Transport Properties and Singlet-Triplet Inst...

Transport Properties and Singlet-Triplet Instabilities in Oligoacenes

Author Name : Dr. Sharon Achamma Abraham

ABSTRACT

Closed shell singlet structures, Openshell singletstructures  and open shell triplet   structures were analysed and  found out using M062X functional and 6-31+G(d,p) basis set in DFT method Gaussian09in Oligoacenes. It is observed that  Closed shell structures energies increases as  number of fused ringsize increases. Open shell singlet structures energiesincreases as  size of  polyacene increases. They are more stabilized than  closed shell configurations.Open shell triplet Structureenergies increases from pentacene to nonacene. This method is used to find out  spin contamination in closed shell structures, open shell singlet states and triplet states. While  0,1 M062X  calculation using  symmetry keywords are seen to stabilize  system for longer polyacenes, open  shell calculations also suffer from large spin-   contamination since,  open   shell   function is not an eigen function of  S² even though it is an eigen function of  S_z component. For example, for  open shell singlet,  expectation value for S² is zero while that for  triplet state is 2.000. The open shell calculations have values for <S²> that exceed  expected exact value. Such a contamination is seen to be moderate for  triplet state. The highest spin  contamination observed  for  triplet state is 2% which is somewhat within  acceptable range.But the magnitude for spin contamination is observed to be huge for  open shell singlet state. This studies has application in spin contamination  and  Fractional spin DFT methods.

Keywords:M062X; Open shell singlet; open shell triplet; spin contamination; DFT; bonding molecular orbital.