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

7.11.6 Job Control for the RASCI1 Implementation

At present the RASCI1 and RASCI2 implementations employ different keywords (which will be reconciled in a future version). This subsection applies to RASCI1 (even electron systems, spin adapted algorithm using exact integrals).

The use of the RAS-CI1 methodology is controlled by setting the $rem variable CORRELATION to RASCI and EXCHANGE should be set to HF. The RASCI1 implementation is only compatible with even numbers of electrons and restricted orbitals, i.e., UNRESTRICTED = FALSE.

The minimum input also requires specifying the desired (non-zero) value for RAS_ROOTS, the number of electrons in the “active” RAS2 space and the number of orbitals in RAS1 and RAS2 subspaces.

RAS_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 RASCI.

RAS_ELEC
       Sets the number of electrons in RAS2 (active electrons).
TYPE:
       INTEGER
DEFAULT:
       None
OPTIONS:
       n User-defined integer, n>0
RECOMMENDATION:
       None. Only works with RASCI.

RAS_ACT
       Sets the number of orbitals in RAS2 (active orbitals).
TYPE:
       INTEGER
DEFAULT:
       None
OPTIONS:
       n User-defined integer, n>0
RECOMMENDATION:
       None. Only works with RASCI.

RAS_OCC
       Sets the number of orbitals in RAS1
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n User-defined integer, n>0
RECOMMENDATION:
       These are the initial doubly occupied orbitals (RAS1) before including hole type of excitations. The RAS1 space starts from the lowest orbital up to RAS_OCC, i.e. no frozen orbitals option available yet. Only works with RASCI.

RAS_DO_HOLE
       Controls the presence of hole excitations in the RAS-CI wave function.
TYPE:
       LOGICAL
DEFAULT:
       TRUE
OPTIONS:
       TRUE Include hole configurations (RAS1 to RAS2 excitations) FALSE Do not include hole configurations
RECOMMENDATION:
       None. Only works with RASCI.

RAS_DO_PART
       Controls the presence of particle excitations in the RAS-CI wave function.
TYPE:
       LOGICAL
DEFAULT:
       TRUE
OPTIONS:
       TRUE Include particle configurations (RAS2 to RAS3 excitations) FALSE Do not include particle configurations
RECOMMENDATION:
       None. Only works with RASCI.

RAS_AMPL_PRINT
       Defines the absolute threshold (×102) for the CI amplitudes to be printed.
TYPE:
       INTEGER
DEFAULT:
       10 0.1 minimum absolute amplitude
OPTIONS:
       n User-defined integer, n0
RECOMMENDATION:
       None. Only works with RASCI.

RAS_ACT_ORB
       Sets the user-selected active orbitals (RAS2 orbitals).
TYPE:
       INTEGER ARRAY
DEFAULT:
       From RAS_OCC+1 to RAS_OCC+RAS_ACT
OPTIONS:
       [i,j,k] The number of orbitals must be equal to the RAS_ACT variable
RECOMMENDATION:
       None. Only works with RASCI.

RAS_NATORB
       Controls the computation of the natural orbital occupancies.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Compute natural orbital occupancies for all states FALSE Do not compute natural orbital occupancies
RECOMMENDATION:
       None. Only works with RASCI.

RAS_NATORB_STATE
       Saves the natural orbitals of the i-th RASCI computed state into the .fchk file.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       i Saves the natural orbitals for the i-th state
RECOMMENDATION:
       None. Only works with RASCI and if GUI=2.

RAS_GUESS_CS
       Controls the number of closed shell guess configurations in RAS-CI.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n Imposes to start with n closed shell guesses
RECOMMENDATION:
       Only relevant for the computation of singlet states. Only works with RASCI.

RAS_SPIN_MULT
       Specifies the spin multiplicity of the roots to be computed
TYPE:
       INTEGER
DEFAULT:
       1 Singlet states
OPTIONS:
       0 Compute any spin multiplicity 2n+1 User-defined integer, n0
RECOMMENDATION:
       Only for RASCI, which at present only allows for the computation of systems with an even number of electrons. Thus, RAS_SPIN_MULT only can take odd values.

