Powell algorithm, similar to Simplex and Dud algorithms, is also an optimization method, which does not require any derivative of the cost function being minimized. In this method, for minimizing a function with N parameters, a user should provide an initial parameter guess as well as N search vectors. The method transforms the multi-dimensional minimization problem into a sequence of one-dimensional minimization problem. It is also an iterative procedure. At each iteration, a new parameter guess is determined by a sequence of line searches, starting from the previous parameter guess. The new parameter guess therefore can be written as a linear combination of all search vectors. Afterwards, the search direction which gives the largest impact is replaced by a new search direction. The iteration is performed until convergence. The line search implemented in OpenDa is the so called Brent's method.