4.3 Basic SCF Job Control

4.3.1 Overview

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:

  • Addition of several useful SCF convergence algorithms and support for user-specified hybrid algorithm (Sect. 4.5.9).

  • More general and user-friendly internal stability analysis and automatic correction for the energy minimum (Sect. 4.5.12).

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 α and β 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 α and β 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.

4.3.1.1 Job Control

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-SCF_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 α/β 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 = πCOMPLEX_MIX/100.
RECOMMENDATION:
       It may help find the stable complex solution (similar idea as SCF_GUESS_MIX).

Example 4.1  Restricted open-shell singlet ROSCF calculation for the first excited state of formaldehyde using GEN_SCFMAN. The first job provides the guess orbitals through a restricted SCF calculation.

$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