Run Modes

The mode keyword controls how OpenMECP manages QM wavefunction checkpoints between optimization steps. Choosing the right run mode can significantly affect SCF convergence speed and stability.

mode = normal    # default

Normal Mode (Default)

mode = normal

Standard MECP optimization. OpenMECP reads the wavefunction checkpoint from the previous step as the SCF initial guess.

• Reads .chk (Gaussian) or .gbw (ORCA) from the previous step
• Recommended for most calculations
• Fastest per-step runtime

Read Mode

mode = read

Reads existing wavefunction checkpoints from disk without running pre-point calculations. Use this when restarting a calculation where checkpoints already exist from a previous run.

• Skips pre-point calculations
• Requires existing checkpoint files
• Useful for restarting after a crash or job time limit

NoRead Mode

mode = noread

Runs a fresh SCF at every step without reading any checkpoint. This is slower but more robust for systems with difficult SCF convergence or when the wavefunction character changes significantly between steps.

• Does not read or write checkpoint files
• Slower (full SCF from scratch each step)
• Use when checkpoint reading causes SCF divergence

Stable Mode

mode = stable

Runs pre-point calculations at the initial geometry before starting the MECP optimization. This checks SCF stability (stable=opt in Gaussian) and produces reliable initial checkpoints.

• Runs stable=opt or equivalent before step 1
• Ensures stable wavefunction from the start
• Recommended for systems with unstable ground-state wavefunctions (e.g., open-shell systems)

InterRead Mode

mode = inter_read

Essential for open-shell singlet (OSS) states.

InterRead runs state B (typically the triplet) first at each step, then copies the triplet wavefunction as the initial guess for state A (the open-shell singlet) and adds guess=(read,mix) (Gaussian) or equivalent. This provides a physically correct broken-symmetry starting point for the singlet.

Example — open-shell singlet/triplet MECP:

mode   = inter_read
mult_a = 1    # Open-shell singlet
mult_b = 3    # Triplet (reference for broken-symmetry guess)

Mode Comparison