Sigma (eV) | |

Aluminium | |

Lithium | 0.4 |

Tellurium | 0.8 |

Copper, Palladium | 0.4 |

Vanadium | 0.2 |

Rhodium | 0.2 |

Potassium | 0.3 |

The method described in the last section has two shortcomings:

- The forces calculated by VASP are a derivative of the free electronic energy F (see section 7.5). Therefore the forces can not be used to obtain the equilibrium groundstate, which corresponds to an energy-minimum of . Nonetheless the error in the forces is generally small and acceptable.
- The parameter must be chosen with great care. If is too large the energy will converge to the wrong value even for an infinite k-point mesh, if is too small the convergence speed with the number of k-points will deteriorate. An optimal choice for for several cases is given in table 2. The only way to get a good is by performing several calculations with different k-point meshes and different parameters for .

In contrast to the Gaussian method the entropy term will be very small for reasonable values of (for instance for the values given in table 2). The is a simple error estimation for the difference between the free energy and the 'physical' energy . can be increased till this error estimation gets too large.