Help with FCIMQC Setup for Cyclo[16]carbon Using NECI — Different Results from SCF vs CASSCF Orbitals

[Marcovit94]

Hi there,

I'm a beginner working with QMC and trying to set up a FCIMQC calculation using NECI for a C₁₆ cyclocarbon, focusing on the full π-electron active space (32 electrons in 16 orbitals). I've generated two different FCIDUMP files from OpenMolcas and get quite different results when using them in NECI.

Approach 1: SCF orbitals

&GATEWAY
 COORD = opted.xyz
 title = test
 group = z
 basis = cc-pvdz

&SEWARD
&SCF
&RASSCF
 Spin = 1
 CIRoot = 1 1 1
 Frozen = 16 0
 Inactive = 16 0
 RAS1 = 0 0
 RAS2 = 16 16
 RAS3 = 0 0
 NActEl = 32 0 0
 DMPO

This setup produces a cleanly converging NECI run with a total energy of: E = -605.72129172258 Ha

Approach 2: CASSCF orbitals

&GATEWAY
 COORD = opted.xyz
 title = test
 group = z
 basis = cc-pvdz

&SEWARD
&SCF
&RASSCF
 Spin = 1
 CIRoot = 1 1 1
 Frozen = 16 0
 Inactive = 21 5
 RAS1 = 0 0
 RAS2 = 6 6
 RAS3 = 0 0
 NActEl = 12 0 0

&RASSCF
 Spin = 1
 CIRoot = 1 1 1
 Frozen = 16 0
 Inactive = 16 0
 RAS1 = 0 0
 RAS2 = 16 16
 RAS3 = 0 0
 NActEl = 32 0 0
 DMPO

This was an attempt to improve the orbital quality by first doing a smaller π-π* CASSCF (12,12) and then using those orbitals for the full space. However, with this FCIDUMP, NECI gives worse and unstable results. The best I got so far is: E = -604.54366646533 Ha This is more than 1.2 Ha higher than the previous result, and I'm not sure why. I have also tried (4,4) and (8,8) active spaces and got the same result.

NECI Input

Here's the NECI input I use for both:

system read
    nonuniformrandexcits pchb delocalised
    electrons 32
endsys

calc
    methods
        method vertex fcimc
    endmethods

    load-balance-interval 1000
    nmcyc              5000
    totalwalkers       10000000

    diagshift           0.0
    startsinglepart      10
    truncinitiator
    addtoinitiator        3
    allrealcoeff
    no-changeref
    realspawncutoff     0.7
    avgrowthrate
    shiftdamp           0.1
    stepsshift            1
    memoryfacpart       5.0
    memoryfacspawn     10.0
    tau-values \
        start user-defined 0.02 \
        max 0.05
    tau-search \
        algorithm conventional \
        stop-condition n-opts 5 \
        maxwalkerbloom 1
endcalc

end

i also tried nonuniformrandexcits pchb localised but have the same exact issue.

My question(s):

1. Why might the CASSCF-generated orbitals yield significantly worse FCIMC performance or higher energy than plain SCF orbitals?
2. Is there something I should change in the NECI input to better accommodate casscf orbitals or improve convergence?
3. Is it common that orbitals from a smaller CASSCF don't help, or possibly hurt, for larger full-valence FCI-type runs?

I’d really appreciate any insights — especially as I’m just starting out with QMC and NECI. Thanks in advance!

I've attached the FCIMCStats output files from both NECI runs for comparison.

FCIMCStats_SCF.txt

FCIMCStats_CASSCF.txt

Best, Marco