As of version 5.1, Q-Chem uses a new SCF package, GEN_SCFMAN, developed by E. J. Sundstrom, P. R. Horn and many other coworkers. In addition to supporting the basic features of the previous SCF package (e.g. restricted, unrestricted and restricted open-shell HF/KS-DFT calculations), many new features are now available in Q-Chem, including:
GEN_SCFMAN also supports a wider range of orbital types, including complex orbitals. A full list of supported orbitals is:
Restricted (R): typically appropriate for closed shell molecules at their equilibrium geometry, where electrons occupy orbitals in pairs.
Unrestricted (U): - appropriate for radicals with an odd number of electrons, and also for molecules with even numbers of electrons where not all electrons are paired, e.g., stretched bonds and diradicals.
Restricted open-shell (RO): for open-shell molecules, where the $\alpha $ and $\beta $ orbitals are constrained to be identical.
Open-shell singlet ROSCF (OS_RO): see the “ROKS" method documented in Section 7.6.
Generalized (G): i.e., each MO is associated with both $\alpha $ and $\beta $ spin components.
The use of complex orbitals (with Hartree-Fock only): restricted (CR), unrestricted (CU), and generalized (CG).
Aspects of an SCF calculation such as the SCF guess, the use of efficient algorithms to construct the Fock matrix like occ-RI-K (see Section 4.6.9), are unaffected by the use of GEN_SCFMAN. Likewise, using GEN_SCFMAN does not make any difference to the post-SCF procedures such as correlated methods, excited state calculations and evaluation of molecular properties.
It should be noted that many special features (e.g. dual-basis SCF, CDFT, etc.) based on Q-Chem’s old SCF code are not yet supported in GEN_SCFMAN. They will become available in the future.
The following two $rem variables must be specified in order to run HF calculations:
METHOD
Specifies the exchange-correlation functional.
TYPE:
STRING
DEFAULT:
No default
OPTIONS:
NAME
Use METHOD = NAME, where NAME is one of the following:
HF for Hartree-Fock theory;
one of the DFT methods listed in Section 5.3.4.;
one of the correlated methods listed in Sections 7.9,
7.10, and 7.8;
RECOMMENDATION:
In general, consult the literature to guide your selection. Our recommendations for DFT are indicated
in bold in Section 5.3.4.
BASIS
Specifies the basis sets to be used.
TYPE:
STRING
DEFAULT:
No default basis set
OPTIONS:
General, Gen
User defined ($basis keyword required).
Symbol
Use standard basis sets as per Chapter 8.
Mixed
Use a mixture of basis sets (see Chapter 8).
RECOMMENDATION:
Consult literature and reviews to aid your selection.
In addition, the following $rem variables can be used to customize the SCF calculation:
GEN_SCFMAN
Use GEN_SCFMAN for the present SCF calculation.
TYPE:
BOOLEAN
DEFAULT:
TRUE
OPTIONS:
FALSE
Use the previous SCF code.
TRUE
Use GEN_SCFMAN.
RECOMMENDATION:
Set to FALSE in cases where features not yet supported by GEN_SCFMAN are needed.
PRINT_ORBITALS
Prints orbital coefficients with atom labels in analysis part of output.
TYPE:
INTEGER/LOGICAL
DEFAULT:
FALSE
OPTIONS:
FALSE
Do not print any orbitals.
TRUE
Prints occupied orbitals plus 5 virtual orbitals.
NVIRT
Number of virtual orbitals to print.
RECOMMENDATION:
Use true unless more virtual orbitals are desired.
SCF_CONVERGENCE
SCF is considered converged when the wave function error is less that
${10}^{-\mathrm{SCF}\mathrm{\_}\mathrm{CONVERGENCE}}$. Adjust the value of THRESH at the same
time. (Starting with Q-Chem 3.0, the DIIS error is measured by the maximum error
rather than the RMS error as in earlier versions.)
TYPE:
INTEGER
DEFAULT:
5
For single point energy calculations.
8
For geometry optimizations and vibrational analysis.
8
For SSG calculations, see Chapter 6.
OPTIONS:
User-defined
RECOMMENDATION:
Tighter criteria for geometry optimization and vibration analysis. Larger
values provide more significant figures, at greater computational cost.
UNRESTRICTED
Controls the use of restricted or unrestricted orbitals.
TYPE:
LOGICAL
DEFAULT:
FALSE
Closed-shell systems.
TRUE
Open-shell systems.
OPTIONS:
FALSE
Constrain the spatial part of the alpha and beta orbitals to be the same.
TRUE
Do not Constrain the spatial part of the alpha and beta orbitals.
RECOMMENDATION:
Use the default unless ROHF is desired. Note that for unrestricted calculations on
systems with an even number of electrons it is usually necessary to break
$\alpha $/$\beta $ symmetry in the initial guess, by using SCF_GUESS_MIX or
providing $occupied information (see Section 4.4 on initial guesses).
The calculations using other more special orbital types are controlled by the following $rem variables (they are not effective if GEN_SCFMAN = FALSE):
OS_ROSCF
Run an open-shell singlet ROSCF calculation with GEN_SCFMAN.
TYPE:
BOOLEAN
DEFAULT:
FALSE
OPTIONS:
TRUE
OS_ROSCF calculation is performed.
FALSE
Do not run OS_ROSCF (it will run a close-shell RSCF calculation instead).
RECOMMENDATION:
Set to TRUE if desired.
GHF
Run a generalized Hartree-Fock calculation with GEN_SCFMAN.
TYPE:
BOOLEAN
DEFAULT:
FALSE
OPTIONS:
TRUE
Run a GHF calculation.
FALSE
Do not use GHF.
RECOMMENDATION:
Set to TRUE if desired.
COMPLEX
Run an SCF calculation with complex MOs using GEN_SCFMAN.
TYPE:
BOOLEAN
DEFAULT:
FALSE
OPTIONS:
TRUE
Use complex orbitals.
FALSE
Use real orbitals.
RECOMMENDATION:
Set to TRUE if desired.
COMPLEX_MIX
Mix a certain percentage of the real part of the HOMO to the
imaginary part of the LUMO.
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
0–100
The mix angle = $\pi \cdot $COMPLEX_MIX/100.
RECOMMENDATION:
It may help find the stable complex solution (similar idea as SCF_GUESS_MIX).
$molecule 0 1 H -0.940372 0.000000 1.268098 H 0.940372 0.000000 1.268098 C 0.000000 0.000000 0.682557 O 0.000000 0.000000 -0.518752 $end $rem GEN_SCFMAN true METHOD wb97x-d BASIS def2-svpd THRESH 14 SCF_CONVERGENCE 9 SYM_IGNORE true $end @@@ $molecule read $end $rem JOBTYPE sp METHOD wb97x-d BASIS def2-svpd GEN_SCFMAN true OS_ROSCF true THRESH 14 SCF_CONVERGENCE 9 SCF_ALGORITHM diis SYM_IGNORE true SCF_GUESS read $end