Can someone explain to me please what does the PURECART command do exactly? Couldn’t find an answer in the manual.
Thank you in advance
It is in the manual, as you can see. Use of Cartesian functions generally garners little additional variational flexibility (e.g., the 6 cartesian d functions are the five L=2 spherical harmonics plus an s function that you probably didn’t need), but on the other hand the integrals are evaluated in cartesian functions for for DFT calculations there’s not much downside to using them and you can probably stick with the defaults. For correlated wave functions that scale badly with respect to number of virtuals, there is more of an argument to use pure spherical harmonics as it is fewer basis functions. There are a few features here and there that only work with Cartesian functions (PURECART=2222).
I would also add that the default for most standard bases (e.g., all cc-pVXZ series) is to use pure angular momentum (5d, 7f, etc). The notable exception is 6-31*, which uses 6 d-functions. It is important to know that in some packages, the defaults are set differently. That is why one needs to specify in the papers whether pure angular momentum or Cartesian polarization functions were used. This whole issue is nicely explained in Szabo.
Thank you so much for the explanation