Saddle Search¶

A saddle search is initiated by making a local displacement of atoms from their position at the minimum of the current state. This displacement can be done using the different strategies indicated by the client_displace_type option, and the following parameters. If the user knows something about the local environment where reactions are likely to take place in the system, this information can be used to make saddle searches more efficient by getting them started in the right part of configuration space.

Displacement Strategies¶

Epicenters and Weight-Based Selection¶

Each saddle search begins by choosing an epicenter — a single atom around which the initial displacement is constructed. The epicenter is selected probabilistically from several strategies, each controlled by a relative weight:

Weight parameter	Strategy
`displace_random_weight`	Pick any atom at random
`displace_least_coordinated_weight`	Pick an atom with the lowest coordination number
`displace_not_FCC_HCP_weight`	Pick an atom whose local environment is neither FCC nor HCP
`displace_under_coordinated_weight`	Pick an atom with coordination ≤ `displace_max_coordination`
`displace_listed_atom_weight`	Pick an atom from `displace_atom_list`
`displace_listed_type_weight`	Pick an atom whose type is in `displace_type_list`

The weights do not need to sum to 1 — they are normalised internally. Setting a weight to 0 disables that strategy. For example, setting only displace_listed_atom_weight = 1.0 and all other weights to 0 ensures that every saddle search starts from an atom in the explicit list.

Radius and Magnitude¶

Once the epicenter is chosen, all atoms within displace_radius of the epicenter are displaced. Each displaced atom receives a random perturbation drawn from a Gaussian distribution with standard deviation displace_magnitude (in Ångströms) independently in each Cartesian direction.

Displacing All Listed Atoms¶

When displace_all_listed is true and a listed-atom strategy is selected, every atom in displace_atom_list (or displace_type_list) is displaced — not just one chosen at random. Atoms within displace_radius of any displaced atom are also included. Set displace_radius to 0 to restrict the displacement strictly to the listed atoms.

Dynamic Atom Lists via Scripts¶

For systems where the relevant atoms change from state to state (e.g. a migrating vacancy), a static list is insufficient. The displace_atom_kmc_state_script option lets you specify a Python script that is executed once per new AKMC state to determine the atom list dynamically. See the Targeted Displacement for Saddle Searches tutorial for worked examples covering vacancy diffusion and adsorbate-on-surface scenarios.

Client-Side Displacement¶

The client_displace_type option selects how the client (C++ code) picks the epicenter when the displacement is performed client-side rather than by the server:

random — uniform random atom
last_atom — the last atom in the configuration
min_coordinated — the atom with the fewest neighbours
not_fcc_or_hcp — an atom whose local structure is neither FCC nor HCP
listed_atoms — an atom from displace_atom_list (parsed from the INI config; no server displacement file needed)
load — read a displacement vector from a file written by the server

Configuration¶

[Saddle Search]