*From*: Jeya Vimalan <jeyavimalan2k() gmail.com>*Subject*: CCL: Formation Energy - Summary*Date*: Fri, 21 Oct 2016 11:20:33 +0530

Dear All,

Thanks for your
response.

I am summarizing the
replies.

vimal

Question
:

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

Dear All,

I have three composition.

A1-B3-C4

A2-B4-C5

A4-B3-C7

I would like to understand

which structures are stable and hence

decided to calculate formation (or Cohesive)
energy.

Lets take the first two cases and

calculate Formation energy by the below
formulae.

Formation Energy of 1 (E1) = Total energy of
A1-B3-C4 -(Energy of 1*A + Energy of 3*B + Energy of 4*C).

Formation Energy of 1 (E2) = Total energy of
A2-B4-C5 -(Energy of 2*A + Energy of 4*B + Energy of 5*C).

Does the direct comparison (E1, E2) makes
sense?

Can i conclude if E1>E2, then composition 1 is
more stable.

I feel, i am missing something.

Your help would be appreciable.

Best regards,

Vimal

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

===

Dear Vimal,

I would say
that your approach is valid, under the condition that the A, B, and C
and

sensible moieties. It reminds a bit of
atomization energies.

However, I would say that if E1 > E2 (e.g. +2
vs. -10) that 2 is more stable, not 1.

Marce

Prof
Dr Marcel Swart, FRSC

ICREA Research Professor at

Institut de Química Computacional i Catàlisi (IQCC)

Univ. Girona (Spain)

COST Action CM1305 (ECOSTBio) chair

Girona Seminar 2016 organizer

IQCC director

Young Academy of Europe (Board) member

ICREA Research Professor at

Institut de Química Computacional i Catàlisi (IQCC)

Univ. Girona (Spain)

COST Action CM1305 (ECOSTBio) chair

Girona Seminar 2016 organizer

IQCC director

Young Academy of Europe (Board) member

====l

Dear
Vimal:

Unfortunately,
the concept of stability is not that straightforward. Thermodynamics does not
provide the final answer. The stability of a molecule not only depends on its
formation energy, but it depends also on the relative energies of its
neighboring local minima on its potential energy surface and the energy barrier
between the minimum and its immediate neighboring minima (kinetic stability). To
begin, I would rather check the depth of minimum energy well by looking at
frequencies. In the next step ab initio/MC/DFT- MD simulations can provide more
insight toward the kinetic stability. Then of course we can define stability
with respect to a particular reaction. Is your molecule stable when it is in the
presence of air/water/light/etc.

You must
be more precise about stability.

I hope my
general answer can help you to solve your problem.

Good luck,

Cina

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

Cina Foroutan-Nejad, PhD

CEITEC-Central European Institute of
Technology

Kamenice 5/A4, Masaryk University, Brno,
Czech Republic

====

Hi
Vimal:

I
am not exactly certain what you want to do but if you want to estimate is
composition A1-B3-C4 is more stable than A2-B4-C5, you should calculate the
energy of this "reaction" ...

A1-B3-C4 + A + B + C
--> A2-B4-C5

Now
you have a mass balanced equation. Assuming the energies of these components are
negative, then if E(Right Hand Side) - E(Left Hand Side) is negative the
LHS is more stable. This means A2-B4-C5 as a compound is more stable
than A1-B3-C4 and an extra A, B, and C.

Regards,

Joe Golab

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

Dear Jeya, how are
you?

The comparison kind
of makes sense, but to get the correct energies it is not enough to calculate
the single points without the thermodynamic corrections (and using the correct
stoichiometry in your equations).

Please, if you can,
take a look at this link[1] (it is in Portuguese, but maybe Google Translator
can deal with this issue for you).

All the best

===

Thanks and best regards,

vimal