## IMA Postdoc Seminar (April 28, 2009)

**Speaker:** Abdul Rehaman Moughal Shahi (Universite de Geneve)

**Title:** II order Perturbation over Restricted Active Space
Wavefunction : An Answer to Chemical Problems Requiring Larger Active
Spaces

**Abstract:**
Ionization potential (IP) is an essential property which helps in
understanding the band structures and electronic states of molecular
wires and rings. We use the newly developed RASSCF/RASPT2 method to
calculate the Ionization potentials of pi-conjugated oligomeric
systems of acetylene (n = 1,5) and phenylene (n = 1,3) . We check for
the convergence of this method with respect to the well established
CASSCF/CASPT2 method. The accuracy of this approach in calculating the
IPs is comparable ~0.1 eV to that of the CASPT2. The heavy reduction
in the number of configuration state functions with this approach is
of great importance in studying the chemical problems, where
CASSCF/CASPT2 calculations are not feasible owing to the large active
spaces. In this study we have been able to study the pentamer of
acetylene and trimer of Phenylene systems which can not be studied
previously with the CASPT2 method