# 6.20.2 Job Control for iFCI

(June 30, 2021)

IFCI_TUPLES
Level of $n$-body expansion to solve. Note that $n>2$ can be computationally costly.
TYPE:
INTEGER
DEFAULT:
Must be set.
OPTIONS:
1 $n=1$ 2 $n=2$ 3 $n=3$ 4 $n=4$
RECOMMENDATION:
Use $n=2$ for initial system analysis, $n>2$ for higher accuracy.

IFCI_READ
Restarts iFCI with existing TUPLES_1E_DATA file, if it exists.
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
0 Start from scratch 1 Restart from previous file
RECOMMENDATION:
Use 0 if no previous run files exist. Use 1 if intending to restart from previous data.

IFCI_TRIPLETS
Set state to solve.
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
0 Singlet 1 Triplet 2 Quintet
RECOMMENDATION:
None

IFCI_ZETA
Convergence for each iFCI increment. Note that the format is $\zeta=$ IFCI_ZETA$/10$.
TYPE:
INTEGER
DEFAULT:
55
OPTIONS:
45 Loose 55 Moderate 65 Tight 75 Tighter 85 Quite tight 95 Maximum
RECOMMENDATION:
Use 65 and increase to 75 to check convergence.

HBCI_EPS1
Determines dimension of HBCI space.
TYPE:
INTEGER
DEFAULT:
1000
OPTIONS:
$N$ HBCI $\varepsilon_{1}$ in $\mu$ E${}_{\mathrm{h}}$
RECOMMENDATION:
Use default or 500 for tighter convergence.

IFCI_TUPLE_THRESH
Collapse near-degenerate geminals within threshold into one body.
TYPE:
INTEGER
DEFAULT:
2500
OPTIONS:
$n$ in $\mu$E${}_{\mathrm{h}}$
RECOMMENDATION:
Use default unless there are sets of highly correlating occupied orbitals.

IFCI_TRIPLES_SCREEN
Cutoff ($\mathcal{C}_{3}$) for determining if a 3-body term is significant.
TYPE:
INTEGER
DEFAULT:
1000
OPTIONS:
$n$ where $\mathcal{C}_{3}=10^{-\zeta}\times n$ in E${}_{\mathrm{h}}$
RECOMMENDATION:
Use the default unless looser (higher $n$) or tighter (lower $n$) consideration of triads for a given system is desired. Setting to 0 computes all triads (costly).

IFCI_QUAD_SCREEN
Cutoff ($\mathcal{C}_{4}$) for determining if a 4-body term is significant.
TYPE:
INTEGER
DEFAULT:
IFCI_TRIPLES_SCREEN
OPTIONS:
$n$ where $\mathcal{C}_{4}=10^{-\zeta}\times n$ in E${}_{\mathrm{h}}$
RECOMMENDATION:
Same as IFCI_TRIPLES_SCREEN but note that $4$-body terms are significantly more costly.

IFCI_STATE_ADD
Adds additional states to HBCI solver when there is degeneracy amongst states.
TYPE:
INTEGER
DEFAULT:
10
OPTIONS:
$n$ Add states within $n$ mE${}_{\mathrm{h}}$
RECOMMENDATION:
Use default unless it is known that degenerate states are present.

IFCI_PRINT
Larger number gives more output.
TYPE:
INTEGER
DEFAULT:
2
OPTIONS:
1 Minimal output 2 Readable output 3 Extra output 4 Excessive output 5+ Bug testing output
RECOMMENDATION:
2 is recommended, 1-3 is appropriate, larger than 4 is unnecessary (consider yourself warned).

IFCI_NO_THRESH
Equivalent to HBCI $\varepsilon_{1}$ for increment-specific NO generation step.
TYPE:
INTEGER
DEFAULT:
1000
OPTIONS:
$n$ in $\mu$E${}_{\mathrm{h}}$
RECOMMENDATION:
Set to equal HBCI_EPS1.

IFCI_REF_ITER
Use HF or PP reference density.
TYPE:
INTEGER
DEFAULT:
1
OPTIONS:
0 HF 1 PP
RECOMMENDATION:
Use 0.

IFCI_OCC
Specifies the number of active occupied orbitals.
TYPE:
INTEGER
DEFAULT:
Full valence.
OPTIONS:
$n$ Include $n$ orbitals in the active space $-1$ Full valence
RECOMMENDATION:
Use full valence active space.