6.19 Complete Active Space Methods

6.19.3 CAS-CI and CASSCF Job Control

(May 16, 2021)

CAS_METHOD
       Indicates whether orbital optimization is requested.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Not running a CAS calculation 1 CAS-CI (no orbital optimization) 2 CASSCF (orbital optimization)
RECOMMENDATION:
       Use 2 for best accuracy, but such computations may become infeasible for large active spaces.

CAS_M_S
       The number of unpaired electrons desired in the CAS wavefunction.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       N for a wavefunction with N unpaired electrons
RECOMMENDATION:
      

CAS_N_ELEC
       Specifies the number of active electrons.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       N include N electrons in the active space -1 include all electrons in the active space
RECOMMENDATION:
       Use the smallest active space possible for the given system.

CAS_N_ORB
       Specifies the number of active orbitals.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       N include N orbitals in the active space -1 include all orbitals in the active space
RECOMMENDATION:
       Use the smallest active space possible for the given system.

CAS_N_ROOTS
       Specifies the number of electronic states to determine.
TYPE:
       INTEGER
DEFAULT:
       1
OPTIONS:
       N solve for N roots of the Hamiltonian
RECOMMENDATION:
      

CAS_THRESH
       Specifies the threshold for matrix elements to be included in the CAS Hamiltonian.
TYPE:
       INTEGER
DEFAULT:
       12
OPTIONS:
       N for a threshold of 10-N
RECOMMENDATION:
      

CAS_SAVE_NAT_ORBS
       Save the CAS natural orbitals in place of the reference orbitals.
TYPE:
       BOOLEAN
DEFAULT:
       FALSE
OPTIONS:
       TRUE overwrite the reference orbitals with CAS natural orbitals FALSE do not save the CAS natural orbitals
RECOMMENDATION:
      

MAX_CASSCF_CYCLES
       Maximum number of orbital optimization cycles for CASSCF.
TYPE:
       INTEGER
DEFAULT:
       50
OPTIONS:
       N set maximum number of optimization cycles to N
RECOMMENDATION:
      

CAS_USE_RI
       Indicates whether the resolution of the identity approximation should be used.
TYPE:
       BOOLEAN
DEFAULT:
       FALSE
OPTIONS:
       FALSE Compute 2-electron integrals analytically TRUE Use the RI approximation for 2-electron integrals
RECOMMENDATION:
       Analytic integrals are more accurate, RI integrals are faster

CAS_DAVIDSON_TOL
       Specifies the tolerance for the Davidson solver used in CAS.
TYPE:
       INTEGER
DEFAULT:
       5
OPTIONS:
       N for a threshold of 10-N
RECOMMENDATION:
       The default should be suitable in most cases

CAS_DAVIDSON_MAXVECTORS
       Specifies the maximum number of vectors to augment the Davidson search space in CAS.
TYPE:
       INTEGER
DEFAULT:
       10
OPTIONS:
       N sets the maximum Davidson subspace size to N+CAS_N_ROOTS
RECOMMENDATION:
       The default should be suitable in most cases

CAS_SOLVER
       Specifies the solver to be used for the active space.
TYPE:
       INTEGER
DEFAULT:
       1
OPTIONS:
       1 CAS-CI/CASSCF 2 ASCI (see Section 6.21) 3 Truncated CI (CIS, CISD, CISDT, etc.)
RECOMMENDATION:
      

TRUNC_CI_LEVEL
       Specifies the order of truncated CI to be used in the active space.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Do not carry out truncated CI 1 CIS 2 CISD 3 CISDT 4 CISDTQ etc.
RECOMMENDATION:
      

Example 6.43  CASCI(6,14) calculation for the ground state of N2.

