Robust DIIS

Standard GDIIS/GEDIIS can occasionally produce unreliable steps when the DIIS matrix is ill-conditioned — for example, near convergence or when the history contains redundant points. The Robust DIIS system adds configurable sanity checks that detect and reject bad steps before they are applied.

Enabling Robust DIIS

use_robust_diis = true

This is the master switch. All options described on this page are inactive unless use_robust_diis = true.

Cosine Check

gdiis_cosine_check = standard    # default

Computes the cosine between the proposed DIIS step and the reference gradient. A low cosine means DIIS is stepping in a direction inconsistent with the local gradient — a sign that the step is unreliable and should be rejected.

Value	Threshold	Description
`none`	—	Skip cosine check
`zero`	cos > 0	Reject only steps pointing uphill
`standard`	cos ≥ 0.71	45° angle limit — balances safety and speed
`variable`	0.1 → 0.7	Adaptive: permissive early, tight near convergence
`strict`	cos ≥ 0.866	30° angle limit — conservative for fragile systems

Coefficient Check

gdiis_coeff_check = regular    # default

Validates that the DIIS interpolation coefficients $c_{i}$ are physically reasonable. Well-conditioned DIIS produces coefficients that sum to 1 with individual values near $[0, 1]$ .

Value	Description
`none`	Accept any coefficients
`regular`	Reject extreme deviations; allows moderate negative $c_{i}$
`force_recent`	Strongly bias toward the latest point; useful after restart
`combined`	Both `regular` bounds and cosine threshold simultaneously

GEDIIS Variants

gediis_variant = auto    # default

Only active when use_robust_diis = true and use_gediis = true.

Variant	B-matrix	Best for
`auto`	Switches automatically	General use; starts with `energy`, switches to `rfo`
`rfo`	Quadratic step overlaps	Near the minimum; quadratic PES approximation
`energy`	Gradient-coordinate products	Far from minimum; non-quadratic PES
`simultaneous`	Combined energy + quadratic	Most sophisticated; blends `rfo` and `energy`

The gediis_sim_switch parameter (default: 0.0025) controls the RMS error threshold at which auto switches from energy to rfo.

Negative Eigenvalue Control

n_neg = 0    # default (minimum search)
n_neg = 1    # transition-state search (one imaginary mode)

Set n_neg = 1 for TS-like MECP searches where the crossing point is a saddle point with one negative Hessian eigenvalue.

Worked Examples

Oscillating Convergence

If the optimizer oscillates or takes wild steps near convergence:

use_robust_diis    = true
gdiis_cosine_check = standard
gdiis_coeff_check  = regular

Fragile System (Flat PES / Near-Dissociation)

use_robust_diis    = true
gdiis_cosine_check = strict

Restart from Checkpoint

use_robust_diis   = true
gdiis_coeff_check = force_recent
restart           = true

Full Robust GEDIIS

use_robust_diis    = true
use_gediis         = true
gediis_variant     = energy
gdiis_cosine_check = standard
gdiis_coeff_check  = regular

OpenMECP Documentation