(225b) Mapping Saddles and Minima on Free Energy Surfaces Using Multiple Climbing Strings

Locating saddle points on high dimensional free energy surfaces is key in characterizing multistate transition events and associated transition rates in complicated molecular-scale systems. Though finding minima is a routine task, locating saddles and measuring their energies relative to their associated minima remain challenging, especially in high-dimensional spaces. We propose here a multistring version of climbing string method to locate multiple saddles in collective variable space by initiating multiple strings in one go. The multistring method employs dynamic strings to locate saddles which propagate by the gradient flow in the path space, with one end fixed and other end climbing across the free energy profile. The method further introduces the static strings to keep the history of prior strings converged to saddles. The inter-string interactions are used to avoid the convergence of multiple strings to a common saddle. The presence of multiple images along the string helps to ensure the direct connectivity between the minimum and saddles and to compute the free energy along the path. Hence, the climbing multistring method can be used to map the network of directly connected stationary points and their associated activation barrier. We demonstrate this method to locate saddle points in two-dimensional and four-dimensional free energy surface of alanine dipeptide and alanine tripeptide, respectively.

References:

1. Ren, W.; Vanden-Eijnden, E. J. Chem. Phys. 2013, 138, 134105

2. Maragliano, L.; Vanden-Eijnden, E. Chem. Phys. Lett. 2006, 426, 168–175.

*Funding for this project was provided by NIH grant number GM 100472.