RESP charge fitting suffers from ill-conditioning of the linear equations,
which means that you may get "degenerate" solutions for the atomic charges.
Some might be unphysical.
Usually, AM1 charges are fine. Or I should say they are compatible with
existing AMBER force fields.
If you REALLY want to use ESP-based charge, you can use our dRESP charge
fitting method ( doi: 10.1002/jcc.23208) taking AM1 charge as an initial
guess.
Or you can hack the resp code by replacing LU decomposition with SVD
analysis and setting the null space to zero.
Good luck,
Ye
On 10/27/2015 04:38 PM, Jérémie Knoops wrote:
> Dear Amber users,
>
> I'm trying to determine the charges for phenylguanidinium.
> I normally use the RED servers (and very often multiconfigurational fit)
> but the q4md website is still down.
> Using RESP and Gaussian with one conformation (minimized structure), I get
> too high absolute values for them (up to 1.1 for a carbon atom,
> see GdPhenyl_resp.mol2).
> It's impossible to parametrize the molecule with those charges, the
> equilibrium bond length is 0.06 Ĺ shorter than the reference bond length.
>
> I tried the am1-bcc scheme, also with one conformation and the charges
> seem reliable (see GdPhenyl_am1-bcc.mol2). I can parametrize the torsion
> with them.
> Do I have to use the am1-bcc charges? Are they really reliable? Are there
> other options?
>
> Thank you for your advice.
>
> *Jérémie Knoops*
>
> *PhD StudentLaboratory for Chemistry of Novel Materials
> <http://morris.umh.ac.be/default.aspx>*
> *University of Mons* <http://portail.umons.ac.be/en2/>
>
>
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Received on Tue Oct 27 2015 - 03:30:03 PDT
