This is briefly documented here:
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. ...
On Fri Oct 4 '13 9:38pm, Antonio Carlos Borin wrote
>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.
> SCFTYP=MCSCF RUNTYP=optimize inttyp=hondo icut=12 d5=.true. nzvar=1
> $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
> CISTEP=GUGA soscf=.t. ISTATE=2
> acurcy=3d-8 ENGTOL=1.0d-12 maxit=150
> $mcaver target=pure jstate=3 conic=2 multiw=.t. $end
> $statpt method=conic nstep=200 $end
> $conic shift0=1d-3 $end