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

7.11.7 Job Control Options for RASCI2

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
TYPE:
       Integer
DEFAULT:
       None
OPTIONS:
       n user defined integer
RECOMMENDATION:
       Set to 1 for an odd number of electrons or a cation, -1 for an anion. Only works with RASCI2.

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.