A quantum Monte Carlo study of zinc-porphyrin: Vertical excitation between the singlet ground state and the lowest-lying singlet excited state

We have calculated the vertical excitation energy between the singlet ground state ( 1 1 A g ) and the lowest-lying singlet excited state ( 1 1 E u ) of Zn-porphyrin employing the fixed-node diffusion Monte Carlo technique. The determinantal parts of trial wave functions were constructed using results from Configuration Interaction and time-dependent Density Functional Theory.