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.

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.

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.

Task: Run the xTB input and compare wall time and SCF behaviour to the AIMD run.