Keywords: user_guide

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Subsections

- - 3.4.0.1 Structural optimization
  - 3.4.0.2 Molecular Dynamics
  - 3.4.0.3 Free-energy surface calculations
  - 3.4.0.4 Variable-cell optimization
  - 3.4.0.5 Variable-cell molecular dynamics

## 3.4 Optimization and dynamics

#### 3.4.0.1 Structural optimization

For fixed-cell optimization, specify `calculation='relax'` and add namelist &IONS. All options for a single SCF calculation apply, plus a few others. You may follow a structural optimization with a non-SCF band-structure calculation. See example 2.

#### 3.4.0.2 Molecular Dynamics

Specify `calculation='md'`, the time step `dt`, and possibly the number of MD stops `nstep`. Use variable `ion_dynamics` in namelist &IONS for a fine-grained control of the kind of dynamics. Other options for setting the initial temperature and for thermalization using velocity rescaling are available. Remember: this is MD on the electronic ground state, not Car-Parrinello MD. See example 3.

#### 3.4.0.3 Free-energy surface calculations

Once `PWscf` is patched with the `PLUMED` plug-in (only v.1.3), it is possible to use most PLUMED functionalities by running `PWscf` as: `./pw.x -plumed` plus the other usual `PWscf` arguments. The input file for `PLUMED` must be found in the specified `outdir` with fixed name `plumed.dat`.

#### 3.4.0.4 Variable-cell optimization

Variable-cell calculations (both optimization and dynamics) are performed with plane waves and G-vectors calculated for the starting cell. Inly the last step, after convergence has been achieved, is performed for the converged structure, with plane waves and G-vectors calculated for the final cell. Small differences between the two last steps are thus to be expected and give an estimate of the convergence of the variable-cell optimization with respect to the plane-wave basis. A large difference means that you are far from convergence in the plane-wave basis set and you need to increase the cutoff(s) `ecutwfc` and/or (if applicable) `ecutrho`.

#### 3.4.0.5 Variable-cell molecular dynamics

"A common mistake many new users make is to set the time step `dt` improperly to the same order of magnitude as for CP algorithm, or not setting `dt` at all. This will produce a ``not evolving dynamics''. Good values for the original RMW (Wentzcovitch) dynamics are `dt` = 50÷70. The choice of the cell mass is a delicate matter. An off-optimal mass will make convergence slower. Too small masses, as well as too long time steps, can make the algorithm unstable. A good cell mass will make the oscillation times for internal degrees of freedom comparable to cell degrees of freedom in non-damped Variable-Cell MD. Test calculations are advisable before extensive calculation. I have tested the damping algorithm that I have developed and it has worked well so far. It allows for a much longer time step (dt= 100÷150) than the RMW one and is much more stable with very small cell masses, which is useful when the cell shape, not the internal degrees of freedom, is far out of equilibrium. It also converges in a smaller number of steps than RMW." (Info from Cesar Da Silva: the new damping algorithm is the default since v. 3.1).

Next: 3.5 Direct interface with Up: 3 Using PWscf Previous: 3.3 Electronic structure calculations Contents