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Table of contents

• Table of contents
• Details
• Keywords
  • The GUESS Keyword
  • The FIELD Keyword
  • The ALG Keyword
  • The INCFOCK and NINCFOCK Keywords
  • Extrapolation Keywords
    • The DIIS and NKEEP Keywords
    • The DAMP, DAMPPARAM, and DAMPERROR Keywords
• Example
• References

Details

The SCF section handles the SCF parameters for the optimization of the reference wave functions specified in QM.REFERENCE. The SCF section is optional, and the default parameters are often sufficient. However, as SCF optimizations can often be non-trivial, it may be the case that these parameters must be adjusted for the optimization.

Keywords

Keyword	Type	Description	Default
GUESS	String	Initial guess for reference wave function	SAD
FIELD	Multiline string	Static external field specification in which to perform optimization	None
ALG	String	Algorithm for SCF procedure	CONVENTIONAL
ENETOL	Double precision float	Convergence tolerance for the change in total energy	10^{-10}
DENTOL	Double precision float	Convergence tolerance for the root-mean-squared change in the one particle density matrix	10^{-8}
MAXITER	Integer	Maximum number of SCF iterations	128
INCFOCK	Boolean	Whether or not to perform an incremental Fock build throughout the SCF for improved screening	TRUE if ERI formation is DIRECT; FALSE otherwise
NINCFOCK	Integer	Number of SCF iterations after which to start the incremental Fock build	20
EXTRAP	Boolean	Whether to do extrapolation of Fock matrix with DIIS and damping	TRUE
DIIS	Boolean	Whether to use direct inversion of the iterative subspace extrapolation (CDIIS specifically)	TRUE
NKEEP	Integer	Number of previous iterations to track for DIIS	10
DAMP	Boolean	Whether to damp Fock matrix changes	TRUE
DAMPPARAM	Double precision float	Damping parameter	0.7
DAMPERROR	Double precision float	Energy difference after which to turn off Fock damping	10^{-3}
PRINTMOS	Boolean	Whether or not to print MO coefficients after SCF procedure	FALSE

The GUESS Keyword

The GUESS keyword defines the method for producing the initial one-particle reduced density matrix (1PDM) in the SCF procedure. Currently there are 5 options, with support for reading in Gaussian fchk files:

1. CORE: Use the 1PDM from the core Hamiltonian

2. SAD: Use the Superposition of Atomic Densities method

3. READMO: Read MO coefficients from ChronusQ restart file (specified with -b) or old restart file (specified with -z).

4. READDEN: Read 1PDM from ChronusQ restart file (specified with -b) or old restart file (specified with -z).

5. FCHKMO: Read MO coefficients from ChronusQ scratch file (specified with -s), where the scratch file should be an fchk file from the Gaussian software package. ChronusQ does not match the default underlying basis representation in Gaussian (e.g., orthogonalization, primitive deletion, etc.) and the recommended Gaussian IOp per basis set is given below.

   • For Dunning correlation-consistant basis sets (e.g., cc-pVDZ): IOp(3/60=-1)

The FIELD Keyword

The ALG Keyword

The ALG keyword specifies the algorithm used for the SCF procedure. Currently CONVENTIONAL first-order optimization is the default. Second-order methods like Newton-Raphson will be implemented in the near future. Additionally, SKIP skips the SCF cycles completely. This can be useful for analyzing the quality of an SCF guess or for saving time when a post-SCF treatment is desired.

The INCFOCK and NINCFOCK Keywords

The INCFOCK keyword is a boolean that turns on the incremental Fock build algorithm. In short, and incremental Fock build uses the difference density between the current and last SCF step to screen which electron-repulsion integrals aren't necessary to compute for certain Fock matrix elements from the coulomb and exchange matrices, speeding up the Fock build timing.

NINCFOCK then controls which SCF step the incremental Fock build procedure starts. The default is set to 20 since smaller density differences offer the greatest speed-up.

Extrapolation Keywords

Currently, ChronusQ uses the commutator direct inversion of the iterative subspace, also known as CDIIS to perform extrapolations of future Fock matrices in the SCF procedure.¹ The commutator used is [\mathbf{F}, \mathbf{P}] where \mathbf{F} is the Fock matrix and \mathbf{P} is the 1PDM.

The DIIS and NKEEP Keywords

The DIIS keyword is just a boolean that tells ChronusQ wether or not to perform CDIIS. The NKEEP keyword is an integer that decides how many previous Fock matrices are used for the extrapolation.

The DAMP, DAMPPARAM, and DAMPERROR Keywords

Fock damping is when the Fock matrix of the previous iteration is mixed in to the current step to damp large oscillations in energy.

1. DAMP is the boolean for choosing to do damping, by default it is TRUE.
2. DAMPARAM is a floating point value between 0 and 1 that represents the fraction of the previous Fock matrix to average into the current step for damping. By default the value is 0.7 meaning that 70% of the previous Fock matrix is mixed in.
3. DAMPERROR is the energy tolerance at which damping is turned off. Since damping is most useful for early steps in the SCF when there can be large changes in the energy and density. By default the damping error threshold is set to 0.001 Hartree.

Example

For example, let's say we want to specify that during the SCF procedure in a ChronusQ job, we will use 12 Fock matrices for extrapolation, keep damping on until the energy change is less than 10^{-5}, and only mix in 50% of the previous Fock matrix for damping. Then, the [SCF] section will look like the following:

[SCF]
nkeep = 12
damperror = 1e-5
dampparam = 0.5

References

1. Pulay, P. (1982). Improved SCF convergence acceleration. Journal of Computational Chemistry, 3(4), 556-560. ↩