SCCDFTB as a bridge between MM and high-level QM

Jan Hermans, hermans@med.unc.edu, Department of Biochemistry and Biophysics,
University of North Carolina, Chapel Hill, NC 27599-7260

Is the greater effort required for use of SCCDFTB justified? Initial results
with crambin and Ace-Ala-NMe showed improvement relative to standard MM. Was
improvement to be expected? Probably, yes: both SCCDFTB and MM force fields have
been calibrated against high-level QM and features such as, varying charge
distribution, should give the former the edge. However, MM force fields are
calibrated also against experimental data. Thus, we have not yet found an
SCCDFTB-based representation of water that is more effective than the
simple-point-charge models. --- Two systems show superiority of SCCDFTB over MM:
torsion of N-methyl-acetamide (NMA) affects the geometry at N; the H..O=C
angle of hydrogen-bonded dimers of NMA varies more widely than with MM. Finally,
simulations with SCCDFTB can be used to produce a sample against which to fit MM
parameters. Energies are recalculated with a high-level QM program, thus the MM
force field is calibrated at that level. These samples can be used also to
compare different formulations of the force field.