AMBER Archive (2007)

Subject: Re: AMBER: HF/6-31G* ESP Charges do not match with ff02 charges

From: mernst_at_tricity.wsu.edu
Date: Fri Jan 19 2007 - 12:59:52 CST

> Quoting mernst_at_tricity.wsu.edu:
>
>> I do not know what version of NWChem you are using. NWChem at least
>> version 4.5-4.7 does
>> not properly implement RESP, though it can give deceptively
>> good-looking results on some
>> molecules. I will guess you want RESP, not plain ESP, as that's how
>> AMBER charges were
>> assigned. NWChem is not suitable for RESP. I know that NWChem 5.0
>> has significantly
>> changed its RESP charge calculations, but they too were flawed
>> (though in a different
>> way) when I used it in beta. If you want to verify that your
>> software and methods are
>> reasonable, obtain the original RESP papers and confirm that you can
>> reproduce results
>> for simple cases like butane and n-methylacetamide. NWChem seemed to
>> give especially
>> unreasonable results on butane when I was doing my own verifications.
>
> Here, is it not possible to ask to NWChem to only (i) optimize a
> structure (ii) compute the MEP and NOT derive the charges; letting the
> RESP program doing the fitting step to generate RESP or ESP charge
> values ?
>
> Did you try to compare the MEP generated by Gaussian, and that
> generated by NWChem ? Are they identical ? Thanks, Francois
That seems like a reasonable approach. I think it is NWChem's built-in fitting with
restraints/constraints that is bad, not its underlying quantum chemistry. I discovered
RED II as I was looking for the source of the NWChem trouble, and immediately began
using it with GAMESS. I did not take the time to parse the NWChem output for use with
the external RESP program.

Matt Ernst

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