Constraint methods for determining pathways and free energy of activated processes |
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Authors: | J Schlitter |
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Institution: | 1.Ruhr-Universit?t Bochum ND04,Bochum,Germany |
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Abstract: | Activated processes from chemical reactions up to conformational transitions of large biomolecules are hampered by barriers
which are overcome only by the input of some free energy of activation. Hence, the characteristic and rate-determining barrier
regions are not sufficiently sampled by usual simulation techniques. Constraints on a reaction coordinate r have turned out to be a suitable means to explore difficult pathways without changing potential function, energy or temperature.
For a dense sequence of values of r, the corresponding sequence of simulations provides a pathway for the process. As only one coordinate among thousands is
fixed during each simulation, the pathway essentially reflects the system’s internal dynamics. From mean forces the free energy
profile can be calculated to obtain reaction rates and and insight in the reaction mechanism. In the last decade, theoretical
tools and computing capacity have been developed to a degree where simulations give impressive qualitative insight in the
processes at quantitative agreement with experiments. Here, we give an introduction to reaction pathways and coordinates,
and develop the theory of free energy as the potential of mean force. We clarify the connection between mean force and constraint
force which is the central quantity evaluated, and discuss the mass metric tensor correction. Well-behaved coordinates without
tensor correction are considered. We discuss the theoretical background and practical implementation on the example of the
reaction coordinate of targeted molecular dynamics simulation. Finally, we compare applications of constraint methods and
other techniques developed for the same purpose, and discuss the limits of the approach. |
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