You are right, $DET is for ALDET CI, while $DRT is for GUGA CI and, therefore, is ignored when ALDET is used.
>2- What is the function of the NSTATE keyword in the $MCQDPT group and what is its relation to the KSTATE keyword? At first glance, it seems that NSTATE is for the inclusion of more than one state in the MCQDPT part of the calculation, but I thought that this is what KSTATE is for. Incidentally, I have performed a few MCQDPT2 calculations with either keyword (NSTATE=2 or KSTATE(1)=1,1), but the differences in energy where negligible.
What does NSTATE in $MCQDPT group mean, is still unclear for me, but KSTATE defines, which states are to be included in the effective Hamiltonian. It is advisable to include no less than 10 states if you are interested in 2 lowest roots. However, Heff cannot include more states than the active space itself has; hence, for very small active spaces (say, CASSCF(2,2)), you should include all states into Heff.
>3- And finally, what does the IROT keyword in the $MCQDPT group do? Its function unfortunately is not described in the most up-to-date Firefly documentation as well as the current GAMESS US documentation.
This question is for Alex, I believe :)
In general, it is advisable to perform CASSCF calculation in parallel on as many nodes as possible (32 cores for 1000 basis functions and up to 100000 CSFs is OK), because MCQDPT can run only on one node in multi-threading mode, while CASSCF in this mode will be very slow. Next, MOREAD the OPTIMIZED ORBITALS for MCQDPT run. MCQDPT will use settings from $DET of $DRT groups, so that the input in $MCQDPT group is reduced to INORB=1 (means use of CASSCF MOs as is) KSTATE(1)=.... EDSHFT=0.02 (reasonable value of ISA energy denominator shift).
In addition, you may use XMCQDPT (http://classic.chem.msu.su/gran/gamess/xmcqdpt.pdf;) its keywords are identical with MCQDPT. Unlike MCQDPT, XMCQDPT lacks some severe drawbacks described in http://classic.chem.msu.su/gran/gamess/mcqdpt-vs-xmcqdpt-in-O3.pdf and http://classic.chem.msu.su/gran/gamess/conic-qdpt.pdf