Effective Hamiltonian formalism projection operator

Extensive introductions to the effective core potential method may be found in Ref. [8-19]. The theoretical foundation of ECP is the so-called Phillips-Kleinman transformation proposed in 1959 [20] and later generalized by Weeks and Rice [21]. In this method, for each valence orbital (pv there is a pseudo-valence orbital Xv that contains components from the core orbitals and the strong orthogonality constraint is realized by applying the projection operator on both the valence hamiltonian and pseudo-valence wave function (pseudo-valence orbitals). In the generalized Phillips-Kleinman formalism [21], the effect of the projection operator can be absorbed in the valence Pock operator and the core-valence interaction (Coulomb and exchange) plus the effect of the projection operator forms the core potential in ECP method. 

Before the effective hamiltonian can be used in actual calculations some means must be found for expressing the terms Gcore [equation (33)] and the projection operator terms in equations (31) or (34) in a form which is convenient for computing matrix elements this is the subject of parameterization, which is dealt with in Section 3. Two other formal problems remain at this level. Firstly there is the need to modify equation (29) and, as a result, equations (31) and (34) if the atomic calculations on the separate atoms are of the open shell kind as is usually the case. In order not to bias the later molecular calculation the core operators and projection terms can be derived for some average of all the possible open-shell configurations,25 although care should be exercised in the choice of the hamiltonian for which the... 

The formal theory of resonances due to Feshbach begins with the decomposition of the Hamiltonian in terms of a projection operator Q [8]. He defines Q as the projection onto the closed-channel space, just like the example of H discussed around Eqs. (4) and (5). Then, QBSs described well by the eigenfunctions Q4> of Eq. (5) with his Q may be called Feshbach resonances." A simplified picture would be that eigenstates Q are supported by some attractive effective potential approaching asymptotically the threshold energy of a closed channel. If this is the case, then the energies EQ of... 

The two previous secfions were devoted to modeling quantum resonances by means of effective Hamiltonians. From the mathematical point of view we have used two principal tools projection operators that permit to focus on a few states of interest and analytic continuation that allows to uncover the complex energies. Because the time-dependent Schrodinger equation is formally equivalent to the Liouville equation, it is attractive to try to solve the Liouville equation using the same tools and thus establishing a link between the dynamics and the nonequilibrium thermodynamics. For that purpose we will briefly recall the definition of the correlation functions which are similar to the survival and transition amplitudes of quantum mechanics. Then two models of regression of a fluctuation and of a chemical kinetic equation including a transition state will be presented. 

The intermediate projection of operators or states upon the vacuum can be used also in order to classify the wave operator (and other perturbation expansions) in a different way. In the Bloch Eq. (16) and the effective Hamiltonian Heg in Eq. (10), namely, the order of the individual terms in the perturbation expansion is determined just by the number of interactions V, which occur in each term. A different classification of the diagrams is obtained in the (so-called) Q-box formalism as applied, for instance, in nuclear physics. In this formalism, the order of a perturbation expansion results from the number of foldings, i.e. the number of projections o)(o upon the vacuum. In each of these new orders, the number of interaction lines is no longer fixed but may vary from application to application. 

As in the previous section, we wish now to eliminate formally the perturber variables as well as the radiation field variables. This is achieved by the same projection operator procedure. A straightforward extension of Eqs. (13)-(18) leads to a redefinition of the effective Hamiltonian for the subspace of the molecular excitations, selected by the projector M. The result is... 

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