Function fixed_sequential_blend_step 

pub fn fixed_sequential_blend_step(
    opt_state: &OptimizationState_blend,
    config: &Config,
) -> DVector<f64>

Fixed-then-GDIIS sequential blend step.

Two-phase approach:

• Phase 1 (far from minimum): 50/50 fixed blend of GDIIS and EDIIS
• Phase 2 (RMS displacement < gediis_switch_step): Pure GDIIS for quadratic final convergence

This avoids the plateau problem by using pure GDIIS near convergence, while keeping the stability of the 50/50 blend in the far region.

Arguments

• opt_state - Experiment optimization state.
• config - Configuration with phase thresholds.

Returns

The new geometry from the selected phase.