Unusual point charges in charge shifting method

Dear developers,

I’m using Tcl-ChemShell to perform QM/MM tasks combining ORCA and DL_poly. The charge shifting method is used, therefore, additional point charges are expected to be found at the end of “pointcharges.xyz” file for ORCA.

I recently noticed that these additional point charges seem so large. For example:

I cut the amide bond -CO-NH- and two point charges should be placed before and after H atom. Their net charge should depend on the charge of N atom, which is -0.4517 in ff14SB force field. There are two MM2 atoms connected to N atom (MM1). The additional point charges near H atom should be:

q(H) = q(N)/2 = 0.4517/2 = 0.2258

However, the printed value in “pointcharges.xyz” is 1.732, which is obviously too large for MM charges. Similar conditions are found in many cases. The largest one I found reached 3.05.

I doubt if there is something wrong with related code. Or is there any special treatment in Tcl-ChemShell?

When you use charge shift method, the charge is shifted from the MM atom (say M1) involved in the cut to it adjacent atom(s) (say M2) and in doing so a dipole is created along the M1-M2 which needs to be compensated. The extra charges in the point charge file are to compensate for dipole, not the shifted charge which is located on the adjacent atom(s).
Ref: QUASI: A general purpose implementation of the QM/MM approach and its application to problems in catalysis

It is highly recommended to cut a C-C bond or similar neutral bonds rather than bonds across elements with different electronegativity - that will cause different charge distribution when cut giving rise to large dipoles. As could be in your case.

Hope this answers your question.