mars. At this point, it would be wise to short any nuclear power stocks (they will be unpopular).
For a while, there will be a boom in blockchain and/or gold based currency. Sooner or later a new central bank is created and the Federal Reserve of Mars will be printing our new fiat currency — Mars-dollars, which unlike it’s predecessors the goldbased dollar and the oil based petro-dollar, will be based on the value of oxygen. A new generation of the Al-Saud family discover a way to refine solidifed material into breatheable oxygen… etc. To conclude, humanity will not change much. The question is if AI will be trusted to take care of the nuclear launch code after an accident of this magnitude, or if their will still be distrust between the organic and not-so-organic. No matter the answer, the event will repeat itself, and so begins a new cycle.
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