# 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. 4.5.8).

• More general and user-friendly internal stability analysis and automatic correction for the energy minimum (Sect. 4.5.9).

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.

## 4.3.1.1 Job Control

The following two $rem variables must be specified in order to run HF calculations: METHOD Specifies the exchange-correlation functional. TYPE: STRING DEFAULT: No default OPTIONS: NAME Use METHOD = NAME, where NAME is one of the following: HF for Hartree-Fock theory; one of the DFT methods listed in Section 5.3.4.; one of the correlated methods listed in Sections 7.8, 7.9, and 7.7; RECOMMENDATION: In general, consult the literature to guide your selection. Our recommendations for DFT are indicated in bold in Section 5.3.4. BASIS Specifies the basis sets to be used. TYPE: STRING DEFAULT: No default basis set OPTIONS: General, Gen User defined ($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 $10^{-\mathrm{SCF\_CONVERGENCE}}$. 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). 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
$end$rem