Subject: RE: CCL:M:? re partial atomic charges

From: "Sal Profeta, Jr. [SP4087]" <[email-masked]>
Date: Wed, 10 Jan 96 23:46:01 -0500

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Recently, there has been a question regarding establishing "partial charge" 
values for lone pairs on atoms such as phenolic oxygens, in the context of 
force field calculations. 

This is actually an old but still bothersome aspect to dealing with FF 
calculations which explicitly use lone pairs. As the individual who 
parameterized MM2 for aliphatic nitrogen systems, I faced a similar problem 
20 years ago.

Peter Shenkin's suggestion to evaluate the bond moment and proceed from 
there is a conservative but effective means to attack the problem. I have 
used this also to parameterize MM2 for quaternary amines, relying on ESP-
derived charges from MP2/6-31G**//6-31G** or similar calculations to do the 
bond moment scaling. Presumably, this can be done as readily with MOPAC ESP 
charges. There may, in fact, be programs to do this automatically out there 
- I know that several years ago, Chandra Singh, when he was at Scripps 
worked on such a scheme. 

Others who may have also done so include the compchem folks at many of the 
large drug firms, like Merck, Pfizer, Lilly, Vertex (Mark - do you have 
one?) - who may be able to share this 'piece' of software with the academic 
community. Also, the folks at the SW vendors, such as Tripos, Biosym/MSI or 
SPARTAN might have a solution. 

Hope this helps.

Cheers,

Sal Profeta, Jr.

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