7.10.5 Job Control for the RASCI1 Implementation‣ 7.10 Restricted Active Space Spin-Flip (RAS-SF) and Configuration Interaction (RAS-CI) ‣ Chapter 7 Open-Shell and Excited-State Methods ‣ Q-Chem 5.2 User’s Manual

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 $h o l e$ 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 ( $\times 10^{2}$ ) for the CI amplitudes to be printed.
TYPE:
       INTEGER
DEFAULT:
       10 0.1 minimum absolute amplitude
OPTIONS:
       $n$ User-defined integer, $n\geq 0$
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, $n\geq 0$
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 ( $\times 10^{3}~{}au$ ) 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- $s r$ DFT states and energies FALSE Do not perform a RASCI- $s r$ DFT 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- $s r$ DFT method.
TYPE:
       INTEGER
DEFAULT:
       400 ( $\omega=0.4$ bohr ${}^{-1}$ )
OPTIONS:
       $n$ Corresponding to $\omega=n/1000$ , in units of bohr ${}^{-1}$
RECOMMENDATION:
       None. Range-separation parameter is typical indicated by $\omega$ or $\mu$ . Only works with RASCI.

RAS_SRDFT_DAMP
       Sets damping factor ( $\alpha<1$ ) in the RAS-CI- $s r$ DFT method.
TYPE:
       REAL
DEFAULT:
       0.5 ( $\alpha=0.5$ )
OPTIONS:
       $\alpha$ Damping factor $0<\alpha<1$
RECOMMENDATION:
       Modify in case of convergence issues along the RAS-CI- $s r$ DFT 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:
       $[i,j,k\ldots]$ 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.

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7.10.5 Job Control for the RASCI1 Implementation