the fit when the number of terms is varied from 91 to 325 (Pearsons r changes from 0.94 to 0.96 in a typical case), indicating that 91 terms suffice to reproduce the ab initio shift surface appropriately. Since this functions relate directly the geometry
K13C is a force constant, 13Cdobs is the experimental chemical shift and
13Cd(f,y)calc is the chemical shift computed through the empirical equation described above. Since energy minimization and molecular dynamics algorithms compute forces from this pseudo-energy term (first derivatives), its inclusion in simulations simply g
E13Cd = K13C * ( 13Cdobs ñ 13 Cd(f,y)calc ) 2
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