In order to be familiar with the method of optimization of force field parameter sets, I did a test with the existed force field parameter sets of AGLC, the corresponding files are top_all22_sugar.inp and par_all22_sugar.inp. The results showed that the dihedral energy functions can be fitted well between ab initio results and charmm results, while the interaction energies between water and AGLC did not give satisfied agreement. So I suspected that I did not calculate the interaction energies correctly, the followed are my calculation process, your responses are eagerly welcome. Thanks a lot in advance!
The empirical calculations followed the examples of Alex's website.
The ab initio calculations went like this:
Firstly, I got the energy E1 of water (with fixed TIP3 geometry) and the energy E2 of optimized AGLC through GAUSSIAN98 with HF/6-31G* basis set. The input file is like this:
#T HF/6-31G* opt
C 1.384 -0.026 -0.056
H 1.666 -0.555 0.870
O 1.833 -0.757 -1.175
Then I calculated the minimized energy E3 of complex of AGLC and water based on fixed relative geometry position. The input file is like this:
#T HF/6-31G* opt=z-matrix
glc H5OHH energy
X 6 R1 5 A1 20 A2
X 25 R1 6 A1 5 A2
O 6 R2 25 A1 26 A3
H 27 R3 6 A4 25 A2
H 27 R3 6 A4 25 A3
The third step is to calculate the interaction energy (E3-(E1+E2))and compare them (scaled by 1.16) with the results of empirical force field calculations.
Is the process is right? Why my results are not as well as those of reported?