# Self-consistent field (SCF) cycles - tolerances, stopping criteria

### This page gives hints on how to control the SCF cycle with the ABINIT package.

##### Copyright (C) 2016-2017 ABINIT group (XG)

Mentioned in help_features#2.4.

### Table of content:

- 1. Introduction.
- 2. Related lesson(s) of the tutorial.
- 3. Related input variables.

### 1. Introduction.

The numerical precision of the calculations depends on many settings, among which the precision in solving the Kohn-Sham self-consistent equation.

Several parameters govern the SCF loop. The maximum number of cycles is given by nstep, but the iterative procedure might be stopped earlier, as soon as the criterion chosen by the user is fulfilled. The user is asked to give a tolerance on some measure of the convergence. The user must choose among toldfe, toldff, tolrff, tolvrs and tolwfr.

- The most theoretically justified for the density/potential self-consistency is tolvrs.
- tolwfr is interesting for non-self-consistent calculations.
- For molecular dynamics (which rely on the accuracy of forces), one might prefer tolrff.

Some input variables relate to the solution of the Schrödinger equation. However, usually the related iterative techniques are well-tuned, so that these input variables (nline and tolrde) are usually used only by experts. However, in cases where the convergence is difficult, it might be interesting to test improving them, as well as modifying nnsclo.

The accuracy variable enables to tune the accuracy of a calculation by setting automatically up to seventeen variables.
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### 2. Related lesson(s) of the tutorial.

- The lesson 2 deals again with the H2 molecule: convergence studies, LDA versus GGA
- The lesson 3 deals with crystalline silicon (an insulator): the definition of a k-point grid, the smearing of the cut-off energy, the computation of a band structure, and again, convergence studies ...
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### 3. Related input variables.

Basic input variables:

... accuracy [ACCURACY]
... nstep [Number of (non-)self-consistent field STEPS]
... toldfe [TOLerance on the DiFference of total Energy]
... toldff [TOLerance on the DiFference of Forces]
... tolrff [TOLerance on the Relative diFference of Forces]
... tolvrs [TOLerance on the potential V(r) ReSidual]
... tolwfr [TOLerance on WaveFunction squared Residual]

Useful input variables:

... nbdbuf [Number of BanDs for the BUFfer]

Input variables for experts:

... nline [Number of LINE minimisations]
... nnsclo [Number of Non-Self Consistent LOops]
... tolrde [TOLerance on the Relative Difference of Eigenenergies]
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