Shifted potential, truncation method

In this paper we use the quaternion method of Evans and Murad to study methane, and report preliminary results using a truncated and shifted site-site Lennard-Jones potential. The multiple time step method was not used in this work, principally because of storage limitations on the PDF 11/70 minicomputer used. The potential model studied involves five Lennard-Jones sites for each molecule, located on the C and H nuclei (C-H bond length 1.10 A). Thus the pair potential is the sum of 25 interactions, 1 between C C, 8 between C...H, and 16 between H,.H... 

The small changes to the force and potential energy, resulting from the change to a shifted force potential function, provide improved accuracy eind stcibility in computer simulations, with negligible changes to structure and time correlation functions, at short to moderate times, calculated from the usual truncated potential. Corrections to calculated thermodynamic properties to account for modifications to the potential can be calculated by a perturbation method similar to that used for the long tail corrections. This matter will be discussed in detail in a separate paper. 

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