Constant pressure molecular dynamics on a hypercylinder.
A Lagrangian formalism for variable-cell-shape molecular dynamics is derived from first principles. It is based on consideration of a crystal as arranged on the surface of hypercylinder in an extended coordinate frame. The artificial curvature along the additional degrees of freedom upsets the balance of forces acting on every atom in a periodically repeating cell. Since the distance between atoms is not the metric tensor the proposed method provides an essential simplification of the equations of motion compared to those of Parinello and Rahman approach. The Lagrangian of the system eliminates the cell orientation from the dynamics, thus avoiding symmetry-breaking effects and physically irrelevant cell rotation. The change from a fully flexible cell to an isotropically flexible cell is realized within the same computational framework. Simulations for the transformation of a model He lattice under isotropic applied pressure are used to illustrate the application of this method.
Ryabov
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Laboratory of Radiation Solid State Physics, Institute of Physics and Power Engineering, 249020 Obninsk, Russia.