Hi Lorenzo,
As Dave said, it is better to check the papers about halogen bond modeling. One related reference could be found here: http://pubs.acs.org/doi/abs/10.1021/ct300180w <http://pubs.acs.org/doi/abs/10.1021/ct300180w>.
Kind regards,
Pengfei
> On Apr 27, 2016, at 8:26 AM, David A Case <david.case.rutgers.edu> wrote:
>
> On Wed, Apr 27, 2016, Lorenzo Gontrani wrote:
>>
>> The system is ICH2CN, and I calculated the electrostatic potential ab
>> initio as usual, obtaining these point charges:
>>
>> ESP charges:
>> 1
>> 1 C -0.898387
>> 2 C 0.631150
>> 3 I 0.032933
>> 4 H 0.353024
>> 5 H 0.353045
>> 6 N -0.471765
>>
>> When I use the ordinary RESP method, I get the following result:
>> 4 I1 i M 3 2 1 1.540 111.208 -180.000 -0.113877
>> 5 C1 c3 M 4 3 2 2.208 151.520 -180.000 -0.101590
>> 6 H1 h1 E 5 4 3 1.090 105.533 -58.378 0.140478
>> 7 H2 h1 E 5 4 3 1.090 105.521 58.407 0.140478
>> 8 C2 c1 M 5 4 3 1.447 112.551 -180.000 0.346442
>> 9 N1 n1 M 8 5 4 1.162 179.005 180.000 -0.411931
>>
>> the charge on nitrile carbon is less than half, and the iodine changes
>> sign..
>
> This is what I would expect: the point of the "R" in "RESP" is to avoid large
> charges on atoms that are partially or nearly buried. This is what you are
> seeing here, where C1 goes for -.89 (ESP) to -.10 (RESP), and C2 goes
> from 0.63 (ESP) to 0.35 (RESP). A consequence is that I goes from being very
> slightly positive to slightly negative.
>
> Be sure to look at the literature on modeling halogen bonds with force
> fields. I don't know the details, but most studies find that off-center
> charges or other strategems (beyond simple atom-based fixed charges) are
> needed to get reasonable results.
>
> ....dac
>
>
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Received on Wed Apr 27 2016 - 07:00:03 PDT
