How to make a SCF run converge

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 * [ABORT]                                                                     *
 *  \___/     SCF run NOT converged. To continue the calculation regardless,   *
 *    |             please set the keyword IGNORE_CONVERGENCE_FAILURE.         *
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 * / \                                                            qs_scf.F:702 *
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Since CP2K 2024.1 version, a failure in SCF convergence in a Quickstep calculation aborts the program by default. This means that after reaching MAX_SCF cycles (50 by default), the value printed under the Convergence column does not meet EPS_SCF. A variety of measures are available for converging the SCF to a reasonable result, which this page discusses, assuming that the reader has read Foreword and FAQ and the other documentations under Density Functional Theory first.

Note

At the moment, the convergence criterion does not take the absolute change in energy into account, and the convergence of the diagonalization algorithm differs from that of the OT algorithm.

Danger

Take your own risk and responsibility for ignoring convergence failure !!

Setting IGNORE_CONVERGENCE_FAILURE will instead emit a warning *** WARNING in qs_scf.F:700 :: SCF run NOT converged *** and proceed to other calculation. Unfortunately, few do realize the necessity to scrutinize subsequent outcomes for accuracy, let alone precision. It can lead to qualitatively and quantitatively incorrect results including but not limited to strange electronic occupation and band structure, unphysical response properties, and wild atomic motion and out-of-control temperature. These are not credible and useful.

Therefore, IGNORE_CONVERGENCE_FAILURE should only be considered as a last resort out of desperation, rather than a universal remedy used on a regular basis or even as the default. Please try achieving SCF convergence on the starting structure in a single-point energy calculation in the first place; any other tasks not preceded by it is like putting the cart before the ponies.

General considerations

The very first ingredient of SCF convergence is a sensible input structure, applicable to every type of computation task including geometry and cell optimization, as elaborated on Starting structure and cell.

On top of that, there is also the net charge and spin multiplicity as specified by keywords CHARGE and MULTIPLICITY respectively that should be set to represent the realistic electronic state. Use the UKS keyword (or equivalently written as LSD) to request for an unrestricted, spin-polarized calculation of open-shell systems.

If the geometry is reasonable, check whether the XC section or the respective section of model Hamiltonian has been set up correctly.

The default of SCF_GUESS is ATOMIC, meaning that the initial wavefunction and density matrix is generated from the atomic density of each kind of atom. In this case the MAGNETIZATION keyword and the BS section can be used to provide the orbital occupation pattern of different spin channels and quantum numbers, which is especially crucial for systems with certain magnetic order like ferromagnetic, ferrimagnetic and antiferromagnetic materials. Relevant literature or materials database entry often give information about the atomic magnetization. As for how the keywords and sections are set up, prior discussions can be found at a google group thread and page 34-36 of a 2015 tutorial.

Setting SCF_GUESS to RESTART and specifying a wavefunction restart file for the keyword WFN_RESTART_FILE_NAME will instead parse the file for the initial density matrix. If some preliminary cheap calculation can converge, restarting from the wavefunction is highly recommended for going to advanced, expensive ones:

Having used the 2-zeta DZVP-MOLOPT-SR-GTH basis set in a gamma-only calculation, restart another gamma-only calculation with the 3-zeta TZVP-MOLOPT-SR-GTH basis set (note that both should use the same pseudopotential);
Having used the pure GGA functional PBE, restart a calculation with hybrid functional PBE0 (which also makes SCREEN_ON_INITIAL_P reasonable);
After a plain calculation, restart another with special external environments such as a periodic electric field or an implicit solvation model;
After a single-point energy evaluation, restart a geometry or cell optimization task, and then after that restart a vibrational analysis task;
After a ground-state calculation, use WFN_MIX to manipulate MO coefficients and restart an excited-state calculation;
…

Note

The format and suffix of wavefunction restart files differ between a gamma-only formalism and a k-point sampling scheme: the former is typically <project>-RESTART.wfn while the latter is typically <project>-RESTART.kp. They cannot be interchanged for the purpose of restarting.

Starting from CP2K version 2026.2, it is possible to produce a <project>-RESTART.kp from a gamma-only calculation by using the Harris functional for energy correction under section DFT/ENERGY_CORRECTION.

Some parameters that control the accuracy for Quickstep calculation, such as EPS_DEFAULT, CUTOFF and REL_CUTOFF, can also help convergence when chosen as good as necessary. The overall time cost is not necessarily increased with the parameters leaning on the more accurate and expensive side: even if each SCF iteration takes longer, the total number of iterations to reach convergence may still be reduced.

Similarly, higher number or density of k-points for the Brillouin-zone sampling may be beneficial, in particular if the cell is small and the system is not insulating. A convergence test for k-points with respect to the target property does not need to start with what is too low to make SCF converge.

For diagonalization

The standard diagonalization algorithm for the Kohn-Sham matrix is activated by setting the DIAGONALIZATION section, with full support for the mixing and smearing techniques.

The mixing procedures of the density matrix in MIXING supports several methods in the keyword METHOD. The default conservative DIRECT_P_MIXING option may be swapped with BROYDEN_MIXING, PULAY_MIXING, KERKER_MIXING, etc.

Fractional occupation of molecular orbitals, or smearing, is enabled by the section SMEAR. It is very useful for systems with small to none band gap and strong static correlation. The possibilities provided by the keyword METHOD include Fermi-Dirac smearing at a certain ELECTRONIC_TEMPERATURE and several broadening methods with width SIGMA; elevated ELECTRONIC_TEMPERATURE or SIGMA can handle difficult systems, but extrapolation to 0 is required to obtain results comparable with what without smearing.

For OT

Alternative to the diagonalization is the orbital transformation (OT) method, activated by setting the OT section. The most important settings are ALGORITHM, LINESEARCH, MINIMIZER, and PRECONDITIONER.