Finding the conical intersection between the excited and ground state in a protonated schiff base with Gaussian09.

Quantum Chemistry tutorial

Introduction

In one of the previous lecture we've learned about the conical intersections. At such intersections, the adiabatic surfaces touch. We have seen that the degeneracy between the electronic states is lifted (to first order) by displacements in so-called branching space. In the remaining (3N-8) internal degrees of freedom, the degeneracy can be maintained, forming a 3N-8 dimensional seam. In this tutorial we will locate the minimum energy point on this seam, the so called minimum energy conical intersection (MECI). 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. 

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) coordinates
• the overall charge of our system
• the method that will be used (Hartree-Fock (HF), Density Functional Theory (DFT), Moeller-Plesset perturbation theory (MPx) etc.)
• a basis set.

With these options determined, Gaussian can compute the electronic wave function. With the wave function at hand, we have access also to the energy and energy gradients with respect to nuclear displacement. With the latter we can search for stationary points, at which these gradients are zero.

In this practical we will locate the minimum energy conical intersection. At this point, the energy gap in the branching space space is zero, while the gradients in the 3N-8 dimensional seam space are zero as well. We will not go into details about the optimization algorithm that we will use to optimize the MECI, but details can be found in Bearpark et al. Chem. Phys. Lett. 223 (1994) 269.

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

tar xzvf conical.tar.gz

However, we will try to do everything by ourselves, starting by creating the formaldimine molecule in Gaussview.

Building the molecule

Open gaussview by typing 

gview.exe

This will open a user interface.

To continue reading click on the following link:

http://wwwuser.gwdg.de/~ggroenh/exercise_CI/im.html