Calculation of rotational partition functions by an efficient Monte Carlo importance sampling technique

被引:12
作者
Tafipolsky, M [1 ]
Schmid, R [1 ]
机构
[1] Ruhr Univ Bochum, Lehrstuhl Anorgan Chem 2, D-44780 Bochum, Germany
关键词
partition function; entropy; generalized inertial tensor; hindered rotor; molecular mechanics; Monte Carlo simulations; importance sampling; alkanes;
D O I
10.1002/jcc.20298
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
The evaluation of the classical rotational partition function represented by a configuration integral over all external and internal rotational degrees of freedom of nonrigid chain polyatomic molecules is described. The method of Pitzer and Gwinn is used to correct the classical partition function for quantum mechanical effects at low temperatures. The internal rotor hindrance and all coupling arising from the external and internal rotational degrees of freedom are explicitly taken into account. Importance sampling Monte Carlo based on the adaptive VEGAS algorithm to perform multidimensional integration is implemented within the TINKER program package. A multidimensional potential energy hypersurface is calculated with the MM3(2000) molecular mechanics force field. Numerical tests are performed on a number of small n-alkanes (from ethane to octane), for which the absolute entropies calculated at three different temperatures are compared both with the experimental values and with the previous theoretical results. The application of a more efficient importance sampling technique developed here results in a substantial reduction of statistical errors in the evaluation of the configuration integral for a given number of Monte Carlo steps. Error estimates for the calculated entropies are given, and possible sources of systematic errors, and their importance for a reliable prediction of the absolute entropy, are discussed. (c) 2005 Wiley Periodicals, Inc.
引用
收藏
页码:1579 / 1591
页数:13
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