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# 6.19.3 CAS-CI and CASSCF Job Control

(December 20, 2021)

CAS_METHOD

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

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

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

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

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

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

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

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

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

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

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

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

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 N${}_{2}$.

$molecule 0 1 N 0.0 0.0 0.0 N 0.0 0.0 1.8$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 N${}_{2}$.

$molecule 0 1 N 0.0 0.0 0.0 N 0.0 0.0 1.8$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 N${}_{2}$ 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 N${}_{2}$ 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