And in the line that fails the code checks if N*(N+1)/2, where N is the number of orbitals is greater than IOFF_MAX and if so it gives the error above. It makes sense since 400**2/2 = 80000.
My question is can I just increase the IOFF_MAX value to some arbitrary number and recompile the code, or the number need to have some special value?
Also do I need to adjust any other parameters together with changing IOFF_MAX?
Increasing IOFF_MAX will run into problems as its defined as an int. Napkin math says you can increase it up to 46,339 without issue (2^15.5 - 1) since its signed. Although, there is a good chance that I may have missed something there.
It looks like the relevant IOFF_MAX is actually defined in the src/detci/globals.h file (we are slowly deprecating globals!). A few comments moved immediately to mind:
I dont think we use IOFF in the detci program anymore. Its more efficient to compute these on the fly these days.
The DETCI program is really a FCI code so its CISD abilities are quite slow.
On that note if you get around the IOFF issue you will hit a complaint that CI is limited to 255 orbitals.
Any particular reason you want to use the DETCI module over FNOCC or the like which has orbital based representations of CISD?
I dont think we store anything bitwise so you may be able to do a sed in the detci folder that replaces all int with long int. There may be a few conversion conflicts that you have to manually fix. @sherrill any comments on that?
Apologies, you have found the one FCI edge case that we didnt really optimize for