Yk. Choe et al., Identifying and removing intruder states in multireference Moller-Plesset perturbation theory, J CHEM PHYS, 114(9), 2001, pp. 3913-3918
The lowest-lying excited states of N-2 and CO are examined for the presence
of intruder states that adversely influence the perturbative convergence i
n computations using multireference Moller-Plesset perturbation theory. Int
ruder states are found in the computations of the (3)Pi (u) state of N-2 an
d the (3)Pi state of CO. Since, in second order, these intruders-under norm
al circumstances-should make a negligible contribution to the target state,
their contributions to the second-order energies are simply removed from t
he perturbation expansion. With the intruder state removed, the vertical ex
citation for the (3)Pi (u) ((3)Pi) state of N-2 (CO) is only 0.23 eV (0.35
eV) below the experimental value, compared to differing by 0.72 eV (0.44 eV
) with the intruder state present. The intruder states are also removed by
performing energy-denominator shifts using a redefined zeroth-order Hamilto
nian H-0 that differs from previous treatments, since only the energy denom
inator associated with the intruder state is shifted. This approach removes
intruders, but makes no other changes to the perturbation expansions. Two-
state models are used to identify the intruder states and suggest remedies.
(C) 2001 American Institute of Physics.