On Wed, Sep 21, 2016, Trang Nguyen wrote:
>
> I've been noticed that in many of my MM-PBSA calculations, the
> electrostatic and solvation energy are compensated with each other; i.e.,
> when one is negative, the other would be positive with relatively close
> absolute values. It's pretty straightforward to explain why the
> electrostatic energy is negative would lead to positive solvation energy.
> But I'm not quite sure about the positive electrostatic and negative
> solvation energy. Any comment or explanation in the later situation is
> appreciated.
Your observation is correct. Think about it this way: solvation screens
electrostatic interactions between atoms. So the electrostatic interaction
terms in the gas-phase are very much smaller in solution, which implies that
the solvation energy is close to being the opposite of the electrostatic
energy. Doesn't matter whether the gas-phase term is positive or negative:
the solvation term will always be of the opposite sign.
This analysis ignores the "self-energy" terms, where i=j, which reflect the
penalty for burial of charges. There is no such contribution in the
gas-phase, but a significant contribution to the solvation free energy. This
means that the anti-correlation between electrostatics and solvation energy is
less perfect that it would have been for just the cross terms.
...dac
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Received on Wed Sep 21 2016 - 05:00:02 PDT
