Why the Lennard−Jones parameters E for anions are usually larger than cations?

From: bay__gulf618_at_sina.com
Date: Mon Jun 15 2015 - 04:02:44 CDT

Hi everybody :
I am simulating the solution systems comprising water and ions. And I note that in most popular classical forcefield, the Lennard−Jones parameters E (or \eps) for anions are usually larger than cations. And the adsorption energy (on metals or graphene) is also higher for anions than cations.
Is there some different treatment between cations and anions when optimize the parameters? or some physical nature lies on the results?
Thank you very much.

Table. L−J parameters E in (kcal/mol)
  Na K Cl
charmm -0.0469 -0.087 -0.15
amber -0.00277 -0.000328 -0.1
opls_aa -0.0028 -0.0003 -0.118

Best regards,
J. Liu

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