Abstract: Although for carbon and hydrogen the basis set of atomic natural orbitals(ANO-S) has been used in the all calculations, the basis set of ANO and effective core potential(ECP) have been used for the bromine, respectively. Using complete active space selfconsistent field(CASSCF) method with these two species correlation consistent basis set, the ground state and low-lying excited states of ethyl bromide molecule and cation were studied. Meanwhile, taking into account the dynamic correlation effects, the second-order perturbation(CASPT2) method was used to obtain more reliable energies. For the ethyl bromide molecule, the optimized geometry of the ground state with ANO basis set agrees better with the experimental values than those with ECP basis set. The adiabatic ionization energies were obtained from the corresponding molecule and cation states optimized, the first and second ionization energies and the energetic gap of the first two electronic states were of somewhat difference with the experimental values at the CASSCF level. The results were improved with the CASPT2 method, in which the first two ionization energies differ from the experimental data by just 0.359 3 and 0.449 6 eV using the ANO basis set. Furthermore, the harmonic frequencies of the title molecule were calculated at the CASSCF/ANO level, the frequencies agree well with the experimental values.

Key words: Ethyl bromide, CASSCF, CASPT2, Ionization potential, Excited state