Internal coordinate Quantum Monte Carlo

Increases in computer power and improvements in algorithms have greatly extended the range of applicability of classical molecular simulation methods. In addition, the recent development of Internal Coordinate Quantum Monte Carlo (ICQMC) has allowed the direct comparison of classical simulations and quantum mechanical results for some systems. In particular, it has provided new insights into the zero point energy problem in many body systems. Classical studies of non-linear dynamics and chaos will be compared to ICQMC results for several systems of interest to nanotechnology applications. The ramifications of these studies for nanotechnology applications will be discussed. 

The recent development of internal coordinate quantum Monte Carlo has made it possible to directly compare classical and quantum calculations for many body systems. Classical molecular dynamics simulations of many body systems may sometimes overestimate vibrational motion due to the leakage of zero point energy. The problem appears to become less severe for more highly connected bond networks and more highly constrained systems. This suggests that current designs of some nanomachine components may be more workable than MD simulations suggest. Further study of classical-quantum correspondence in many body systems is necessary to resolve these concerns. 

A series of Monte Carlo computer simulation studies of the structure and properties of molecular liquids and solutions have recently been carried out in this Laboratory.The calculations employ the canonical ensemble Monte Carlo-Metropolis method based on analytical pairwise potential functions representative of ab initio quantum mechanical calculations of the intennolecular interactions. A number of thermodynamic properties including internal energies and radial distribution functions were determined and are reported herein. The results are analyzed for the structure of the statistical state of the systems by means of quasicomponent distribution functions for coordination number and binding energy. Significant molecular structures contributing to the statistical state of each system are identified and displayed in stereographic form. 

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