6.17 Complete Active Space Methods

6.17.3 CAS-CI and CASSCF Job Control

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 6.18) 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 6.35  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
   jobtype  sp
   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

Example 6.36  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
   jobtype    sp
   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

Example 6.37  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

Example 6.38  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
   jobtype  sp
   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