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mona

Member

Registered: 2026-09-28

Posts: 2

RESP charges with equivalence constraints and charge constraint

Dear developers,

Thank you for providing Multiwfn and so many examples how to use it.
I would like to determine RESP charges for a larger molecule to build the topology for a MD simulation with the AMBER force field. To this end, I built fragments of the molecule with methyl caps. For each fragment, I would like to constrain the total charges of the caps to zero, but I also want to use equivalence constraints on other methyl- or CH2-groups in the same fragment. As I understand, when I use the standard 2-stage RESP fit (7 18, loading the equivalence and charge constraint files with 5 1 and 6 1, then 1), the total charge of the cap methyls is not zero in the end, because the constraints are only applied in the first stage. So my idea is to do the following:
1) One stage fitting without equivalence constraint but charge constraint (methyl caps: 0) (7 18 5 0 6 1 2)
2) Again one stage fitting, with tighter parameter a, charge constraints: heavy atoms except the ones used in equivalence constraint file and except the carbons with equivalent hydrogens = results from first-stage, cap methyls = zero; equivalence constraint: all non-cap methyl-Hs, Hs of CH2 groups and other equivalent atoms (n 4 2  0.001 0 5 1 6 1 2 y)
Could you please confirm if this is the right way to do it?

Thank you for your help.

Last edited by mona (2026-09-28 08:54:50)

sobereva

Tian Lu (Multiwfn developer)

From: Athens, Greece

Registered: 2017-09-11

Posts: 2,289

Website

Re: RESP charges with equivalence constraints and charge constraint

Hello,

It looks reasonable, please just try it.

mona

Member

Registered: 2026-09-28

Posts: 2

Re: RESP charges with equivalence constraints and charge constraint

Thank you!