7.12 Restricted Active Space Spin-Flip (RAS-SF) and Configuration Interaction (RAS-CI)

7.12.9 Job Control Options for RASCI2

(May 16, 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 n<-99 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.