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 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
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 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
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
Contols 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. Please
see Visualization of Natural Transition Orbitals section for more information.