Subject: Re: dipolar moments

From: [email-masked] (Joel Polowin)
Date: Fri, 2 Aug 96 16:44:59 -0400

> From: Massimo Trotta <[email-masked]>
> Date: Thu, 1 Aug 1996 12:21:01 +0200 (MET DST)
> 
> 	I'm studying the dipolar moments of some organic molecules in the
>  ground and first excited states using semi-empirical calculations
> (on Hyperchem 4.0) and I have a couple of question regarding those calculations:
> a) I decided to use ZINDO/1 as semiempirical method (available methods are
> CNDO, INDO, MINDO3, MNDO, AM1, PM3, ZINDO/1 and ZINDO/s), but the choice was 
> not made on how appropriate is the method for evaluating dipolar moment. Am I
> doing right? is there any reason for choosing this or that SE method for the
> dipolar moment calculations?

The advantage of ZINDO/1 is that it is parameterized for transition metals.
For general work on organic compounds, you are probably better off using
AM1 or PM3.  For dipole calculations, AM1 is likely to give slightly
better accuracy, on average.  This may not be true for a particular set
of molecules which are all rather similar but have some relatively minor
structural differences.

> b) In evaluating the dipole moment in the excited state I select the option
> _Next Lowest_ in the SE option dialog box. Should I work with _Unrestricted 
> Hartree Fox_ to get more accurate number or _Restrictes HF_ gives the same
> accuracy as well?.

For closed-shell systems you should use RHF; for open-shell systems you
should use UHF.  Please note that "Lowest" and "Next Lowest" refer to the
specified multiplicity -- for example, the lowest and second-lowest
singlet state -- and not to all states.  For a molecule whose ground
state is a singlet, the first excited state is probably "Lowest" of
multiplicity 3, not "Next Lowest" of multiplicity 1.

> c) How can I display, in hyperchem or with some other software (freeware)
> the dipolar moments as spacial vectors overlapped on the molecules?

This cannot be done with current versions of HyperChem -- at least, not
easily.  I suppose that you could get the dipole vector from HyperChem
by using the "dipole-moment-components" script command, or from a log
file, and use those values to specify coordinates for the atoms in a
diatomic molecule.  One atom could be placed at coordinates in Angstroms
given by the dipole components in Debyes, and the other atom at the
coordinates multiplied by -1; the original molecule could be centred on
the origin, and then the new diatomic molecule would show the dipole.

Joel

------------
Joel Polowin, Ph.D.   Manager, Scientific Support
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