$molecule
   0 1
   N   0.0   0.0   0.0
   N   0.0   0.0   1.8
$end

$rem
   EXCHANGE          hf
   BASIS             cc-pvtz
   CAS_METHOD        1    !1 for CAS-CI, 2 for CASSCF
   CAS_M_S           0    !M_s value*2
   ASCI_DIAG         2    !Arma Sparse=0, Davidson=1, Eigen Sparse=2
   CAS_N_ELEC        6    !N_elec
   CAS_N_ORB         14   !N_orb
   CAS_N_ROOTS       1    !N_roots
   CAS_SOLVER        1    !2=ASCI, 1=Olsen, 0=naive
   THRESH            14
   MAX_SCF_CYCLES    400
   SCF_CONVERGENCE   6
   MEM_TOTAL         4000
   MEM_STATIC        1000
   SYMMETRY          false
   SYM_IGNORE        true
   SCF_ALGORITHM     diis_gdm
$end

View output

Example 6.44  CASSCF(6,6) calculation for the ground state of N2.

$molecule
   0 1
   N   0.0   0.0   0.0
   N   0.0   0.0   1.8
$end

$rem
   EXCHANGE          hf
   BASIS             cc-pvtz
   CAS_METHOD        2    !1 for CAS-CI, 2 for CASSCF
   CAS_M_S           0    !M_s value*2
   ASCI_DIAG         2    !Arma Sparse=0, Davidson=1, Eigen Sparse=2
   CAS_N_ELEC        6    !N_elec
   CAS_N_ORB         6    !N_orb
   CAS_N_ROOTS       1    !N_roots
   THRESH            14
   MAX_SCF_CYCLES    400
   SCF_CONVERGENCE   6
   MEM_TOTAL         4000
   MEM_STATIC        1000
   SYMMETRY          false
   SYM_IGNORE        true
   SCF_ALGORITHM     diis_gdm
$end

View output

Example 6.45  Geometry optimization of the ground state of N2 at the CASSCF(6,6)/cc-pVTZ level of theory.

$molecule
   0 1
   N   0.0   0.0   0.0
   N   0.0   0.0   1.3
$end

$rem
   JOBTYPE             opt
   EXCHANGE            hf
   BASIS               cc-pvtz
   CAS_METHOD          2    !1 for CAS-CI, 2 for CASSCF
   CAS_M_S             0    !M_s value*2
   ASCI_DIAG           2    !Arma Sparse=0, Davidson=1, Eigen Sparse=2
   CAS_N_ELEC          6    !N_elec
   CAS_N_ORB           6    !N_orb
   CAS_N_ROOTS         1    !N_roots
   CAS_SAVE_NAT_ORBS   true !overwrite MOs with CAS natural orbs
   THRESH              14
   MAX_SCF_CYCLES      400
   SCF_CONVERGENCE     6
   MEM_TOTAL           4000
   MEM_STATIC          1000
   SYMMETRY            false
   SYM_IGNORE          true
   SCF_ALGORITHM       diis_gdm
$end

View output

Example 6.46  Truncated CI (S, D, T) calculation for the ground state N2 using the CASCI routines.

$molecule
   0 1
   N   0.0   0.0   0.0
   N   0.0   0.0   1.1
$end

$rem
   EXCHANGE          hf
   BASIS             cc-pvdz
   CAS_METHOD        1   !1 for CAS-CI, 2 for CASSCF
   CAS_M_S           0   !M_s value*2
   ASCI_DIAG         2   !Arma Sparse=0, Davidson=1, Eigen Sparse=2
   CAS_N_ELEC        6   !N_elec
   CAS_N_ORB        -1   !N_orb (-1: include all orbitals in active space)
   CAS_N_ROOTS       1   !N_roots
   CAS_SOLVER        3   !2=ASCI, 1=Olsen, 0=naive, 3=truncated CI
   TRUNC_CI_LEVEL    3   !include up to triples excitations
   THRESH            14
   MAX_SCF_CYCLES    400
   SCF_CONVERGENCE   6
   MEM_TOTAL         4000
   MEM_STATIC        1000
   SYMMETRY          false
   SYM_IGNORE        true
   SCF_ALGORITHM     diis_gdm
$end

View output