Genetic algorithms (GA) [31, 32, 64] form another class of well-known stochastic methods.
The idea of the GA stems from Darwin’s theory of evolution. Degrees of freedom of the
ligand are encoded as binary strings called genes. These genes make up the ‘chromosome’
which actually represents the pose of the ligand. Mutation and crossover are two kinds of
genetic operators in GA. Mutation makes random changes to the genes; crossover exchanges
genes between two chromosomes. When the genetic operators affect the genes, the result is
a new ligand structure. New structures will be assessed by scoring function, and the ones
that survived (i.e., exceeded a threshold) can be used for the next generation. Genetic
algorithms have been used in AutoDock [31], GOLD [65], DIVALI [66] and DARWIN
[67].
Molecular dynamics (MD) [68-70] is widely used as a powerful simulation method in many
fields of molecular modeling. In the context of docking, by moving each atom separately in
the field of the rest atoms, MD simulation represents the flexibility of both the ligand and
protein more effectively than other algorithms. However, the disadvantage of MD
simulations is that they progress in very small steps and thus have difficulties in stepping
over high energy conformational barriers, which may lead to inadequate sampling. On the
other hand, MD simulations are often efficient at local optimization. Thus a current strategy
is to use random search in order to identify the conformation of the ligand, followed by the
further subtle MD simulations.