Strain
Overview
Strain is an evolutionary operation that generates a new structure (offspring) by applying a small random distortion to the lattice of a parent structure. It helps to explore nearby regions of the configuration space while preserving atomic connectivity and composition. This operator is useful for fine-tuning structural candidates and escaping local minima.
How it works
The lattice vectors are $ \mathbf{a} $ transformed to $ \mathbf{a}' $ by applying a strain matrix, as follows:
$$ \mathbf{a}' = \begin{pmatrix} 1 + \eta_1 & \frac{1}{2} \eta_6 & \frac{1}{2} \eta_5 \\ \frac{1}{2} \eta_6 & 1 + \eta_2 & \frac{1}{2} \eta_4 \\ \frac{1}{2} \eta_5 & \frac{1}{2} \eta_4 & 1 + \eta_3 \end{pmatrix} \mathbf{a}. $$Here,
$ \eta_i $ are given by a Gaussinan distribution
$ \mathcal{N}\left( 0, \ \sigma_{\mathrm{st}}^2 \right) $.
$ \sigma_{\mathrm{st}} $ is specified by the input parameter sigma_st (by default, sigma_st = 0.5).
As shown in the figure below, the lattice is deformed and then rescaled to restore the original volume.
Finally, the minimum interatomic distance constraint is checked.
