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arXiv 1107.2967cond-mat.stat-mechphysics.chem-phphysics.comp-ph
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使用非平衡涨落定理理解和纠正平衡和非平衡离散朗之万动力学模拟中的错误

Using nonequilibrium fluctuation theorems to understand and correct errors in equilibrium and nonequilibrium discrete Langevin dynamics simulations

David A. Sivak, John D. Chodera, Gavin E. Crooks

摘要:

计算模拟朗之万动力学的常用算法必须离散运动的随机微分方程。这些有限时间步长积分器必然有几个共同的实际问题:微观可逆性被违反,采样的平稳分布不同于所需的平衡分布,以及在非平衡模拟中积累的工作不能直接用于基于非平衡工作定理的估计。在这里,我们表明,即使与时间无关的哈密顿量,有限时间步长朗之万积分可以被认为是一个驱动,非平衡物理过程。一旦定义了一个适当的类功量--这里称为影子功--最近发展起来的非平衡涨落定理就可以用来测量或校正由于使用有限时间步长而引入的误差。特别是,我们证明了修改估计的基础上非平衡工作定理,包括这个影子工作删除的时间步长依赖于自由能的估计误差。我们还量化,第一次,采样的静态分布和所需的平衡分布之间的偏差的大小,平衡Langevin模拟溶剂化系统的大小不同。虽然这些偏差可能很大,但它们可以通过Metropolization完全消除,或者通过时间步长的小幅减少而大大减少。通过与驱动过程的这种联系,非平衡涨落定理的进一步发展可以为处理有限时间步长积分器中的误差提供额外的分析工具。

英文摘要:

Common algorithms for computationally simulating Langevin dynamics must discretize the stochastic differential equations of motion. These resulting finite time step integrators necessarily have several practical issues in common: Microscopic reversibility is violated, the sampled stationary distribution differs from the desired equilibrium distribution, and the work accumulated in nonequilibrium simulations is not directly usable in estimators based on nonequilibrium work theorems. Here, we show that even with a time-independent Hamiltonian, finite time step Langevin integrators can be thought of as a driven, nonequilibrium physical process. Once an appropriate work-like quantity is defined -- here called the shadow work -- recently developed nonequilibrium fluctuation theorems can be used to measure or correct for the errors introduced by the use of finite time steps. In particular, we demonstrate that amending estimators based on nonequilibrium work theorems to include this shadow work removes the time step dependent error from estimates of free energies. We also quantify, for the first time, the magnitude of deviations between the sampled stationary distribution and the desired equilibrium distribution for equilibrium Langevin simulations of solvated systems of varying size. While these deviations can be large, they can be eliminated altogether by Metropolization or greatly diminished by small reductions in the time step. Through this connection with driven processes, further developments in nonequilibrium fluctuation theorems can provide additional analytical tools for dealing with errors in finite time step integrators.

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