Thanks Dave and Pengfei for the very valuable info!!
On 04/27/2016 03:39 PM, Pengfei Li wrote:
> 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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--
==========================================
Lorenzo Gontrani
Research associate - EDXD group
University of Rome "La Sapienza"
ResearcherID: L-6061-2014
GSM +39 338 7615798
Email lorenzo DOT gontrani AT uniroma1 DOT it
Webpage: http://webcaminiti/gontrani.html
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Received on Wed Apr 27 2016 - 07:00:04 PDT
