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Alex Granovsky
gran@classic.chem.msu.su

This is briefly documented here:

http://classic.chem.msu.su/gran/gamess/Locating_Conical_Intersections_and_Interstate_Crossings_with_Firefly.html

Some extras are:

   The $conic input group

               This group controls the detailed behavior of the 
$statpt method=CONIC Lagrange multiplier based CIs/ISCs optimizer 
called CONIC. In addition to the parameters described below, CONIC is 
also controlled by the parameters of the $statpt group.

               shift                     The final target value of 
Δ = Ej - Ei The default value is 0.0001 Hartree. For true CIs, it is 
almost impossible to reliably converge calculations to values of Δ 
less than ca. 0.0001 Hartree. For ISCs, it is possible to use tighter 
convergence criteria, up to shift=1.0d-7 or 1.0d-8. The difference in 
convergence behavior between true CIs and ISCs is related to non-zero 
non-adiabatic coupling between quasi-degenerate states in the case of 
CIs and the absence of this coupling in the case of ISCs.


               shift0                   The intermediate target value 
of Δ = Ej - Ei The default value is 0.0001 Hartree. For CIs, the 
recommended value is 0.001 Hartree. For ISCs, the recommendation is to 
always set shift0 to the same value as shift. The CONIC optimizer 
first tries to converge CIs/ISCs to Δ values equal to or below of 
shift0 using loosened values of various convergence thresholds. 
During this process (which is internally called "phase 1"), the CONIC 
optimizer gathers information on the behavior of the particular system 
of interest. Upon achieving the initial convergence, the optimizer 
switches to the original values of various convergence thresholds and 
continue optimization trying to achieve tighter convergence with the 
value of Δ equal to or below of shift. This is internally called 
"phase 2". Here, the previously gathered information is used to 
achieve the final convergence. In particular, the first few 
optimization steps of phase 2 attempt to rapidly decrease Δ using 
directed steps along the "most significant" modes while the rest of 
the steps take care of the final relaxation of the "less significant" 
modes. 

...

Kind regards,
Alex Granovsky


On Fri Oct 4 '13 9:38pm, Antonio Carlos Borin wrote
---------------------------------------------------
>Dear users,

>I´d like to better understand the meaning of "shift0=1d-3" in the $conic section (see input below).

>It seems to control the energy difference between the 2 electronic states involved in the "conical intersection". Is that right?

>I could not find it in the manual; sorry if i´m wrong.

>Best
>Antonio

>$CONTRL
>   SCFTYP=MCSCF RUNTYP=optimize inttyp=hondo icut=12 d5=.true. nzvar=1
> $END
> $SYSTEM TIMLIM=9999999 MWORDS=100 $END
> $moorth nostf=1 nozero=1 tole=0 tolz=0 $end
> $BASIS gbasis=ano-vdzp extfil=.t. $END
> $GUESS GUESS=MOREAD NORB=140 $END
> $trans altpar=.t. mptran=2 dirtrf=.t. mode=112 $end
> $DRT NMCC=22 NDOC=7 NVAL=3 GROUP=C1 FORS=.t. $END
> $GUGDIA NSTATE=4 ITERMX=100 cvgtol=1d-8 memmax=999999 $end
> $GUGDM2 cutoff=1d-15 WSTATE(1)=1,1,1,1 $end
> $GUGEM  CUTOFF=1.0D-20 $END
> $MCSCF
>    CISTEP=GUGA soscf=.t. ISTATE=2
>    acurcy=3d-8 ENGTOL=1.0d-12 maxit=150
> $END
> $mcaver target=pure jstate=3 conic=2 multiw=.t. $end
> $statpt method=conic nstep=200 $end
> $conic shift0=1d-3 $end
>
>
>