Dear All,

I am summarizing the replies.
vimal

Question :

=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Dear = All,

I have three composition.

A1-B3-C4
A2-B4= -C5
A4-B3-C7

= I would like to understand
which structures are stable a= nd hence
decided to calculate formation (or Cohesive) en= ergy.

Lets take the first two ca= ses and=C2=A0
calculate Formation energy by the below fo= rmulae.

= Formation Energy of 1 (E1) =3D Total energy of A1-B3-C4 -(Energy of 1*A + E= nergy of 3*B + Energy of 4*C).
Formation Energy of 1 (E2= ) =3D Total energy of A2-B4-C5 -(Energy of 2*A + Energy of 4*B + Energy of = 5*C).

Do= es the direct comparison (E1, E2) makes sense?
Can i con= clude if E1>E2, then composition 1 is more stable.
I feel, i am missing something.
Yo= ur help would be appreciable.

Be= st regards,
Vimal
=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D
=3D=3D=3D
Dear Vimal,

I would say that your approach is valid, u= nder the condition that the A, B, and C and
sensible moieties. It reminds a bit of atomization energies.

Howe= ver, I would say that if E1 > E2 (e.g. =C2=A0+2 =C2=A0vs. -10) that 2 is= more stable, not 1.

Marce

Prof Dr Marcel Swart, FRSC
ICREA Research Professor at
Institut de Qu=C3=ADmica Computacional= i Cat=C3=A0lisi (IQCC)
Univ. Girona (Spain)

COST Action CM1305 (= ECOSTBio) chair
Girona Seminar 2016 organizer
IQCC director
Young = Academy of Europe (Board) member

=3D=3D=3D=3Dl

Dear Vim= al:

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 lo= cal minima on its potential energy surface and the energy barrier between t= he minimum and its immediate neighboring minima (kinetic stability). To beg= in, I would rather check the depth of minimum energy well by looking at fre= quencies. In the next step ab initio/MC/DFT- MD simulations can provide mor= e insight toward the kinetic stability. Then of course we can define stabil= ity 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.=C2= =A0

I hope my general answer can h= elp you to solve your problem.

Good lu= ck,
Cina

--------------------------------------------------------= ------
CEITEC-Central European Institute of Technology
Kamenice 5/A4, Masaryk University, Br= no, Czech Republic

=3D=3D=3D=3D
Hi Vimal:

I am not exactly certain what you wan= t 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 equat= ion. Assuming the energies of these components are negative, then if E(Righ= t Hand Side) - E(Left Hand Side) is negative=C2=A0the LHS is more stable. T= his means=C2=A0A2-B4-C5 as a compound is more stable than=C2=A0A1-B3-C4 and= an extra A, B, and C.

Regards,
Joe Golab
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

Dear Jeya, how a= re you?
The comparison kind of makes = sense, but to get the correct energies it is not enough to calculate the si= ngle points without the thermodynamic corrections (and using the correct st= oichiometry in your equations).
Please, if you= can, take a look at this link[1] (it is in Portuguese, but maybe Google Tr= anslator can deal with this issue for you).
All the best
<= br>
=3D=3D=3D

Thanks= and best regards,
vimal
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