Active space specification and convergence details are controlled by the $rem variables described in Section 6.17.3. During the course of an ASCI calculation, a file named “wf_data” is created in the scratch directory containing information on the determinants and weights in the ASCI wavefunction.
ASCI_DIAG
Specifies the diagonalization procedure.
TYPE:
INTEGER
DEFAULT:
2
OPTIONS:
1
Davidson solver
2
Eigen sparse matrix solver
RECOMMENDATION:
Use 2 for best trade-off of speed and memory usage. If memory usage becomes to great, switch to 1.
ASCI_NDETS
Specifies the number of determinants to include in the ASCI wavefunction.
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
for a wavefunction with determinants
RECOMMENDATION:
Typical ASCI expansions range from 50,000 to 2,000,000 determinants depending on active space size, complexity of problem, and desired accuracy
ASCI_CDETS
Specifies the number of determinants to search over during ASCI wavefunction growth steps.
TYPE:
INTEGER
DEFAULT:
-5
OPTIONS:
search from the top determinants
search from the top determinants whose cumulative weight in the wavefunction corresponds to
RECOMMENDATION:
Using a dynamically determined value () gives better results.
ASCI_USE_NAT_ORBS
Specifies whether rotation to a natural orbital basis should be carried out between growth steps.
TYPE:
BOOLEAN
DEFAULT:
TRUE
OPTIONS:
TRUE
rotate to a natural orbital basis between growth wavefunction growth steps
FALSE
do not rotate to a natural orbital basis
RECOMMENDATION:
Natural orbital rotations significantly improve the compactness and therefore accuracy of the ASCI wavefunction.
ASCI_DAVIDSON_GUESS
Specifies the truncated CI guess used for ASCI’s Davidson solver.
TYPE:
INTEGER
DEFAULT:
2
OPTIONS:
Order of the truncated CI to solve explicitly ASCI Davidson guess.
RECOMMENDATION:
Accurate excited states and rapid convergence of the ground state benefit from a good zero-order guess for the low energy spectrum. The default is often sufficient.
ASCI_SKIP_PT2
Specifies whether ASCI PT2 correction should be calculated.
TYPE:
BOOLEAN
DEFAULT:
FALSE
OPTIONS:
FALSE
compute ASCI PT2 contribution
TRUE
do not compute ASCI PT2 contribution
RECOMMENDATION:
The PT2 correction is essential to obtaining converged ASCI energies.
ASCI_RESTART
Specifies whether to initialize the ASCI wavefunction with the “wf_data” file.
TYPE:
BOOLEAN
DEFAULT:
FALSE
OPTIONS:
TRUE
read CI coefficients from the “wf_data” file
FALSE
do not read the CI coefficients from disk
RECOMMENDATION:
ASCI_SPIN_PURIFY
Indicates whether or not the ASCI wavefunction should be augmented with missing
determinants to ensure a spin-pure state.
TYPE:
BOOLEAN
DEFAULT:
FALSE
OPTIONS:
TRUE
augment the wavefunction with determinants to ensure a spin eigenstate
FALSE
do not augment the wavefunction
RECOMMENDATION:
Example 6.39 CASCI calculation for the lowest triplet () state of N. The adaptive sampling method is employed to tackle a large active space (14e, 32o). The PT2 correction is performed on top of the ASCI wavefunction.
$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 2 !M_s value*2 (2: triplet) ASCI_DIAG 2 !Arma Sparse=0, Davidson=1, Eigen Sparse=2 CAS_N_ELEC 14 !N_elec CAS_N_ORB 32 !N_orb CAS_N_ROOTS 1 !N_roots CAS_SOLVER 2 !2=ASCI, 1=Olsen, 0=naive ASCI_NDETS 28000 !Number of ASCI Determinants THRESH 14 max_scf_cycles 400 SCF_CONVERGENCE 8 mem_total 4000 mem_static 1000 ROSCF true symmetry false sym_ignore true scf_algorithm diis_gdm $end
Example 6.40 ASCI-SCF (using adaptive sapling for CAS) calculation for the ground state of N with a large active space (14e, 32o). The PT2 correction is not performed in this case.
$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 14 !N_elec CAS_N_ORB 32 !N_orb CAS_N_ROOTS 1 !N_roots CAS_SOLVER 2 !2=ASCI, 1=Olsen, 0=naive ASCI_NDETS 28000 !Number of ASCI Determinants ASCI_SKIP_PT2 1 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