Accurate estimation of singlet-triplet gap of strongly correlated systems by CCSD(T) method using improved orbitals

An accurate estimation of the singlet-triplet gap is key to various important chemical phenomena ranging from the designing of solar energy cell to organic light emitting diodes. The present work estimates the singlet-triplet gap for a few biradical systems (1,4 - phenylenedinitrene, biphenyl-4,4'-dinitrene, benzyne isomers, naphthalene and anthracene), which are known to have high multireference character. To estimate the singlet triplet gap for such systems, we have improved the reference orbitals for CCSD(T) calculation using different DFT functionals. Interestingly, the strategy improved the performance of the CCSD(T) method drastically for multireference systems. It has also been found that, the performance of this method does not depend much on the choice of the DFT functionals.