Subject: Re: CCL:PCM calculations..

From: Per-Ola Norrby <[email-masked]>
Date: Tue, 21 Oct 1997 08:58:21 +0200

Dear Qiang Cui,

>Hi, folks. Got a few questions concerning PCM for solvation effect,
>I'd appreciate any comments from the experts ouut there.

>2. In a recent CPL paper by Tomasi et al., a new implementation of PCM,
>the so-called PCM/DIR has been reported. ANy one has any experience with
>it? or at least how to get it?

	I've been using it, I can recommend it highly.  The problem is
getting it...  The only way I know is to contact Tomasi directly.  For a
while they gave it away freely, now I'm not so sure.  In my experience,
it's a lot more accurate than all the PCM implementations in Gaussian94.  I
have some cases with relative aqueous pKa for ionic systems (not yet
published) where PCM/DIR was within a pKa unit whereas SCIPCM was 8 pKa
units off.  Of the methods we've been interested in, only the
Cramer-Truhlar SM5 yielded an accuracy comparable to PCM/DIR.

>3. Is there any new development in the gradient calculations with PCM
>recently? Or, any other methods has analytical gradients? (besides
>Truhlar's g-Born and Troung's GCOSMO)

	The latest version of PCM/DIR includes analytical gradients.  We've
tested it, it's OK but not perfectly stable.  We're trying it for
zwitterionic systems, that may be stretching it a bit...

>4. In several papers by Tomasi et al.,the SCIPCM of Frisch et al. was
>shown to be not as accurate for charged solute as for neutral ones, I
>wonder if any one has similar experience on this issue?

	That's general for any solvation model.  PCM/DIR seems to be better
than most in this respect (see above).

>5. Finally, I'd appreciate any comments on solvation effect calculations
>of highly charged systems.

	See above, any ionic system is problematic, and anions are
generally worse than cations.  PCM/DIR accounts to some extent for the
tailing of electron density outside the cavity, it alleviates some of the
problems.  There are some indications you should use smaller basis sets for
the solvation calculations, at least not use diffuse functions.  See, for
example, T. Clark, Faraday Discuss. Chem. Soc., 1984, 78, 203-212.

	Regards,

	Per-Ola Norrby

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 *  Per-Ola Norrby, Associate Professor
 *  The Royal Danish School of Pharmacy, Dept. of Med. Chem.
 *  Universitetsparken 2, DK 2100 Copenhagen, Denmark
 *  tel. +45-35376777-506, +45-35370850    fax +45-35372209
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