How to calculate the IP(ionisation potential and EA(elctron affinity) on a neutral molecule. Is this possible?

Yossa Kamsi Richard Arnaud

•  3.49
• University of Yaounde I

I am using GAUSSIAN09 and would like to calculate the IP(ionisation potential and EA(elctron affinity) on a neutral molecule. Is this possible?

I found some where that, The ionization potential (IP) and electron affinity (EA) for the ground state configuration can be computed using:

IP=E(cation) - E(neutra) and EA= E(neutral) - E(anion)

But I dont know how to calculate the E(cation) and E(neutral) using GAUSSIAN 09 code. I need your Help. The E(neutral) i considered is the one I obtain after the geometry optimization of my molecule, do I have reason to considered it like E(neutral)???

some one told me that I have to add or remove one electron on the neutral form of my molecule and perform a single point calculation on each in other to have E(anion) and E(cation)  respectivelly but I don't know how to modifiered the number of electron in gaussian. I need help.

Rafik Karaman

Added an answer

Dear Yossa,

For the calculations of the ionization potential:

The ionization energy (IE) is qualitatively defined as the amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated gaseous atom to form a cation. It is quantitatively expressed in symbols as:

X + energy → X+ + e−

where X is any atom or molecule capable of being ionized, X+ is that atom or molecule with an electron removed, and e− is the removed electron.

Therefore, you should calculate both: the neutral molecule (X) and for the charges molecule (X+).

The fifth line in the neutral molecule's input file  be run in Gaussian you should be:

0     1

Whereas, 

The fifth line in the charged molecule's input file  be run in Gaussian you should be:

1     1

For the calculations of the electron affinity:

Electron affinity is defined as the change in energy (in kcal/mole) of a neutral atom (in the gaseous phase) when an electron is added to the atom to form a negative ion. In other words, the neutral atom's likelihood of gaining an electron.

X  + e− → X- + energy

Therefore, you should calculate both: the neutral molecule (X) and for the charges molecule (X-).

The fifth line in the neutral molecule's input file  be run in Gaussian you should be:

0     1

Whereas, 

The fifth line in the charged molecule's input file  be run in Gaussian you should be:

-1     1

Please note that I assigned the multiplicity as 1 (singlet), however, you can also use a triplet state by changing the multiplicity.

The following is an example of an input file (in the bold line: the first number is for charge and the second for multiplicity):

The following is a typical Gaussian input file for cationic formaldehyde (B3LYP/6-31G(d,p) level) as an example:

%mem=6000000

%chk=/scratch/test1.chk

#P HF/6-31G(d) scf=tight

test1 HF/6-31G(d) sp formaldehyde

1 1

C1

O2 1 r2

H3 1 r3 2 a3

H4 1 r4 2 a4 3 d4

r2=1.20

r3=1.0

r4=1.0

a3=120.

a4=120.

d4=180

Hoping this will be helpful,

Rafik

Reference: https://www.researchgate.net/post/I_am_using_GAUSSIAN09_and_would_like_to_calculate_the_IPionisation_potential_and_EAelctron_affinity_on_a_neutral_molecule_Is_this_possible