Monte Carlo simulation of diffusion and reaction in water radiolysis – a study of reactant `jump through' and jump distances |
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Authors: | R N Hamm J E Turner M G Stabin |
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Institution: | (1) Life Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA, US;(2) Environmental Health Sciences Division, Oak Ridge Institute for Science and Education, Oak Ridge, TN 37831, USA, US |
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Abstract: | 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 l%–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.
Received: 8 September 1997 / Accepted in revised form: 27 October 1997 |
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