Front-door intruders

There is another trouble known as intruder states, i.e. states which are of unexpectedly low energy. How can these states appear Firstly, the Cl states known as front door intruders appear, if some im-... 

Fig. 14.1. Points of degeneracy in the complex plane. The conjugate points (fz.fj) corre.spond to a front-door intruder and the points ( 4. 4) to a back-door ininider.

In Figure 14.4, we have illustrated the behaviour of the two-state model for two sets of parameters, which represent a back-door intruder dominated by high-order excitations (to the left) and a front-door intruder dominated by low-order excitations (to the right). The numerical values of the parameters were obtained from the Mpller-Plesset calculations discussed in Section 14.5.5. For the high-excitation back-door intruder, the parameters are yS — a = 12.32, S = —0.00034 and... 

To understand this behaviour, let us examine the avoided crossing at z = 0.8, which arises from an interaction between the two lowest states of the physical Hamiltonian (a possible low-excitation front-door intruder). In the subspace spanned by these states, we obtain the following partitioned Hamiltonian in the diagonal basis of the zero-order Hamiltonian ... 

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