Exploring homolytic bond cleavage of a hydrogen molecule with Gaussian09

Quantum Chemistry tutorial

Introduction

In the previous lecture we've learned about the theoretical basis of electronic structure methods, and in particular about the Hartree-Fock method. Now it is time to jump into the electronic structure business and perform a couple of calculations by yourself. The package we are going to use is called Gaussian. It is among the modern electronic structure codes available and provides an easy-to-use interface called Gaussview that allows for a user-friendly access to quantum chemistry. Additionaly the Gaussian program provides many post-Hartree-Fock methods. 

To learn how to use the program by yourself or maybe extend your knowledge beyond the scope of this tutorial one can check out the links on the following site. 

To start a calculation we basically need four things:

• the structure of the molecule of interest in (cartesian or internal) coordinates
• the overall charge and spin state of the system
• the method that will be used (Hartree-Fock (HF), Density Functional Theory (DFT), Moeller-Plesset perturbation theory (MPx) etc.)
• a basis set.

The files needed for this practical can be downloaded as an archive here and unpacked by typing

tar xzvf h2.tar.gz

However, you don't need these, as all files required can be created with the information below.

The H-H molecule

Our calculations will be on the simple H2 molecule, which is the smallest molecule that exhibits all fundamental aspects of chemical bonding (why?).

Single point energy calculation

A simple input for the electronic structure calculation of H2 is given below:

%chk=h2_SP
#P RHF 6-31G 

h2 molecule

0 1
h 
h 1 r

r 1.0

The first line starting with %chk specifies that we want to keep the checkpoint file. You are free to choose a file name; I used h2_SP (standing for single Point). Orbitals are stored on this file and can be visualized with Gaussview. The #P tells Gaussian to print all information during the computation. Then, the method is specified, which is Restricted Hartree Fock (RHF) and the basis (6-31G) is selected, using the so-called Pople notation. G stands for "Gaussian", so a GTO basis with 6 gaussian functions for the inner (non-valence) electrons and 4 (3+1) for the valence electrons is used. Then comes a whileline followed by a title string and another whiteline. The next two integers represent the total charge (0 in this case), and spin multiplicity (i.e. the eigenvalue of the S*S operator, which is (S(S+1)=1, thus Singlet). For the geometry we use a Z-matrix. In the case of a diatomic, one needs to specify only the interatomic distance r, which we set at 1.0 Angstrom.

Now we can do the electronic structure calculation by typing

g09 h2.com 

During the calculation Gaussian writes an output file that communicates the most important infos to the user. You can extract the following information:

• number of SCF cycles needed for convergence
• the HF energy (what units are being used?)
• the dipole moment

Additionaly you could extract the individual HF energies after each SCF cycle and plot them against the number of cycles. I assume you know how at least one program to visualize data


To continue reading click on the following link:

http://wwwuser.gwdg.de/~ggroenh/exercise_H2/h2.html