Pyrazine excited states revisited using the extended multi-state complete active space second-order perturbation method.

We demonstrate that the recently developed extended multi-state complete active space second-order perturbation theory (XMS-CASPT2) [Shiozaki et al., J. Chem. Phys., 2011, 135, 081106] provides qualitatively correct potential energy surfaces for low-lying excited singlet states of pyrazine, while the potential energy surfaces of the standard MS-CASPT2 methods are ill-behaved near the crossing point of two reference potential energy surfaces. The XMS-CASPT2 method is based on the extended multi-configuration quasi-degenerate perturbation theory proposed earlier by Granovsky [J. Chem. Phys., 2011, 134, 214113]. We show that the conical intersection at the XMS-CASPT2 level can be described without artifacts if the entire method is invariant with respect to any unitary rotations of the reference functions. The photoabsorption spectra of the 1(1)B(3u) and 1(1)B(2u) states of pyrazine are simulated, based on a vibronic-coupling model Hamiltonian. The XMS-CASPT2 spectrum of the 1(1)B(3u) band is found to be comparable to the one computed by a more expensive multireference configuration interaction (MRCI) method, while the XMS-CASPT2 simulation of the 1(1)B(2u) band is slightly inferior to the MRCI one.