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
RAS_N_ROOTS
      
Sets the number of RAS-CI roots to be computed.
TYPE:
      
INTEGER
DEFAULT:
      
None
OPTIONS:
      
 Compute  RAS-CI states
RECOMMENDATION:
      
None. Only works with RASCI2
RAS_ACT_OCC
RAS_ACT_OCC
      
Sets the number of occupied orbitals to enter the RAS active space.
TYPE:
      
Integer
DEFAULT:
      
None
OPTIONS:
      
user defined integer
RECOMMENDATION:
      
None. Only works with RASCI2
RAS_ACT_VIR
RAS_ACT_VIR
      
Sets the number of virtual orbitals to enter the RAS active space.
TYPE:
      
Integer
DEFAULT:
      
None
OPTIONS:
      
user defined integer
RECOMMENDATION:
      
None. Only works with RASCI2.
RAS_ACT_DIFF
RAS_ACT_DIFF
      
Sets the number of  versus  electrons and therefore 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
anions
 triggers RAS2-SF at DDCI level of excitations
 and  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 MAX_CIS_CYCLES 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
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
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
NTO_PAIRS
      
Controls the writing of hole/particle NTO pairs for SOC transitions calculated
within the RASCI2 SOC analysis section.
TYPE:
      
Integer
DEFAULT:
      
0
OPTIONS:
      
Write  NTO pairs per SOC transition.
RECOMMENDATION:
      
If activated (), a minimum of two NTO pairs will be printed for each
transition. Increase the value of  if additional NTOs are desired.
See Section 7.14.3 for information on visualizing NTOs.
RAS_PCM_INIT
RAS_PCM_INIT
      
Triggers state-specific PCM solvation within RAS-SF.
TYPE:
      
LOGICAL
DEFAULT:
      
FALSE
no solvation (vacuum boundary conditions)
OPTIONS:
      
TRUE
include solvation
RECOMMENDATION:
      
This triggers the nonequilibrium version of state-specific solvation. Additional keywords are necessary
for the equilibrium version.