Jussi, You have a point here. Let me rephrase my question then:

Can you calculate the Gibbs energy of a system when you are not at a minimum or a TS? In other words, is there a simple way to calculate a potential GIBBS energy surface?

I'm not a specialist on this, but physically having a stretched molecule may only mean being at the geometrical limit of a high vibrationally state. In principle, there should be an equivalent to E+ZPE for each point of the curve of a reaction (as long as it is permitted by
> the uncertainty principle). Can someone share some light on this?

Thanks,

Sebastian

face="Arial" size="2">

**From:** "Jussi Lehtola jussi.lehtola,,helsinki.fi" <owner-chemistry- -ccl.net>

**To:** "Kozuch, Sebastian " <kozuchs- -yahoo.com>

**Sent:** Saturday, November 3, 2012 7:10 PM

**Subject:** CCL: ZPE in non-stationary points

>

Sent to CCL by: Jussi Lehtola [jussi.lehtola]^[helsinki.fi]

On Sat, 3 Nov 2012 16:05:34 -0700 (PDT)

"Sebastian Kozuch kozuchs]~[yahoo.com" <owner-chemistry{:}ccl.net> wrote:

> Dear all,

> Does anyone know of a (simple) method to calculate ZPE and maybe

> Gibbs energies for geometries that are not stationary points (i.e.

> not a stable intermediate or a TS)? How valid is a typical frequency

> calculation for these geometries?

Please elaborate on what you mean.

Zero point vibrations only make sense in cases where the potential can

be expanded locally as a Taylor series as

V(r) ~ V(r0) + (r-r0)*d2V/dr2*(r-r0)

where d2V/dr2 is the Hessian computed at r0. This means that you must

be in a stationary point, since the first derivative (gradient) needs > to

vanish.

Secondly, any stationary point will not do, since otherwise you will

have a saddle point, meaning that vibrations do not exist in some

directions, instead the system is just unstable: when the system is

pushed in this direction, it will not start to oscillate around the

configuration in the stationary point, instead the perturbation will

just start growing.

To calculate ZPE you need a bound system. This is not the case even for

all stationary points -- and even less for non-stationary points.

--

--------------------------------------------------------

Mr. Jussi Lehtola, M. Sc. Doctoral Student

jussi.lehtola{:}helsinki.fi Department of Physics

http://www.helsinki.fi/~jzlehtol University of Helsinki

Office phone: +358 9 191 50 632 Finland

--------------------------------------------------------

Jussi Lehtola, FM > Tohtorikoulutettava

jussi.lehtola{:}helsinki.fi Fysiikan laitos

http://www.helsinki.fi/~jzlehtol Helsingin Yliopisto

Työpuhelin: (0)9 191 50 632

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