Theoretical Study on the Properties of Small Molecules with the Block Effective Hamiltonian Theory and Multireference...

The effective Hamiltonian theory is extended from the original three blocks in our previous study to the four blocks and finally to the n-block. The four block and n-block-effective Hamiltonian are derived. The vertical and adiabatic excitation energies of H₂O, NH₂, and CH₂ are calculated with three-block-effective Hamiltonian theory (BEHT3), two types of multireference perturbation theories, and multireference configuration interaction (MRCI). The BEHT3 energies of the ground state and vertical and adiabatic excited states of H₂O, NH₂, and CH₂ converge with the size of model space from below to the corresponding MRCI energies. On the other hand, the BEHT3 excitation energies of these molecules converge with the size of model space from above to the corresponding experimental energies or energies calculated with higher-level theoretical models such as full configuration interaction. The effect of basis set on the electron affinities of F, PH₂, and SH calculated with the complete active space self-consistent field (CASSCF), the second- and third-order Møller-Plesset perturbation theory (MP2, MP3), coupled-cluster with single and double excitations (CCSD), CCSD plus the perturbation correction of triple excitations (CCSD(T)) and BEHT3 are investigated. It is found that the diffuse basis function plays an important role in achieving accurate electron affinity results. Among all theoretical models considered here, the performance of BEHT3 is best.