A new perturbation approach is proposed that enhances the low-order, perturbative convergence by modifying the zeroth-order Hamiltonian in a manner that enlarges any small-energy denominators that may otherwise appear in the perturbative expansion. This intruder state avoidance (ISA) method can be used in conjunction with any perturbative approach, but is most applicable to cases where small energy denominators arise from orthogonal-space states - so-called intruder states - that should, under normal circumstances, make a negligible contribution to the target state of interests. This ISA method is used with multireference Moøller-Plesset (MRMP) perturbation theory on potential energy curves that are otherwise plagued by singularities when treated with (conventional) MRMP; calculation are performed on the 13Σu- state of O2; and the 21Δ, 31Δ, 23Δ, and 33Δ states of AgH. This approach is also applied to other calculations where MRMP is influenced by intruder states; calculations are performed on the 3∏u state of N2, the 3∏ state of CO, and the 21A′ state of formamide. A number of calculations are also performed to illustrate that this approach has little or no effect on MRMP when intruder states are not present in perturbative calculations; vertical excitation energies are computed for the low-lying states of N2, C2, CO, formamide, and benzene; the adiabatic 1A1-3B1 energy separation in CH2, and the spectroscopic parameters of O2 are also calculated. Vertical excitation energies are also performed on the Q and B bands states of free-base, chlorin, and zinc-chlorin porphyrin, where somewhat larger couplings exists, and - as anticipated - a larger deviation is found between MRMP and ISA-MRMP.
- Excitation energy
- Intruder state problem
- Multireference perturbation theory
- Potential energy surface (PES)