Simulated annealing

[Input: recipes/moleculardynamics/annealSW/]

By heating up a system, and then allowing it to slowly cool, stable minima can be found. If the cooling is sufficiently (logarithmically) slow, then the global minima will be found, however this is not usually practical. More realistically, several heating and cooling experiments can find some stable structures, or investigate the stability of known geometries.

DFTB+ can set a sequence of temperatures during an MD calculation inside the thermostat input block

Temperature [Kelvin] = TemperatureProfile {
  constant      1    100.0
  linear      499   5500.0
  constant    200   5500.0
  linear      500    100.0
}

Here, a starting temperature of 1 K is heated over 500 steps up to 5500 K, held at that temperature for 200 steps and then cooled down again to 100 K in 500 steps.

The first example anneals away a Stone-Wales defect in a graphene sheet. This is acheived by heating the system up to a high (5000K) temperature where C-C bonds start to break, holding it at these temperatures for a while and then cooling to a low temperature. The high temperature is above the usual disociation temperature of graphene, but since only a relatively short (2.4 ps) time is computed, using a higher temperature accelerates the annealing.

Annealing to multiple minima

[Input: recipes/moleculardynamics/annealV2/]

The annealing process can also be used to find alternative local minina. Here two vacancies in a graphene sheet are heated. Depending on the initial conditions, this system anneals to different final structures. The starting velocities are chosen at random, so depending on the seed value for the random number generator (several different cases are given in the input file) a different final defect geometry is obtained by the same cycle of heating and cooling.