Mechanism of photoswitching in diarylethenes involves the light-initiated symmetry-allowed disrotatory electrocyclic reaction. Here we propose a computationally inexpensive Density Functional Theory (DFT) based method that is able to produce accurate potential surfaces for the excited states. The method includes constrained optimization of the geometry for the ground and two excited singlet states along the ring-closing reaction coordinate using the Slater Transition State method, followed by single-point energy evaluation. The ground state energy is calculated with the broken-symmetry unrestricted Kohn-Sham formalism (UDFT). The first excited state energy is obtained by adding the UDFT ground state energy to the excitation energy of the pure singlet obtained in the linear response Time-Dependent (TD) DFT restricted Kohn-Sham formalism. The excitation energy of the double excited state is calculated using a recently proposed (Mikhailov, I. A.; Tafur, S.; Masunov, A. E. Phys. Rev. A 77, 012510, 2008)
Tamm-Dancoff approximation to the second order response TD-DFT.