Subject: Re[2]: CCL:Re[2]: how to calculate proton dissociation energy in GAUSSIAN03

From: Qiang Lu <qiangl/at/uci.edu>
Date: Tue, 29 Jun 2004 15:35:25 -0700

Hi Errol,

   Sorry, I made a mistake. One structure is HIS/CYS3-/ZN2+ and the
other one is HIS/CYS3/ZN2+. The only difference is the three hydrogen
which are dissociated from thiol groups. HIS contains imidazole group.
The energy with CYS3- is -3517.614525 au and with CYS3 is -3516.510491 au.
The energy is the sum of electronic and thermal Enthalpies. They have
the same basis set. I will ask the professor about the triplet issue.

-- 
Best regards,
 Qiang                            mailto:qiangl/at/uci.edu

=================Original message text===============
2004 June 29

hello,

If by energy you mean just the total energy given at the end of a 
calculation(this is electronic energy plus internuclear repilsion), like 
-293.173976 or whatever, then generally the bigger the molecule the more 
negative this is (experiment with a few small molecules; bigger basis 
sets also give more negative energies). So Cys3 etc should have a more 
negative (lower, bigger only if you ignore the minus sign)  energy than 
Cys etc. I can't really answer the spin question since I don't know 
exactly what rour structure looks like; I suggest you discuss it with 
one of the professors. But I don't think it should be triplet.

Good luck.

EL
====

Qiang Lu wrote:

>Hi Errol,
>
>   I did a optimization on both structure. The final results show that
>CYS3-/ZN2+ has higher energy than CYS/ZN2+. That seems not reasonable.
>I also tried CYS2-/CYS/ZN2+ and CYS-/CYS2/ZN2+, but their final structure
>are no longer as good a tetrahedral structure as that in the crystal
>structure. So I guess it might be due to the spin. There is a fourth
>ligand HIS binds to ZN2+. Triplet spin caused by the Nitrogen in the
>imidozole group could give a different result from that of singlet spin.
>I find this in the reference J. Phys. Chem. B, vol. 103, No. 41, 1999.
>But I don't know how to determine when it should be triplet and when it
>should be singlet in the QM calculation other than using final energy.
>Or in another way, I don't know how the whole structure become triplet state.
>
>  
>

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