I am trying to use the fchk produced by Q-chem to perform the NBO calculations in Gaussian (these are very specific calculations and cannot be done using Q-chem and NBO7 code). I have two questions:
(1) During the SP calculations in Q-chem (GUI=2) two fchk files are generated - *.FChk and *.0.FChk or *.1.FChk. Which one contains the converged orbitals/wavefunction?
(2) Sometimes, when i use unfchk tool from gaussian and use the resulting unformated checkpoint to perform e.g. population analysis in gaussian [ # Guess(Read,Only) ChkBasis Geom(AllCheckpoint) Pop(NBO7Read) ] it works perfectly reproducing the numbers from native Q-chem+NBO7 calculations, but sometimes there is a clear problem with the basis set transformations and I simply cannot re-use fchk from Q-chem (cubegen on such fchk files produces absurd results). Do you know if there is a way to ensure that Gaussian is using the exact orbitals/wavefunction from Q-chem calculations? Maybe i should go with external basis sets?
Hmm… I have never seen two .FChk files before. Is it possible that one comes from Q-Chem and one from NBO? (This is a wild guess.)
Regarding your discrepancies: are you sure that both programs are using a consistent choice of pure spherical harmonics vs. Cartesian functions? Defaults can be different, and differ by basis set. On the Q-Chem side this is controllable via PURECART in the $rem (see manual). Simple test is to use a basis like STO-3G that has only s and p functions (where this is not an issue) and see if results are identical in that case. It’s also true that Q-Chem and Gaussian order the d functions differently; not sure if that is accounted for in your file transfer between programs or not.
Regarding question (1) about multiple fchk files. The two files with numerical suffixes (*.0.fchk and *.1.fchk) are temporary formatted checkpoint files generated by a job with multiple inputs (separated by @@@). The total checkpoint file *.fchk is a concatenation of the two partial files. It’s strange that the partial files are not removed (they should be). Either way, you will find all the final quantities in the *.fchk files.