Improved Minimum Mode Following Method for Finding First Order Saddle Points.

The minimum mode following method for finding first order saddle points on an energy surface is used, for example, in simulations of long time scale evolution of materials and surfaces of solids. Such simulations are increasingly being carried out in combination with computationally demanding electronic structure calculations of atomic interactions, so it is essential to reduce as much as possible the number of function evaluations needed to find the relevant saddle points. Several improvements to the method are presented here and tested on a benchmark system involving rearrangements of a heptamer island on a close packed crystal surface. Instead of using a uniform or Gaussian random initial displacement of the atoms, as has typically been done previously, the starting points are arranged evenly on the surface of a hypersphere and its radius is adjusted during the sampling of the saddle points. This increases the diversity of saddle points found and reduces the chances of reconverging on previously located saddle points. The minimum mode is estimated using the Davidson method, and it is shown that significant savings in the number of function evaluations can be obtained by assuming the minimum mode is unchanged until the atomic displacement exceeds a threshold value. The number of function evaluations needed for a recently published benchmark (S. T. Chill et al. J. Chem. Theory Comput. 2014, 10, 5476) is reduced to less than a third with the improved method as compared with the best previously reported results.