Simulation techniques free energy methods

The titration coordinates evolve along with the dynamics of the conformational degrees of freedom, r, in simulations with GB implicit solvent models [37, 57], An extended Hamiltonian formalism, in analogy to the A dynamics technique developed for free energy calculations [50], is used to propagate the titration coordinates. The deprotonated and protonated states are those, for which the A value is approximately 1 or 0 (end-point states), respectively. Thus, in contrast to the acidostat method, where A represents the extent of deprotonation, is estimated from the relative occupancy of the states with A 1 (see later discussions). The extended Hamiltonian in the CPHMD method is a sum of the following terms [42],... 

Also in the early 1990s, a somewhat related method for calculating free energy differences was proposed by Ron Elber and coworkers [92, 93]. It relies on simulating multiple, noninteracting replicas that differ only locally. As a result, the method is applicable to systems that undergo only local modification - e.g., point mutations in proteins. For this reason, it has been called the locally enhanced sampling (LES) technique. 

In this chapter, we will examine in depth the characteristic errors of two free energy techniques and present improved methods based on a better understanding of their behavior. The two techniques examined are free energy perturbation (FEP) [2] and nonequilibrium work (NEW) based on Jarzynski s equality [3-6]. These techniques are discussed in Chaps. 2 and 5. The FEP method is one of the most popular approaches for computing free energy differences in molecular simulation see, e.g., [1, 7-10]. The recently developed NEW method, which is closely related to FEP, is gaining popularity in both simulation [11-18] and experimental applications [19-21],... 

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