Hamm R N, Turner J E, Stabin M G
Life Sciences Division, Oak Ridge National Laboratory, TN 37831, USA.
Radiat Environ Biophys. 1998 Feb;36(4):229-34. doi: 10.1007/s004110050076.
In Monte Carlo simulations of water radiolysis, the diffusion of reactants can be approximated by "jumping" all species randomly, to represent the passage of a short period of time, and then checking their separations. If, at the end of a jump, two reactant species are within a distance equal to the reaction radius for the pair, they are allowed to react in the model. In principle, the possibility exists that two reactants could "jump through" one another and end up with a separation larger than the reaction radius with no reaction being scored. Ignoring this possibility would thus reduce the rate of reaction below that intended by such a model. By making the jump times and jump distances shorter, any error introduced by 'jump through' is made smaller. This paper reports numerical results of a systematic study of 'jump through' in Monte Carlo simulations of water radiolysis. With a nominal jump time of 3 ps, it is found that more than 40% of the reactions of the hydrated electron with itself and of the H atom with itself occur when reactions during 'jump through' are allowed. For all other reactions, for which the effect is smaller, the contributions of 'jump through' lie in the range 1%-16% of the total. Corrections to computed rate constants for two reactions are evaluated for jump times between 0.1 and 30 ps. It is concluded that jump-through corrections are desirable in such models for jump times that exceed about 1 ps or even less. In a separate study, we find that giving all species of a given type the same size jump in a random direction yields results that are indistinguishable from those when the jump sizes are selected from a Gaussian distribution. In this comparison, the constant jump size is taken to be the root-mean-square jump size from the Gaussian distribution.
在水辐射分解的蒙特卡罗模拟中，反应物的扩散可以通过随机“跳跃”所有物种来近似，以表示短时间的流逝，然后检查它们之间的距离。如果在一次跳跃结束时，两种反应物物种之间的距离等于该对物种的反应半径，则它们在模型中可以发生反应。原则上，存在两种反应物可能“相互穿过”并最终分离距离大于反应半径而未记录任何反应的可能性。忽略这种可能性会使反应速率低于该模型预期的速率。通过使跳跃时间和跳跃距离更短，“穿过”引入的任何误差都会更小。本文报告了在水辐射分解的蒙特卡罗模拟中对“穿过”进行系统研究的数值结果。在标称跳跃时间为3皮秒的情况下，发现当允许“穿过”过程中的反应时，水合电子自身以及氢原子自身的反应中有超过40%会发生。对于所有其他影响较小的反应，“穿过”的贡献占总数的1% - 16%。针对0.1到30皮秒之间的跳跃时间，评估了对两种反应计算得到的速率常数的校正。得出的结论是，对于超过约1皮秒甚至更短的跳跃时间，在这样的模型中进行“穿过”校正很有必要。在另一项研究中，我们发现给给定类型的所有物种在随机方向上赋予相同大小的跳跃所产生的结果与从高斯分布中选择跳跃大小时的结果无法区分。在这种比较中，恒定跳跃大小被取为高斯分布的均方根跳跃大小。