RAS_PT2
       Perform second-order perturbative correction to RAS-CI energy
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Compute RASCI(2) energy corrections FALSE Do not compute RASCI(2) energy corrections
RECOMMENDATION:
       None. Only works with RASCI.

RAS_PT2_PARTITION
       Specifies the partitioning scheme in RASCI(2)
TYPE:
       INTEGER
DEFAULT:
       1 Davidson-Kapuy (DK) partitioning
OPTIONS:
       2 Epstein-Nesbet (EN) partitioning 0 Do both DK and EN partitionings
RECOMMENDATION:
       Only for RASCI if RAS_PT2 is set to true.

RAS_PT2_VSHIFT
       Defines the energy level shift (×103au) in RASCI(2)
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n User-defined integer
RECOMMENDATION:
       Only for RASCI if RAS_PT2 is set to true.

RAS_SRDFT
       Perform short-range density functional RAS-CI calculation
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Compute RASCI-srDFT states and energies FALSE Do not perform a RASCI-srDFT calculation
RECOMMENDATION:
       None. Only works with RASCI. RAS_SRDFT_EXC and RAS_SRDFT_COR need to be set.

RAS_SRDFT_EXC
       Define short-range exchange functional
TYPE:
       STRING
DEFAULT:
       No default
OPTIONS:
       NAME Use RAS_SRDFT_EXC = NAME, where NAME is one of the short-range exchange functionals listed in Section 5.3.2
RECOMMENDATION:
       None. Only works with RASCI.

RAS_SRDFT_COR
       Define short-range correlation functional
TYPE:
       STRING
DEFAULT:
       No default
OPTIONS:
       NAME Use RAS_SRDFT_COR = NAME, where NAME is one of the short-range correlation functionals listed in Section 5.3.3
RECOMMENDATION:
       None. Only works with RASCI.

RAS_OMEGA
       Sets the Coulomb range-separation parameter within the RAS-CI-srDFT method.
TYPE:
       INTEGER
DEFAULT:
       400 (ω=0.4 bohr-1)
OPTIONS:
       n Corresponding to ω=n/1000, in units of bohr-1
RECOMMENDATION:
       None. Range-separation parameter is typical indicated by ω or μ. Only works with RASCI.

RAS_SRDFT_DAMP
       Sets damping factor (α<1) in the RAS-CI-srDFT method.
TYPE:
       REAL
DEFAULT:
       0.5 (α=0.5)
OPTIONS:
       α Damping factor 0<α<1
RECOMMENDATION:
       Modify in case of convergence issues along the RAS-CI-srDFT iterations. Only works with RASCI

RAS_SRDFT_SA_ROOTS
       Sets the list of roots used to build the state averaged reference density in RAS-CI-srDFT.
TYPE:
       INTEGER ARRAY
DEFAULT:
       All computed states
OPTIONS:
       [i,j,k] List of states.
RECOMMENDATION:
       None. Only works with RASCI

RAS_NFRAG
       If n>0 activates the excitation analysis in RASCI
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       n Number of fragments to be considered
RECOMMENDATION:
       Only for RASCI. The printed information level is controlled by RAS_PRINT

RAS_NFRAG_ATOMS
       Sets the number of atoms in each fragment.
TYPE:
       INTEGER ARRAY
DEFAULT:
       None
OPTIONS:
       [i,j,k] The sum of the numbers must be equal to the total number of atoms in the systems
RECOMMENDATION:
       None. Only works with RASCI.

RAS_FRAG_SETS
       Sets the number of atoms in each fragment.
TYPE:
       INTEGER ARRAY
DEFAULT:
       [NOcc,NAct,NVir] Number of orbitals within RAS1, RAS2 and RAS3 spaces
OPTIONS:
       [i,j,k] Defines sets of canonical MOs to be localized into n fragments
RECOMMENDATION:
       Setting within RAS1, RAS2 and RAS3 spaces alleviates the computational cost of the localization procedure. It might also result in improved fragment orbitals. Only works with RASCI.