Exercise 2: Geometry optimization of the L-alanine crystal

To perform a geometry optimisation, we need to add a third main section to the input file.

MOTION: contains all information related to structural changes during a calculation, such as geometry optimisation, cell optimisation, or molecular dynamics.

The relevant input for geometry optimisation is:

&MOTION
  &GEO_OPT
    OPTIMIZER BFGS
    MAX_ITER 100
  &END GEO_OPT
&END MOTION

The subsection GEO_OPT controls how the geometry optimisation is carried out. Here, the keyword OPTIMIZER specifies the optimisation algorithm; in this example we use BFGS, a quasi-Newton method that is efficient for molecular and solid-state systems. The keyword MAX_ITER sets the maximum number of optimisation steps (here 100). The optimisation will stop earlier if the convergence criteria are reached.

Task: Run the geometry optimisation of the L-alanine crystal. What do you have to do except adding the MOTION section? After the run, inspect the optimised structure using Mercury, VMD, VESTA, or another molecular visualisation program.