EHF theory of chemical reactions Part 4. UNO CASSCF, UNO CASPT2 and R(U)HF coupled-cluster (CC) wavefunctions

Abstract

Spin-restricted and spin-unrestricted post-Hartree-Fock methods are compared by depicting potential energy curves of several hydrogen clusters (H 2-H 6), hydrogen fluoride and fluorine molecules. In the former approach, RHF, RMP, RHF coupled-cluster (CC), UNO CASSCF and UNO CASPT2 are utilized, whereas the latter approach includes UHF MP n (UMP n) and UHF coupled-cluster (UCC), approximate spin-projected UMP n and UCC (APUMP n and APUCC) and Löwdin-type full spin-projected UMP n (PMP n). Characteristics (shapes in particular) of the potential curves are discussed in relation to the roles of electron correlation corrections revealed by both methods. The UNO CASSCF followed by PT2 provides reasonable potential curves in the whole region. The spin contamination error is remarkable in the case of the UMP n wavefunction, leading to the necessity of spin projection. The resulting PMP2 and APUMP2 can be used for larger clusters and are regarded as alternatives to UNO CASPT2. The spin contamination error is not so serious for UCCSD(T), at least in the weak and intermediate correlation regime, whereas it is important in the magnetic (dissociation) region where APUCCSD(T) works well in depicting the potential energy curves. It is regarded as an approximate version of UNO CASSCF CC SD(T). Implications of the calculated results are discussed in order to clarify the applicabilities of the post-HF methods to chemical reactions in the ground and lower-excited states.