XtalOpt is a free and truly open source evolutionary algorithm designed for a priori crystal structure prediction.
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XtalOpt is implemented as an extension to the Avogadro molecular editor.
Check out a few of the features:
User Interface Mutation OperatorsScreenshot of Avogadro visualizing a 16 x TiO2 cell.
The "Progress" tab
\centering \vec{\textrm{v}}_{\textrm{new}} = \vec{\textrm{v}} \begin{bmatrix} 1+\varepsilon_{11} & \frac{\varepsilon_{12}}{2} & \frac{\varepsilon_{13}}{2} \\ \frac{\varepsilon_{12}}{2} & 1+\varepsilon_{22} & \frac{\varepsilon_{32}}{2} \\ \frac{\varepsilon_{13}}{2} & \frac{\varepsilon_{23}}{2} & 1+\varepsilon_{33} \end{bmatrix}
\Delta z = \rho\cos(2\pi\mu x+\theta_{x})\cos(2\pi\eta y+\theta_{y})~~~~~\mu\in\mathbb{Z},~\eta\in\mathbb{Z}