Subject: Re: CCL:MM2 parameters

From: [email-masked] (Per-Ola Norrby)
Date: Mon, 30 Sep 1996 10:25:33 +0200

Chaya Duraiswami wrote:
>Hi! I would like to have MM2 torsion parameters (i.e. v1,v2, & v3
>values) for the following bonds:
>
>Csp2 - Csp3 - Csp - Csp
>Osp3 - Csp3 - Csp - Csp
>Csp3 - Csp - Csp - Csp2
>Csp2 - Csp2 - Csp - Csp
>Osp3 - Csp2 - Csp - Csp
>Csp2 - Osp3 - Csp2 - Csp

        All except the last one are linear so even though the program
requires parameters, they should be set to 0.0.  You can't very well rotate
around a bond when one of the central atoms is sp, can you ?-)

        Note that if you are using a "real", modern MM2, with
pi-calculations, you may encounter a nasty bug here.  Some interfaces set
up the calculation so that the sp-carbons are considered conjugated.  Since
the torsion is used in determining overlap, you will get a division-by-zero
error for linear atoms, which will cause the program to crach.  The
workaround is to exclude the sp-carbons from the list of conjugated atoms.
MM2 was, as far as I know, parameterized for non-conjugated acetylenes
anyway.

        For the last parameter, shouldn't you rat´her use the special
furan-O atom type?  I believe it's 41 or something close.

        Best Regards,

        Per-Ola Norrby

        PS.  If you are NOT using Allingers MM2, you should specify the
source, as there are many MM2 implementations out there, not all as good as
the original.

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 *  Per-Ola Norrby
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