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(June 30, 2021)

At present the RASCI1 and RASCI2 implementations employ different keywords (which will be reconciled in a future version). This subsection applies to RASCI2 (even and odd electron systems, determinant-driven algorithm using the resolution of the identity approximation).

The use of the RAS-CI2 methodology is controlled by setting the CORRELATION = RASCI2 and EXCHANGE = HF. The minimum input also requires specifying the desired (non-zero) value for RAS_N_ROOTS, and the number of active occupied and virtual orbital comprising the “active” RAS2 space. RASCI2 calculations also require specification of an auxiliary basis via AUX_BASIS.

RAS_N_ROOTS

Sets the number of RAS-CI roots to be computed.

TYPE:

INTEGER

DEFAULT:

None

OPTIONS:

$n$
$n>0$ Compute $n$ RAS-CI states

RECOMMENDATION:

None. Only works with RASCI2

RAS_ACT_OCC

Sets the number of occupied orbitals to enter the RAS active space.

TYPE:

Integer

DEFAULT:

None

OPTIONS:

$n$
user defined integer

RECOMMENDATION:

None. Only works with RASCI2

RAS_ACT_VIR

Sets the number of virtual orbitals to enter the RAS active space.

TYPE:

Integer

DEFAULT:

None

OPTIONS:

$n$
user defined integer

RECOMMENDATION:

None. Only works with RASCI2.

RAS_ACT_DIFF

Sets the number of alpha vs. beta electrons.
Controls the level of excitations used in calculations.

TYPE:

Integer

DEFAULT:

None

OPTIONS:

1
odd number of electrons or cations
0
even number of electrons
-1
anions
$n$
$$ triggers RAS2-SF at DDCI level of excitations
$n=-451$ and $n=-452$ triggers restart mechanism that restores the last best guess for each state to the number of states requested

RECOMMENDATION:

Set to 0 would be appropriate for most singlet systems.
Only works with RASCI2.

Note that other *$rem* variables that can be used to control the evaluation of
RASCI2 calculations are SET_ITER for the maximum number of Davidson
iterations, and N_FROZEN_CORE and N_FROZEN_VIRTUAL to
exclude core and/or virtual orbitals from the RASCI2 calculation. Please see
the sample inputs below for more details.

RAS_CALC_SOC

Controls whether to calculate the SOC constants for RAS2 jobs only.

TYPE:

Integer/Logical

DEFAULT:

False

OPTIONS:

False
Do not perform the SOC calculation.
True
Perform the SOC calculation.

RECOMMENDATION:

This *$rem* variable is used to control the Spin-Orbit Coupling (SOC) analysis section.

Note: For SOC calculations, RASCI2 method by default computes both the
one-electron and two-electron mean-field contributions. SOC-NTO analyses can be
triggered after the SOC analysis. Two additional parameters,
MOLDEN_FORMAT and NTO_PAIRS, are needed for the NTO
analysis. Details of the two variables are provided in the following *$rem*
sections:

MOLDEN_FORMAT

Sets the output format of NTOs in RASCI2 SOC analysis to MolDen format.

TYPE:

Logical

DEFAULT:

False

OPTIONS:

True
Append MolDen input file at the end of the Q-Chem output file.

RECOMMENDATION:

Currently, SOC-NTO analysis in RASCI2 only works with MolDen. Other
visualization tools are not supported at the moment. Please see the Visualizing
Orbitals Using MolDen section for more information.

NTO_PAIRS

Contols the writing of hole/particle NTO pairs for SOC transitions calculated
within the RASCI2 SOC analysis section.

TYPE:

Integer

DEFAULT:

0

OPTIONS:

$N$
Write $N$ NTO pairs per SOC transition.

RECOMMENDATION:

If activated ($N>0$), a minimum of two NTO pairs will be printed for each
transition. Increase the value of $N$ if additional NTOs are desired. Please
see Visualization of Natural Transition Orbitals section for more information.