I have a question about pair interaction energy calculations.

From: 송 연호 (syh6076_at_kangwon.ac.kr)
Date: Wed Oct 30 2013 - 02:14:32 CDT

 
Dear all
 
Hello.
 
I have a question about pair interaction energy calculations for my system that is composed of one K+, 80 DMPC lipids and several thousand water molecules.
 
I have calculated the pair interaction energy between +1 charged atom (K+ ion) and -0.6 charged atoms (N atoms at head group of 80 DMPC lipids), Distance between two molecules is approximately 25 Å. The calculation shows that electrostatic energy is nearly +128 kcal/mol/Å.
 
I have also calculated the pair interaction energy between +1 charged atom (K+ ion) and total +0.4 charged groups ((CH3)3 groups at head group of 80 lipids), Distance between two molecules are nearly 25 Å. The resulting electrostatic energy is around -256 kcal/mol/Å.

I don’t understand these two results. I'm wondering why the electrostatic interaction energy between + and - charged atoms has a positive value and the one between + and + charged groups has a negative value

The net-charge of my system is +1 (because of a k+ ion) and I use PME parameters. The force field that I used is CHARMM27 for all the atoms.
 
Thanks in advance
Yeonho Song.

This archive was generated by hypermail 2.1.6 : Wed Dec 31 2014 - 23:21:50 CST