Third-order multireference perturbation theory The CASPT3 method

Rayleigh-Schrödinger perturbation theory is applied to compute second and third-order correlation energies using complete active space self-consistent field (CASSCF) zeroth-order wavefunctions. The first-order wavefunction is expanded in a basis of internally contracted configurations. The zeroth-order Hamiltonian is a sum of one-electron effective Fock operators, which are invariant to unitary transformations among the active orbitals. Comparisons with FCI data are made for the singlet-triplet splitting of CH2 and the barrier height and exothermicity of the F + H2 reaction. Potential energy functions and spectroscopic data are computed for C2, CN, CO, CF,N2, NO, O2, and F2 using large basis sets. It is demonstrated that the third-order results (CASPT3) are significantly more accurate than the second-order (CASPT2) ones. The equilibrium distances and harmonic frequencies obtained with CASPT3 are found to be as accurate as MRCI and RCCSD(T) values, while dissociation energies are generally somewhat too small. Modifications of the zeroth-order Hamiltonian as recently proposed by Andersson are found to have a much smaller effect on the third-order energies than on the second-order ones.