Electronic structure of azobenzene: ground and first excited singlet states

QMC techniques are used to obtain energies at selected points on the potential energy surfaces of a photoswitchable molecule, azobenzene (AB), along the torsion pathway (CNNC dihedral angle), in the ground and first excited singlet states. We study the excitation energies of well separable cis- and trans-conformers, and th Inst. Phys., Slovak Acad. Scie energy of the transition state located at 90\r{ }. By a careful QMC optimization of the Slater-Jastrow wavefunctions with up to 500 determinants, chemical accuracy is obtained. Our results not only outperform all the available quantum chemistry results such as CAS-SCF, CAS-PT2, as well as DFT results with proper spin symmetry taken into account (ROKS), but open also a credible window to possible correction/reinterpretation of the available experimental data.