Hamiltonians state specific methods

The requirement needed to incorporate the solvent effects into a state-specific (multireference) method is fulfilled by using the effective Hamiltonian defined in Equation (1.159). The only specificity to take into account is that in order to calculate Va we have to know the density matrix of the electronic state of interest (see the contribution by Cammi for more details). Such nonlinear character of Va is generally solved through an iterative procedure [35] at each iteration the solvent-induced component of the effective Hamiltonian is computed by exploiting Equation (1.157) with the apparent charges determined from the standard ASC equation with the first order density matrix of the preceding step. At each iteration n the free energy of each state K is obtained as... 

At the core of the analysis and methods that are discussed in this Chapter is the consistent consideration of the fact that the form of each resonance wavefunction is = fl I o+Xas (Eq. (4.1) of text), if necessary, the extension to multi-dimensional forms is obvious. Depending on the formalism, the coefficient a and the asymptotic part, Xas, are functions of either the energy (real or complex) or the time. The many-body square-integrable, %, represents the localized part of the decaying (unstable) state, i.e., the unstable wavepacket which is assumed to be prepared at f = 0. its energy, Eo, is real and embedded inside the continuous spectrum, it is a minimum of the average value of the corresponding state-specific effective Hamiltonian that keeps all particles bound. 

In order to unify, in fhe spirit of quantum defect theory, the treatment of discrefe and confinuous spectra in the presence of discrete Rydberg and valence states and of resonances, Komninos and Nicolaides [82, 83] developed K-mafrix-based Cl formalism that includes the bound states and the Rydberg series, and where the state-specific correlated wavefunc-tions (of the multi-state o) can be obtained by the methods of the SSA. The validity and practicality of fhis unified Cl approach was first demonstrated with the He P° Rydberg series of resonances very close to the n = 2 threshold [76], and subsequently in advanced and detailed computations in the fine-structure spectrum of A1 using fhe Breit-Pauli Hamiltonian [84, 85], which were later verified by experiment (See the references in Ref. [85]). 

The early attempts to develop MR perturbation methods focused on the use of an effective Hamiltonian determined from the Bloch equation [7, 18, 24, 27]. The main drawback of these theories is the intrader state problem, i.e., the appearance of close to zero denominators which give nonphysical contribution to the effective Hamiltonian matrix elements, especially for large CAS spaces where the high lying model functions are energetically not separated from the outer space determinants. To tackle with this problem the application of incomplete model spaces [18, 27], various level shift-based techniques [14, 23, 30, 45] and the concept of intermediate Hamiltonian [26] were intensively studied, but the most common solution is to use a state-specific description, where only a single target state is described [3-5, 8,16,17, 29,44]. 

The last fundamental aspect characterizing PCM methods, i.e. their quantum mechanical formulation, is presented by Cammi for molecular systems in their ground electronic states and by Mennucci for electronically excited states. In both contributions, particular attention is devoted to the specific aspect characterizing PCM (and similar) approaches, namely the necessity to introduce an effective nonlinear Hamiltonian which describes the solute under the effect of the interactions with its environment and determines how these interactions affect the solute electronic wavefunction and properties. 

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