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The quantum information theoretic measures in terms of Shannon entropy and Fisher entropy (both in position and momentum spaces) on the ground, excited as well as virtual states arising out of the two-photon transitions (1s--> nl; n=2-4, l=0,2) of H atom embedded in classical weakly coupled plasma environment are done for the first time. Fourth order time dependent perturbation theory is adopted within a variational framework for calculating the two photon excitation energies and their respective wavefunctions from an analysis of the pole positions of the non linear response of the system. The representation of virtual state follows from an analysis of the linear response at such poles using a novel method developed by us. Ground and perturbed state wave functions of appropriate symmetries are represented by linear combination of Slater-type orbitals. The analytic form of the momentum space wave functions of ground, excited and virtual states are determined by taking Fourier transformation of the respective position space wave functions. The quantum information measures give interesting insights on the delocalization patterns of the all the real and virtual states under question \textit{w.r.t.} the increase in plasma strength. The estimated data values are found to be in excellent agreement with the few existing in literature for the ground as well as excited states participating in the two-photon transitions. Such data for the virtual states are completely new and can be set as benchmark for future works in related disciplines.

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The stability of two-electron Zee system trapped inside an impenetrable spherical cavity are analyzed using explicitly correlated multi-exponent Hylleraas type basis set in the framework of Ritz variational method. The wavefunction is considered to be consistent with the Dirichlet's boundary condition. Four different Z\in [1,(Z_c)_b,0.25,0.0] values are considered, where (Z_c)_b denotes the critical nuclear charge beyond which the system is embedded in discrete positive energy continuum due to the impenetrable nature of the cavity. The energy contribution due to total correlation (in the presence of both radial and angular correlation) effect, the radial correlation limit and angular correlation limit are also studied in details. The thermodynamic pressure felt by the two-electron Zee system inside the cavity is estimated and a formula replicating the behaviour between the pressure and volume of the cavity is deduced by fitting procedure. Different geometrical properties e.g. radial moments [\langle r_1^p\rangle, \langle r_1r_2\rangle, \langle r_{12}^p\rangle, \langle r_< \rangle, \langle r_>\rangle, p = 1-3], angular moments [\langle\cos\theta_1\rangle, \langle\cos\theta_{12}\rangle] and related important physical quantities are determined. The variation of Kirkwood and Buckingham polarizabilities w.r.t. the pressure felt by the two-electron Zee system are analyzed. The one-electron radial density is estimated for each pair of (Z,R), which has been employed to generate the electrostatic potential as well as different information theoretical measures like Shannon entropy, Fisher entropy, R'{e}yni entropy, Tsalis entropy and Onicescu informational energy. The chosen set of information theoretical measures have been found to be the sensitive tools for describing the changes in the electronic structure due to the spatial confinement. An interesting interplay between the electronic and the nuclear contribution to the classical electrostatic potential is observed leading to the shift in the position of its characteristic minimum due to the compression. Wherever possible, a comparison is made in order to ascertain a high accuracy of our numerical results. The procedure of analytic evaluation of the integrals needed to estimate the atomic properties under consideration are discussed in details.