Subject: Re: CCL:force constant and zmatrix

From: Joerg Grunenberg <Joerg.Grunenberg~at~tu-bs.de>
Date: Tue, 30 Mar 2004 09:13:18 +0200

Hello Guanglei,

the literature is full of coordinate-dependent force constants.

Since force constants are the energy second derivatives they are 
arranged in a matrix containing both, non-zero diagonal and non-zero 
off-diagonal elements. Changing the coordinate system means changing the 
coupling constants. There are two ways to solve the problem:

1) diagonalize your force constant matrix. You will get unique force 
constants but they will describe delocalized  displacements...
or
2) build the inverse of your force constant matrix. You will get unique 
force constants in a compliance matrix.

See for example:

J. Grunenberg, N. Goldberg, J. Am. Chem. Soc. 2000, 12, 6045
J. Grunenberg, R. Streubel, G. v. Frantzius and W. Marten  J. Chem. 
Phys. 2003, 119

Greetings,

Joerg

Guanglei Cui schrieb:

> Hi,
>
> I tried some frequency calculation with Gaussian03 to obtain the force 
> constants of certain internal degree of freedom. However, the results 
> depended greatly on how the input z-matrix is defined. My question is 
> why this projection is z-matrix-definition dependent, and what is the 
> best way to do such calculations. Thanks in advance.
>
> Guanglei
>
-- 
Jvrg Grunenberg, Org. Chemie, Hagenring 30, 38106 Braunschweig
email: Joerg.Grunenberg~at~tu-bs.de phone: +49 531 391 5252 
URL: http://www.oc.tu-bs.de/Grunenberg

---

From: hinsen!at!cnrs-orleans.fr
Date: Tue, 30 Mar 2004 08:59:40 +0200

> best way to do such calculations. Thanks in advance.
Force constants are partial derivatives, their full specification  
includes the full set of coordinates of the system, i.e. also those  
that are kept fixed. To take a simple (and unrealistic) example,  
consider a quadrilateral arrangement of bonded atoms

	* ---a---*
	|       |
	b       d
	|       |
	*---c---*

The force constant for a obviously depends on what you keep fixed: if  
you keep the angles fixed, then c will change along with a, if you keep  
the other distances fixed, then all the angles will change.
--
------------------------------------------------------------------------ 
-------
Konrad Hinsen                            | E-Mail:  
hinsen!at!cnrs-orleans.fr
Centre de Biophysique Moleculaire (CNRS) | Tel.: +33-2.38.25.56.24
Rue Charles Sadron                       | Fax:  +33-2.38.63.15.17
45071 Orleans Cedex 2                    | Deutsch/Esperanto/English/
France                                   | Nederlands/Francais
------------------------------------------------------------------------ 
-------

---

From: "Qiang Lu" <qiangl(at)uci.edu>
Date: Tue, 30 Mar 2004 10:25:37 -0800

Dear all,

I wander is it ok to use the force constant directly come from QM calculation.
I heard of that it is not good to use QM force constant directly into MM calculations.
Does anyone know about this?

Qiang Lu

======= 2004-03-29 23:13:18 DzTZ@4PEVPP45@#:=======

>email: Joerg.Grunenberg(at)tu-bs.de phone: +49 531 391 5252 
>-= This is automatically added to each message by the mailing script =-
>To send e-mail to subscribers of CCL put the string CCL: on your Subject: line
>and send your message to:  CHEMISTRY(at)ccl.net
>
>Send your subscription/unsubscription requests to: CHEMISTRY-REQUEST(at)ccl.net 
>HOME Page: http://www.ccl.net   | Jobs Page: http://www.ccl.net/jobs 
>
>If your mail is bouncing from CCL.NET domain send it to the maintainer:
>Jan Labanowski,  jlabanow(at)nd.edu (read about it on CCL Home Page)
>-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>
>
>
>
>

= = = = = = = = = = = = = = = = = = = =

!!!!!!!!!!!!!!!!VB
@q#!

!!!!!!!!!!!!!!!!Qiang Lu
!!!!!!!!!!!!!!!!qiangl(at)uci.edu
!!!!!!!!!!!!!!!!!!!!2004-03-30