Michael R. Salazar, Union University
A suite of programs called Accelerated Molecular Dynamics with Chemistry (AMolDC) has been written, tested, and employed in order to perform adaptive, multilevel QM/MM simulations for complex chemical processes in the gas-phase. A paper that examines the properties of this new method was published recently.1 The method is formulated to give a time-dependent, multilevel representation of the total potential that is derived from spatially-resolved quantum mechanical (QM) regions. Ref. 1 shows that the AMolDC method scales linearly with system size due to the fact that at a constant temperature and pressure, the average system size will remain approximately constant regardless of the number of atoms in the simulation.
Fig. 1 Accuracy in the interpolant as a function of underlying grid density
Fig. 2 Computational studies of the AMolDC program on hydrated organic clusters.
Second, AMolDC was rewritten to submit the QM jobs in parallel and timing tests were performed. Shown in Figure 1 is a log-log plot of CPU time for various system sizes of small condensed phase system of water, n-propanol (np), cyclohexanol (ch), and orthoxylene (ox). The systems varied in sizes from 1 molecule of each to 6 molecules of each, giving system sizes from 52 atoms to 312 atoms. The parallel QM calculations were submitted over an 18 node cluster. Figure 1 shows the tremendous cost savings associated with performing interpolations (symbols) over performing parallel QM calulations with no interpolations (lines). Levels of theory from B3LYP with cc-pVTZ basis sets to HF with 6-31G** basis sets were used for the various groups formed in these simulations. Figure 1 also demonstrates that system sizes of multiple hundreds of atoms may be studied with total CPU times of about 6 hours while interpolating on DFT and HF potential and force surfaces.
Two manuscripts have been written and submitted to the Journal of Chemical Physics. The first manuscript is on the interpolation module, how to perform accurate and fast interpolations for large chemical systems. The second paper is using this interpolation methodology within AMolDC to perform computational studies of small hydrated organic clusters. The papers were submitted together as back-to-back publications in the same issue.
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