>Yes, the LIIC path was first constructed between cis-HBDI and CI, and then constructed between CI and trans-HBDI, thus the provided curve consists of two parts. However, the MCSCF curve is smooth and the CI point has the smallest delta energy between S0 and S1,
Note if there were a cusp on MCSCF LIIC curves it would be
effectively masked as you are at MCSCF CI point where there
is already a cusp.
> whereas this is not the case for XMC-QDPT2 curve, where the delta energy of CI point is larger than that of the neighbour points. This is unusually and indicates that the optimized CI point by MCSCF is not the "real" CI in XMC-QDPT2. So, what is the source for this difference between MCSCF and XMC-QDPT2? Is it because lack of dynamic electron correlation effects in MCSCF calculations?
Yes, the lack of dynamic electron correlation can cause this.
You can find an example in this paper:
>On Thu May 9 '13 3:06am, Alex Granovsky wrote
>>Dear Panwang Zhou,
>>I do not think it is discontinuous, it is just not smooth.
>>It seems it simply consists of two parts and I think
>>there is nothing wrong with these results.
>>As far as I understand there are two independent
>>segments of LIIC path rather than a continuous
>>interpolation between three geometries. In this
>>situation I would expect a non-zero angle between
>>two parts of the path i.e. the path is not a smooth
>>curve but has a break at CI point. As path is not a
>>smooth curve, it is natural for energy profiles
>>to have a break along such a path.
>>On Wed May 8 '13 11:50am, Panwang Zhou wrote
>>>Recently, I performed a MCSCF(16,14) calculation then following a XMC-QDPT2 calculation on the constructed LIIC curve among the optimized cis-HBDI, CI and trans-HBDI structures, the obtained MCSCF curve is smooth and continuous whereas the obtained XMC-QDPT2 curve is discontinuous at the CI point (see attachment). I'm not clear why the XMC-QDPT2 curve is discontinuous and how to resolve this problem?
>>>The following is the part of input file:
>>> $CONTRL SCFTYP=MCSCF RUNTYP=ENERGY EXETYP=RUN MAXIT=50 ICHARG=-1
>>> MULT=1 FSTINT=.T. GENCON=.T. INTTYP=HONDO NOSYM=1
>>> ICUT=11 ITOL=30 WIDE=.T. MPLEVL=2 $END
>>> $SYSTEM MWORDS=480 TIMLIM=60000.0 KDIAG=0 NOJAC=100 $END
>>> $SYSTEM MKLNP=1 NP=12 $END
>>> $SMP SMPPAR=.T. HTTNP=1 $END
>>> $SCF DIRSCF=.T. FDIFF=.F. NCONV=8 $END
>>> $P2P P2P=.T. DLB=.T. $END
>>> $TRANS MPTRAN=2 DIRTRF=.T. AOINTS=DIST ALTPAR=.T. MODE=112 $END
>>> $MCSCF CISTEP=ALDET FULLNR=.F. SOSCF=.T. MAXIT=100 $END
>>> $MCSCF IFORB=.T. $END
>>> $DET NCORE=49 NACT=14 NELS=16 NSTATE=6 WSTATE(1)=1,1 DISTCI=12 $END
>>> $XMCQDPT NSTATE=2 EDSHFT=0.02 THRGEN=1D-12 MXBASE=90 $END
>>> $XMCQDPT HALLOC=.T. $END
>>> $XMCQDPT IFORB(1)=-1,1,1 WSTATE(1)=1,1,-0 AVECOE(1)=1,1,-0 $END
>>> $BASIS GBASIS=N31 NGAUSS=6 NDFUNC=1 NPFUNC=1 DIFFSP=.TRUE. $END
>>> $GUESS GUESS=MOREAD NORB=359 $END
>>> $MCQFIT $END
>>>I have tried to increas the NSTATE in XMCQDPT group, however, the required memory is larger than 480 MWORDS, thus the calculation failed.