====== Exercise 4: Molecular dynamics of water ======

In this exercise we perform a short ab initio molecular dynamics (AIMD) simulation of bulk liquid water and then switch to a semiempirical model (xTB) to reduce the computational cost. The input ''water_dft.inp'' and coordinate file ''water.xyz'' are provided in the GitHub repository.

===== AIMD (DFT) run =====

Use the AIMD input provided in the repository (see ''water_dft.inp'' and ''water.xyz''). This setup performs an NVT simulation at 300K with PBE+D3, GPW, GLE thermostat, and writes trajectory and energy files.

<note classic>
**Task:** Run a 20-step AIMD for water using the provided input. How long does it take on your machine? Inspect ''water_dft-pos-1.xyz'' and ''water_dft-1.ener''.
</note>

===== Switching to a faster model (xTB) =====

A second input is provided that uses the xTB method (see ''water_xtb.inp''). This keeps the same MD settings (ensemble, thermostat, timestep, system) but changes the electronic structure method for speed.

<note classic>
**Task:** Run the xTB input and compare wall time and SCF behaviour to the AIMD run.
</note>