Figure 9. Larger coarsening factors are available in MLE.
Thus far, the results have focused on the performance of the MLE approximation for a variety of SPME calculations, emphasizing what values of I(mm) and Tcut are necessary to achieve accurate results with a coarsening factor Cyz of 2, but Cyz is itself a tunable parameter of MLE. These plots show the accuracy of the MLE approximation for the streptavidin and cyclooxygenase-2 test cases (in cubic and triclinic unit cells, respectively) for numerous coarsening factors as shown in each diagram. The coarsening factors are limited to common factors of the mesh sizes in the and directions, but we do not expect this to be a serious limitation in practice. In these tests, I(mm) was fixed at 8. The AMBER default SPME paramaters, or the variant with anisotropic interpolation discussed in previous figures and the maint text, were used for the SPME calculations as indicated in each diagram. While larger values of Cyz require larger values of Tcut to produce accurate results, MLE with Cyz as high as 6 can imply modest additional error with Tcut as low as 7 if anisotropic particle ⇋ mesh interpolation is used. As shown in Table 2, a third, intermediate mesh level, typically with Cyz = 2, is helpful for reducing the computation and communication burden of larger values of Tcut, making it possible to efficiently coarsen the reciprocal space mesh in stages.