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Hydrogen molecule †

In this tutorial, how to execute pw.x and how to converge computational parameters, such as kinetic energy cutoff and box size are described. How to optimize the molecular geometry is also described.

Preliminary †

Let us begin with H2, the simplest molecule. In a standard plane-wave pseudopotential code, three dimensional periodic boundary condition is employed, and an isolated molecule is modeled by placing it in a large box in such a way that the electrostatic interactions with neighboring images can be negligible. If the box is large enough, the Brillouin zone becomes small (as it scales as (2p/a)**3), thus the Brillouin zone is sampled only at the Gamma point [(0,0,0)]. In this case, the accuracy of the calculation is controlled by increasing the kinetic energy cutoff, which corresponds to increasing the real-space mesh (resolution of the wave function in real space) and by increasing the box size. The latter is to confirm that the interaction between image molecules is negligibly small and model an isolated state.

SCF calculation †

Convergence study †

Convergence wrt cutoff †

Convergence wrt box size †

Optimizing the bond length (1) †

Optimizing the bond length (2) †