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Sum and density of states of polyatomic systems with hindered rotors
Author(s) -
Forst Wendell
Publication year - 1996
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/(sici)1096-987x(199606)17:8<954::aid-jcc4>3.0.co;2-q
Subject(s) - inversion (geology) , partition (number theory) , quantum , statistical physics , simple (philosophy) , partition function (quantum field theory) , quantum mechanics , mathematics , physics , combinatorics , paleontology , philosophy , epistemology , structural basin , biology
The density or sum of states for a collection of independent oscillators, free rotors, and one‐dimensional hindered rotors is obtained with good accuracy by numerical inversion of the corresponding total partition function by the method of steepest descents. The hindered‐rotor partition functions are used in both classical and quantum forms, the latter in the approximation proposed by Truhlar [ J. Comput. Chem., 12, 266 (1991)]. The numerical inversion compares well with analytical results obtained in a simple artificial case and also with an exact count of states in a large ethane‐like system. Inversion of the hindered‐rotor classical partition function is shown to lead to a somewhat different energy dependence of the sum or density of states, relative to the quantum counterpart, which is considered to be a more realistic representation. The routines presented are simple and fast enough to be of use in microcanonical rate calculations. © 1996 by John Wiley & Sons, Inc.