I am simulating SPC/E water model. I assigned charges with fix qeq method instead of set command. I observed several issues I am not understanding.
- I used fix qeq/point to assign charge to 1 molecule of water without invoking any pair potential for run 0. The charge on O and H were -0.2 and 0.1, respectively for this simulation. And, after NVE dynamics run with shake, the potential energy was -13 Kcal/mol. The parameter files are from Goddard’s (1991) http://pubs.acs.org/doi/pdf/10.1021/j100161a070 paper,
1 8.741 13.364 0 0 0
2 4.528 13.89 0 0 0
- I used fix qeq/point to assign charge on O and H. But, this time I used chi and eta parameters from ffield.reax.CHO file. I multiplied eta by 2. The charge on O and H were -0.125 and 0.0625, respectively, and the potential energy was -5 Kcal/mol.
param file (ffield.reax.CHO):
1 8.5 17.9978 0 0 0
2 5.32 14.87 0 0 0
- I used set command to assign charges O=-0.8476 and H=0.4238. After NVE dynamics run with shake, the potential energy was -211 Kcal/mol.
According to Goddard’s paper, charge on H atom is 0.35, even then it’s well below SPC/E model’s charge. Am I applying fix qeq incorrectly? Potential energy is very low in method (1) and (2) compared to method 3. If we take, SPC/E model’s charge as reference point, the first two methods are clearing giving me wrong answers.
Here’s my input file: