Function select_diis_step 

pub fn select_diis_step(
    opt_state: &mut OptimizationState,
    config: &Config,
    step: usize,
) -> DVector<f64>

Selects and runs the appropriate DIIS/GEDIIS step based on configuration.

This is a shared dispatch function to eliminate the three copies of DIIS dispatch logic that were previously duplicated across Normal/Read/Noread modes in main.rs (which had subtle differences between them).

Arguments

• opt_state - Mutable optimization state
• config - Configuration with optimizer settings
• step - Current optimization step number (1-indexed, for printouts)

Returns

New geometry coordinates from the selected optimizer.