Example 1 is supposed to be an ISC, but MCSCF is done with GUGA, and in DRT group only NDOC and NVAL are specified, no NALP orbitals. So although MULT is nowhere specified in this input, Firefly calculates two singlet states.
Example 2 uses ALDET, and thus singlets and triplets are calculated simultaneously. Also, MULT is not specified in $CONTRL. Indeed the two lowest roots are one singlet and one triplet. However, using only these two roots, Firefly stops with the error message that it could find only one state with S=0. So it implicitely assumes crossing between two singlets.
So what do I have to do to get the crossing between S0 and T1?
Here is the relevant part of the input:
$SYSTEM TIMLIM=1420 MWORDS=200 NOJAC=1 KDIAG=0 $END
$P2P p2p=.t. dlb=.t. $END
$CONTRL SCFTYP=MCSCF RUNTYP=OPTIMIZE INTTYP=HONDO ICUT=11 D5=.T. NZVAR=1
EXETYP=RUN COORD=UNIQUE NZVAR=1 D5=.T. $END
$BASIS GBASIS=cc-pvdz extfil=.t. $END
$MOORTH NOSTF=1 NOZERO=1 TOLE=0 TOLZ=0 SYMS=1 SYMDEN=1 SYMVEC=1 $END
$MCSCF CISTEP=ALDET SOSCF=.T. FOCAS=.F. MAXIT=100 ACURCY=1D-6 ISTATE=1 $END
$DET GROUP=C1 NCORE=63 NACT=12 NELS=12 NSTATE=2 WSTATE(1)=1,1
ITERMX=1000 CVGTOL=1D-6 $END
$TRANS CUTTRF=1D-12 DIRTRF=.T. $END
$MCAVER JSTATE=2 CONIC=2 $END
! $TRACK TOL=1.4 FREEZE=.T. STICKY=.F. UPDATE=.T. RESET=.F. DELCIV=.F. $END
$STATPT NSTEP=100 OPTTOL=1.0D-4 METHOD=CONIC $END
$ZMAT DLC=.T. AUTO=.T. $END
$GUESS GUESS=MOREAD NORB=75 $END
I commented $TRACK out and removed NTRACK=2 in $MCSCF, if I leave it in, Firefly stops with the error message:
Fatal: not enough CI roots to track!
ADDRESS 0x00C47D1E HAS INITIATED PROGRAM ABORT BECAUSE OF FATAL ERROR(S)
So apparently it is also here looking for two singlet roots. With the input above, the error message is
***** ERROR *****
THIS RUN FOUND 1 CI EIGENVECTORS WITH S= 0.00,
BUT YOU REQUESTED STATE AVERAGING OF 2 ROOTS.
PLEASE EXAMINE YOUR CHOICE OF -NSTATE- INPUT DATA.
Fatal error in DETDM2, error code = -2
If I set NSTATE=4 as in the example, MCSCF does not converge. Roots 3 and 4 are very close, but far away from roots 1 and 2, so I really do not need them.